ID-194 Diagnosing Plant Problems

COOPERATIVE EXTENSION SERVICE • UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, LEXINGTON, KY, 40546

DID-19i4agnosing Plant Problems

Kentucky Master Gardener Manual Chapter 7

By James L. Green, former extension horticulture specialist, Oregon State University; Otis Maloy, retired extension plant
pathologist, Washington State University; and Joseph Capizzi, extension entomologist emeritus, Oregon State University. Edited
for Kentucky by John Hartman, extension plant pathologist (retired), and Lee Townsend, extension entomologist, University of Kentucky.

In this chapter: To determine what factors have damaged a plant, you’ll need
to systematically and carefully observe the plant, its environ-
Diagnostic Terms.................................................. 96 ment, and other plants in the area, then put all the pieces
Plant Identification and Appearance............ 97 together to reconstruct the event(s) that produced the damage. You
Damage Patterns.................................................. 98 must make an accurate diagnosis before taking corrective action.
Development of Damage over Time............. 99 Even if no corrective measures are available, it is good to know what
Distinguishing among Living the problem is and what its future development might be.
Causes of Damage.............................................100
A Step-by-Step Method of Diagnosis .........102 Factors causing plant damage can be grouped into two major
Distinguishing among Nonliving categories:
Causes of Damage.............................................105 • Living organisms such as pathogens (fungi, bacteria, viruses, and
For More Information........................................108
nematodes) and pests (insects, mites, mollusks, mammals, and
birds)
• Nonliving factors such as mechanical damage (for example,
breakage or abrasions), environmental conditions (such as
extremes of temperature, light, moisture, or oxygen), and chemi-
cals (such as herbicides or nutritional disorders)
Some pathogens, insects, and nonliving factors cause damage
only if a plant is weakened by other primary factors. For example,
borers generally attack only trees that already are suffering moisture
or other physical stress.
It’s frequently not enough to use symptoms alone for diagnosis,
because completely different factors may cause similar symptoms
on the same plant. In diagnosing plant damage, you can follow a
series of deductive steps, gathering clues from the general situation
down to an individual plant or plant part to determine the most
probable cause of the damage.
The first step is to identify the plant and the problem. Then,
attempt to distinguish between living and nonliving damaging fac-
tors based on observed damage patterns, development of patterns
over time, and other diagnostic clues. Once you have limited the
probable causes of the damage, you can obtain further information
to confirm your diagnosis from reference books, plant pathologists,
entomologists, horticulturists, and/or laboratory analyses.
These diagnostic steps are described in detail on the following
pages.

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EXTENSION

96 • Diagnosing Plant Problems Chapter 7

Diagnostic Terms Symptoms Necrosis—Death of plant tissue.
Phyllody—A change from normal flower
General Abscission—The dropping of leaves, flow- structures to leafy structures.
ers, or fruit by a plant. Rot—Decomposition and destruction of
Bacterium—A single-celled, microscopic Blight—Rapid, extensive discoloration, tissue.
organism having a cell wall but no chlo- wilting, and death of plant tissue. Rugose—Wrinkled.
rophyll. Reproduces by cell division. Blotch—A blot or spot (usually superfi- Russet—Yellowish-brown or reddish-
Fungus—A plant organism that lacks chlo- cial and irregular in shape and size) on brown scar tissue on a fruit’s surface.
rophyll, reproduces via spores, and usu- leaves, shoots, or fruit. Scab—A crustlike disease lesion.
ally has filamentous growth. Examples Canker—A dead place on the bark and Stippling—Small, light green or chlorotic
are molds, yeasts, and mushrooms. cortex of twigs, stems, or trunks; often specks.
Larva—Immature stage of an insect with discolored and either raised or sunken. Water-soaked—Lesions that appear wet
complete metamorphosis that is special- Catfacing—Disfigurement or malformation and dark and usually are sunken and/or
ized for feeding. A larva looks very differ- of a fruit. Fruits typically affected include translucent.
ent than the adult stage (for example, a tomatoes and strawberries. Wilt—Lack of turgor and drooping of
caterpillar and a moth). Chlorosis—An abnormal yellowish-white leaves from lack of water.
Host—A plant afflicted with a disease or or gray color of plant parts resulting from Witches’ broom—Abnormal brushlike devel-
insect pest. incomplete destruction of chlorophyll. opment of many weak shoots.
Nematode—A microscopic roundworm, Defoliation—The unnatural loss of a plant’s
usually living in the soil. Many feed leaves, generally to the detriment of the Signs
on plant roots and can be disease plant’s health. Can be caused by high
pathogens. wind, excess heat, drought, frost, chemi- Bacterial slime—A gooey or dried mass of
Nymph—Immature stage of an insect with cals, insects, or disease. bacterial cells that oozes out of plant
gradual metamorphosis. It looks very Desiccation—Drying out of plant tissue. tissues.
much like the adult but is smaller. For Dieback—Progressive death of shoots, Conk—A fungal fruiting structure (such as
instance, a grasshopper nymph looks like branches, or roots, generally starting at shelf or bracket fungi) formed on rotting
the adult but does not have wings. the tips. woody plants.
Pathogen—Any organism that causes Dwarfing—The underdevelopment of any Cyst—The swollen, egg-containing female
disease. Generally applied to bacteria, plant organ. body of certain nematodes; can be seen
viruses, fungi, and nematodes. Enation—Epidermal outgrowths on leaves on the outside of infected roots.
Phytotoxic—Toxic to a plant (phyto = or stems. Frass—Sawdust-like material associated
plant). Epinasty—An abnormal downward-curv- with insects chewing into plant tissues.
Sign—Direct evidence of a damaging ing growth or movement of a leaf, leaf Fruiting body—A fungal structure that
factor (for example, a pest or pathogen part, or stem. contains or bears spores.
itself, secretions, insect webbing, chemi- Etiolation—Yellow, long, spindly growth Girdling—The cutting, removing, or
cal residues, records of weather extremes resulting from insufficient light. clamping of bark all the way around a
or chemical applications). Fasciation—A distortion of a plant that trunk or branch, sometimes caused by
Symptom—A change in a plant’s growth results in thin, flattened, and sometimes insect feeding.
or appearance in response to living or curved shoots. Honeydew—A sticky substance excreted
nonliving damaging factors. Flagging—Wilting and/or death of plant by aphids and some other insects.
Vector—A transmitter or carrier of disease. parts, usually starting from the tip(s) of Mine—Tunnel or cavity created by insects
Virus—An infectious agent too small one or a few branches or stems. that feed within leaves or needles.
to see with a compound microscope. Gall—An abnormal, localized swelling on Depending on the insect, it can appear
Multiplies only in living cells. leaf, stem, or root tissue. as a winding serpentine mine or as an
Mosaic—Nonuniform foliage coloration irregularly rounded blotch mine.
with a more or less distinct intermingling Mycelia—Masses of fungal threads
of normal green and light green or yel- (hyphae), which compose the vegetative
lowish patches. body of a fungus.
Mottle—An irregular pattern of light and
dark areas.

Chapter 7 Diagnosing Plant Problems • 97

Plant Identification rhododendron cultivars, are covered
and Appearance by conspicuous, fuzzy, epidermal hairs.
They sometimes are thought to be evi-
First, determine whether a problem truly dence of disease, but they are a normal
exists. It is essential to identify the plant part of the leaf.
(genus, species, and cultivar or variety) to • Some plants have variegated foliage
know what it should look like. Use personal that may resemble symptoms of viral
knowledge or plant reference books or diseases.
consult experts. • Premature leaf or needle drop by ever-
green plants (for example, Ficus ben-
If possible, compare the damaged plant jamina) frequently causes alarm. These
with a healthy or normal plant of the same plants normally retain their leaves or
species and variety. Normal plant parts or needles for three to six years and gradu-
seasonal changes sometimes are mistakenly ally lose the oldest ones during each
assumed to be evidence of disease. For growing season (Figure 1a). This normal
example: leaf drop is obscured by the production
• The ‘Sunburst’ honeylocust might seem of new leaves.
However, prolonged drought or other
to be suffering from a nutrient deficiency stress may cause the whole tree to tempo-
because of its chlorotic, yellowgreen leaf rarily turn yellow and may accelerate leaf
color. However, it was selected for this loss, but that is not a reason for concern.
genetic characteristic, and the color is The leaves that drop or turn yellow are the
normal. oldest ones, and their dropping protects the
• The brown, sporeproducing bodies on tree by reducing water loss.
the lower surface of fern leaves are a If new leaves drop, however, there prob-
fern’s normal propagative organs, not ably is a problem (Figure 1b). The cause may
disease spores or insects. be a pathogen, insect, chemical deficiency,
• The small, brown, clublike tips that toxicity, or root problem.
develop on arborvitae foliage in early In describing a plant abnormality,
spring are male flowers, not deformed distinguish between symptoms and signs.
shoots. Symptoms are changes in a plant’s growth
• Small galls on the roots of legumes such or appearance (for example, galls, blotches,
as beans and peas most likely are nitro- or wilting) in response to living or nonliving
gen-fixing nodules essential to normal damaging factors. Many factors produce
development, not symptoms of root- the same symptoms, so symptoms do not
knot nematode infection. produce a definitive diagnosis.
• The leaves of some plants, such as some

Figure 1. Normal ver-
sus abnormal needle
or leaf drop from
evergreens: (a) If drop
is confined to older
leaves, it is normal.
(b) If newly produced
leaves are lost, there is
a problem.

98 • Diagnosing Plant Problems Chapter 7

Figure 2. If all or a major portion of Figure 3. Nonuniform damage patterns Figure 4. Random, scattered damage on
a tree or shrub dies, suspect a root on a tree canopy. conifer needles.
problem.

Signs are direct evidence of the damag- Damage Patterns
ing factor (for example, the pest or patho-
gen itself, secretions, chemical residues, or Patterns are excellent diagnostic clues
records of weather extremes). A combina- and are where you start making the distinc-
tion of signs and symptoms is more likely tion between living and nonliving causes of
to produce a definitive diagnosis than are plant damage.
symptoms alone.
Nonuniform or Random
Examine the entire plant. In defining a Damage (Living Factors)
plant problem, it is essential to determine
the true primary problem, because the plant With problems caused by living organ-
part exhibiting obvious symptoms may isms such as pathogens or insects, there
not be the part experiencing damage. For usually is no uniform pattern of damage. It
example, some root problems cause foliage may appear randomly on parts of a plant or
symptoms. In this case, the primary prob- on some plants in a group.
lem is damage to roots, not foliage.
If scattered damage occurs in the plant
In general, if the entire top of a plant or canopy, suspect a problem in the foliage or
entire branches look abnormal, examine the aerial environment, not the roots (Figure 3).
plant downward to find the primary dam- If scattered branches gradually decline and
age (Figure 2). Look for the factor causing eventually die, suspect a canker pathogen,
the damage at the edge of the symptomatic shoot blight, or borers. Verticillium wilt, on
area, and always examine the roots. the other hand, is characteristically one-
sided on a tree or shrub and can develop
relatively quickly—in a month or so on
some species (for example, some maples).

Figure 5. Shoot dieback caused by a Figure 6. Uniform death of needle tips. Cracked
fungal infection. bark

Figure 7. Shoot dieback caused by
winter injury.

Chapter 7 Diagnosing Plant Problems • 99

Similarly, living organisms usually dam- Damage caused by nonliving factors
age leaves or needles in a random pattern usually results in a sharp margin or edge
(Figure 4). For example, conifer needles between affected and healthy tissue.
usually are affected over varying lengths However, if bark and wood are cracked,
and often appear straw yellow or light tan. suspect winter injury (Figure 7), in which
Black fruiting fungal bodies may be present dieback often is gradual rather than sudden.
on diseased needles. Often, damage appears
first on lower and inner parts of the canopy, Development of
where humidity is higher, and then pro- Damage over Time
gresses upward and outward.
Another clue for distinguishing between
With living damaging factors, there may living and nonliving factors is what you see
not be a sharp line between affected and when you observe the pattern over time.
healthy tissues. Bacterial shoot blight is an
exception, however. With this disease, the Sudden Decline
margin between affected and healthy tis- (Nonliving Factors)
sue often is irregular and sunken. Bacterial
shoot blight usually causes the shoot tip to Sudden decline generally is caused by a
wilt and bend over. nonliving factor such as a toxic chemical or
extreme weather. All affected leaves might
With fungal shoot dieback, there may die immediately after a chemical application
be small, pinlike projections or bumps over that is poisonous to plants. If branches die
the surface of dead bark (Figure 5). These suddenly, especially if affected branches are
structures are the spore-producing parts of concentrated on one side of the plant, suspect
the fungi. Note, however, that small, woody weather, animal damage, or chemical drift.
bumps normally radiate from all sides of
spruce twigs where old needles were attached. If a nonliving damaging factor is not
removed, damage will intensify. For exam-
Uniform Damage ple, if a toxic chemical remains in the soil or
(Nonliving Factors) air, plant damage within the contaminated
area will continue to develop, but it won’t
Damage patterns produced by nonliving spread to uncontaminated areas.
factors, such as frost or toxic chemicals, gener-
ally are more regular. For example, they: Sudden decline can be associated with
• May appear on all leaves of a certain age living factors, such as when a plant is weak-
ened by disease or insects and then sub-
(for example, those forming the plant jected to an otherwise survivable stress (for
canopy when a toxic spray is applied) example, Phytophthora root rot followed by
• May affect all leaves with a certain expo- a normal summer drought period).
sure (such as those on the southwest side
of a plant that are not shaded by other Gradual Decline (Living Factors)
leaves)
• Likely will appear on more than one Gradual decline of an entire plant or a
type or species of plant. Look for similar major portion of it usually is caused by a liv-
damage patterns on weeds, neighboring ing factor (for example, Armillaria root rot,
plants, etc. Verticillium wilt, or root weevils. However,
it can also be the result of marginally surviv-
Air pollutants frequently cause tip burn able conditions such as nutrient-deficient
on conifers, as do certain soil-applied soil.
herbicides and excess fertilizer (Figure 6).
Freezing may have a similar effect. All nee- Living organisms multiply and grow
dles at a specific growth stage usually are with time; therefore, they rarely afflict all
affected, and usually each needle is affected of the host plant or plants at once. Damage
to the same length. Affected tissue usually is generally appears first on one part of a plant
reddish brown. and spreads. Likewise, it progresses from

100 • Diagnosing Plant Problems Chapter 7

Table 1. Symptoms and signs of fungal and bacterial leaf spots.

Abnormality Fungal Bacterial
Watersoaking
Not common Common with rots of thick and succulent leaves and with
Texture initial appearance of angular leaf spots
Odor
Usually dry; may be papery Some dry, some slimy to sticky; may be papery when dry
Pattern
Usually none Foul (putrid to earthy, acrid) odor usually associated with
Disintegration rots of fleshy plant parts
Color changes
Irregular to circular; may Irregular to angular; often restricted by large veins
Pathogen have concentric rings
structures
Uncommon Common with rots

Common: red, yellow, Less common, but may have irregular, yellow halos
purple halos

Common: mycelia, spores, Wet or dried slime at edge of canker or leaf spot
spore structures

plant to plant. For example, gradual shoot Fungal diseases
decline with retention of dead leaves usually The presence of fungal mycelia and
indicates damage by a living factor.
fruiting bodies are the best clue to a fungal
Bacterial shoot blight and Phytophthora disease. Fruiting bodies range in size from
canker are exceptions to the gradual decline microscopic to those easily detectable with
rule. They can cause rapid dieback. the naked eye. They are found within the
leaf spot or stem rot area on an infected
Distinguishing among plant. Each type of fungus has its own
Living Causes of Damage characteristic structures, which are used by
plant pathologists for identification.
To identify the type of living factor that
has damaged a plant, closely examine symp- Fungal leaf spots and stem rots are char-
toms and signs. Symptoms are the modified acterized by various symptoms: dry texture,
appearance of the affected plant, such as concentric rings, and discoloration. Spots
necrotic tissues, chlorosis, cankers, galls, or generally have distinct margins. They vary
leaf distortion. Signs are direct evidence of in size and usually are round and occasion-
the actual organism. Examples are insects, ally elongated (Figure 8).
fungal mycelia, spores, egg masses, insect
frass, and mite webbing. Signs can be clues Figure 8. Fungal leaf spots.
for identifying the specific organism that
produced the damage. A combination of
both symptoms and signs may be necessary
for the initial distinction between disease
and insect/mite damage.

Symptoms and Signs of Disease

Differentiating between bacterial and
fungal pathogens, especially those that
cause leaf spots, is not always clear-cut, but
certain symptoms are distinctive. Table 1
lists key distinguishing characteristics of
fungal and bacterial leaf spots.

Chapter 7 Diagnosing Plant Problems • 101

Concentric rings result as the mycelium Erwinia blight is an example of a bacte-
grows outward from the point of initial rial disease that causes water-soaking. The
infection (much like the crocheting of a water-soaked appearance occurs when
doily). Leaf color ranges from tan (died bacteria dissolve the material holding plant
first) in the center to darker brown (recently cells together, thus destroying leaf or stem
died) to very dark. The outer ring may have integrity. Some fungi also produce water-
a light yellow, chlorotic edge where the soaking, but usually not as extensively as
infection is advancing. Margins of fungal Erwinia blight.
leaf spots and stem rots can be brightly
discolored. Initially, bacterial leaf spot symptoms are
confined between leaf veins, resulting in
Bacterial diseases discrete, angular spots with straight sides
Bacteria enter plant tissues through (Figure 9). Many bacterial leaf spots, such
as Xanthomonas leaf spot on philodendron
wounds or natural openings in leaves, (also called red edge disease), expand until
stems, roots, or fruit. Once they enter a they reach a large leaf vein. The vein fre-
plant, they reproduce rapidly and release quently inhibits the bacteria from spreading
enzymes and toxins that kill plant cells. farther.

Bacterial galls Bacterial leaf spot’s color usually is
Crown gall bacteria genetically engineer uniform, though a chlorotic halo often
surrounds a spot. Spots may enlarge and
their host plant to make galls and amino coalesce. In final stages, cracks may form
acids, thus giving the bacteria a place to in the plant tissue; disintegration follows.
live and the chemicals they need to grow Some lesions may exude bacteria-filled
and reproduce. The galls are characterized fluid.
by hard, unspecialized plant cells. When
young, galls resemble a head of cauliflower, A few bacterial leaf blights, particularly
but they harden with age. on thick or spongy-textured leaves, are
slimy and may have a rotten odor.
Bacterial leaf spot
These bacteria usually enter through Vascular wilt
In some cases, bacteria plug a plant’s
leaf stomata. A common symptom is a
watersoaked appearance, which is usually water-conducting vascular tissue. The result
revealed by holding an infected leaf to light. is yellowing, wilting, browning, and death of
The tissue may become translucent, papery, leaves, stems, and roots.
and tan when dry.

Figure 9. Bacterial leaf spots. Figure 10. Typical signs of viral infection: (a) Vein
banding (b) mosaic.

(a)

(b)

102 • Diagnosing Plant Problems Chapter 7

A Step-by-Step Method of Diagnosis Viral diseases
Viruses are submicroscopic entities
1. Determine that a real problem exists, then define it.
a. Identify the plant. Establish what a “normal” plant would that infect individual plant cells. They are
obligate parasites, meaning that they can
look like at this time of year. Describe the abnormality replicate only within a host’s cell. As a virus
(symptoms and signs). proliferates, it moves on to infect other cells.
b. Examine the entire plant and its community. Locate the
primary problem and the plant part where initial damage Because a virus commandeers its host
occurred. cell to manufacture viruses identical to
2. Look for patterns. Is the damage on more than one plant? On itself, the cell cannot function and grow
more than one plant species? normally. Chlorophyll production may stop,
a. A non-uniform damage pattern (irregular or random pat- causing necrosis or the yellowing or blanch-
tern of damage) is indicative of living factors (pathogens, ing of chlorosis. In some cases, cells may
insects, mites, or other animals). grow and divide rapidly; in others, they may
b. A uniform damage pattern (such as damage on all leaves of grow very slowly or stop dividing, causing
a certain age or all plants in an area) indicates nonliving fac- distortion or stunting. The symptoms of
tors (mechanical, physical, or chemical). most viral diseases fall into four categories:
3. Outline development of damage over time.
a. Progressive spread of the damage on a plant or onto other Lack of chlorophyll in normally green organs—
plants indicates damage caused by living organisms. A common first symptom is vein clearing,
b. Damage that does not spread to other parts of the affected in which veins look somewhat translu-
plant or to other plants, and a clear line between damaged cent or transparent but interveinal tissue
and undamaged tissues, indicate damage caused by nonliv- remains green. In vein banding, a dark
ing factors. green, light green, or yellow band of tissue
4. Gather information to identify possible causes of damage. appears along the veins (Figure 10a). (When
a. Distinguish among living factors. leaf veins remain green but tissue between
1. Symptoms and signs of pathogens veins becomes chlorotic, the cause may
2. Symptoms and signs of insects, mites, and other animals be deficiency of a nutrient such as iron.)
b. Distinguish among nonliving factors. Virus-infected foliage also may be mottled
1. Mechanical factors green and yellow, mosaic, ringed, or a rather
2. Physical factors uniform yellow (Figure 10b).

a. Temperature extremes Stunting or other growth inhibition—As
b. Light extremes chlorophyll is lost, reduced photosynthesis
c. Oxygen and moisture extremes leads to shorter internodes (spaces between
3. Chemical factors nodes), smaller leaves and blossoms, and
a. Pesticide or pollutant phytotoxicities lower yield.
b. Nutritional disorders
c. Use references. You may need laboratory analyses to narrow Distortions—Strangely formed leaves and
the range of probable causes. flowers, witches’ brooms, or rosettes result
5. Synthesize information to determine probable causes. from non-uniform or uncontrolled growth.

Necrotic areas or lesions—Viruses require
their host’s survival for their own procre-
ation, so they rarely cause death. Necrosis
usually is confined to discrete areas of the
plant.

These symptoms can be valuable clues
for virus identification but are easily con-
fused with symptoms of nutritional disor-
ders, chemical injury, or damage caused
by mites or insects. In addition, because
of their extremely small size, viruses
are not visible to the unaided eye. Virus
particles are detectable only through an
electron microscope or with special stains
and a compound (high-magnification)
microscope.

Chapter 7 Diagnosing Plant Problems • 103

Viruses are transmitted from plant to insect pest. Knowing an insect’s life cycle
plant by insects, mites, fungi, nematodes, (complete or simple metamorphosis) also is
rubbing, abrasion, grafting, or other important when attempting to identify an
mechanical means. They occasionally are insect or design a control program.
transmitted in seed.
Feeding habits
Nematodes General groups may be distinguished
Nematodes are microscopic round-
by their method of feeding. Beetles and
worms that damage plant tissues as they caterpillars use chewing mouthparts to eat
feed. Many feed on or in root tissues, but a portions of leaves, while aphids and plant-
few feed on foliage or other aboveground hoppers remove plant sap with tubelike
organs. sucking mouthparts. Thrips rasp leaf and
flower tissue; plant feeding mites remove
Root nematodes the contents of cells, leaving a stippling pat-
Root-infesting nematodes damage root tern or tiny white spots on leaves. Use the
following clues to find the cause of chew-
systems, causing aboveground symptoms— ing/rasping damage:
commonly moisture and nutrient stress and • Caterpillars and some beetles consume
general stunting. Root-lesion nematodes
(Pratylenchus spp.) and burrowing nema- entire leaves, leaving only the toughest
todes (Radopholus similis) destroy root veins.
tissues as they feed. Rootknot nematodes • If distinct portions of the leaf are miss-
(Meloidogyne spp.) inject growth-regulating ing, the cause could be black vine weevil
substances into root tissues, stimulating (the adults cut distinct notches from leaf
growth of large, tender cells that become margins); leaf cutter bees (circular holes
permanent feeding sites. As root tissues cut from margins); or beetles, chafers,
grow around these sites, they form visible weevils, or grasshoppers (small, ran-
swollen galls or knots. Other root nema- domly scattered holes).
todes stunt growth, apparently by killing • Damaged (especially skeletonized) leaf
root meristems. surfaces may indicate slugs, leaf beetle
larvae, pear slugs (pear sawfly larvae),
Shoot nematodes (Aphelenchoides spp.) elm leaf beetles, or thrips.
Shoot (foliar) nematodes feed inside • Leaves tied with silken threads or rolled
into tubes often harbor leafrollers or leaf-
leaves between major veins, causing chlo- tiers (for example, omnivorous leaftiers).
rosis and necrosis. The tissue first collapses • If there is a discolored or swollen area on
in wedge-shaped areas between the larger a leaf, hold it up to the light to look for
veins and then changes color. Injury most insects or frass in the damaged area. The
often is seen at the base of older foliage. culprit may be leafminers, which feed
between upper and lower leaf surfaces
Symptoms and Signs (for example, boxwood, holly, birch, and
of Insect Pests elm leafminers).
• If petioles are weakened and leaves fall
Finding the insects feeding on a plant in early summer, suspect petiole and leaf
is the surest way to identify a problem. stalk borers (for example, maple petiole
However, you may have to rely on other borers), which burrow into petioles near
clues if the pest is no longer present. These leaf blades or bases. Cut open a petiole to
clues include feeding location on the plant, look for a small moth or sawfly larva.
type of feeding damage, time of year, and • If a twig’s bark is girdled (cut, removed,
types of plants being damaged. The loca- or clamped all the way around), the cul-
tion and type of feeding damage are the prit may be vine weevil or twig-girdling
most important clues in identifying an beetle.

104 • Diagnosing Plant Problems Chapter 7

• If a plant or specific branch is in general • Leaf and stem distortion plus off-color
decline, examine it closely for frass, pitch, foliage is the result of aphids (for exam-
or holes in the bark. These signs are ple, rose aphids, black cherry aphids, or
caused by borers, which feed under the leaf curl plum aphids). It is often is con-
bark in cambium tissue, solid wood, or fused with injury from growth-regulating
xylem tissue. Examples are bark beetles, chemicals.
round and flatheaded borers, and clear-
wing borers. • Galls (swellings on leaf or stem tissue)
may be caused by various species of
• General decline of a plant along with aphids, wasps, midges, and mites.
chewed roots may indicate damage by
root weevil larvae or white grubs. • Twigs that look like they’ve been split by
Sap removal by sucking insects can cause a sharp instrument are the result of egg
laying (oviposition) by sucking insects
wilting and occasionally damage to leaf such as treehoppers and cicadas. Cicadas
veins. A few inject saliva or toxic sub- in particular often split a branch enough
stances that cause discoloration or distorted to kill its tip.
growth. Insects that secrete phytotoxic
substances are called toxicgenic (toxin • General decline of an entire plant or
producing). The resulting plant damage is plant part, as indicated by poor color,
called phytotoxemia or toxemia. reduced growth, or dieback, may be the
result of root, stem, or branch feeders
Spotting or stippling occurs when chlo- such as scale insects or mealybugs.
rophyll is destroyed at the feeding site but
toxins do not diffuse throughout the leaf. • Accumulations of shiny honeydew
Aphids, leafhoppers, planthoppers, and and sooty mold indicate infestations of
spider mites bugs commonly cause this type aphids or soft scales.
of injury.
• Stunted tip growth of trees and shrubs
Severe toxemias develop when toxic may be due to armored scales.
saliva causes leaves to curl and pucker
around an insect. Severe aphid infestations Insect life cycles
may cause this type of damage. Most insects develop from egg to adult

In some cases, toxic effects spread through gradual (also called simple or
throughout the plant, resulting in reduced incomplete) metamorphosis or complete
growth and chlorosis. This condition is metamorphosis. Gradual metamorpho-
known as systemic toxemia. Psyllid yellows sis has three stages—egg, nymph, adult.
of potatoes and tomatoes and infestations Nymphs resemble adults and feed and
of scale insects or mealybugs may cause behave much like them. Examples include
systemic toxemia. grasshoppers and aphids.

Look for these clues when determining Complete metamorphosis has four
the cause of sucking damage: stages—egg, larva (specialized feeding
• General (uniform) stippling, flecking, stage), pupa, and adult. Examples include
moths and beetles. Larvae look very differ-
or chlorotic patterns on spruce needles ent from the adult stage.
usually are caused by spider mite dam-
age. Lace bugs may be the cause on other In both types of metamorphosis, the
plants. nymphs or larvae increase in size in distinct
• Random stipple patterns on leaves are steps when they molt (shed their external
caused by leafhoppers and mites. skeleton). Discarded exoskeletons found
near feeding sites on plants may be helpful
in identifying the pest. All growth is com-
pleted before the adult stage.

Chapter 7 Diagnosing Plant Problems • 105

Symptoms and Signs of Distinguishing among
Other Animal Damage Nonliving Causes of Damage

A variety of other living organisms can Uniform patterns of damage indicate
damage plants. For example: that a nonliving factor is the probable cause.
The three broad categories of nonliving fac-
Arachnids—Arachnids have eight legs tors include the following:
(insects have six) and sucking mouthparts.
Spider mites are an example. They have a Mechanical factors—abrasion and bruising
simple life cycle and often cause leaf stip- from construction or maintenance equip-
pling, with damaged leaves then turning ment, handling during transplanting, or
pale on the underside. Severe infestation lawn mowers.
causes leaf bronzing and death. Foliage may
appear dirty, due to the presence of small, Physical factors—environmental or
fine webbing mixed with eggs and frass on weather changes such as temperature
the underside of leaves. Eriophyid mites, extremes, light, moisture, or wind.
on the other hand, cause distorted new
growth, rolled leaf margins, and swollen Chemical factors—pesticide applications,
veins. Symptoms of arachnid damage often aerial and soil pollutants, and nutritional
are confused with damage from growth disorders.
regulators.
Additional clues, as discussed below, will
Crustacea—Sowbugs and pillbugs usually help you determine which of these factors
feed on decaying vegetation. Unless popula- caused the damage.
tions are high, they are not considered
damaging to live plants. Mechanical Factors

Mollusca—Slugs and snails feed on low- First, consider whether there has been
growing foliage and fruit growing in shaded, recent excavation, construction, or paving
humid areas. They use rasping mouthparts on the site. Then examine the damaged
to scratch and loosen soft tissue. Look for plant. Close examination often reveals
slime trails on foliage and the surrounding whether stems or roots were broken or
soil. girdled and whether leaves were bruised,
punctured, or broken. For example, if stems
Miscellaneous animals—Millipedes (arthro- break because a plant is dropped during
pods) feed on decaying plants. They have transplanting, rapid wilting will occur above
many small legs, are brownish or white in the break.
color, and vary in size from 1⁄2 to 2 inches
long. They are not considered injurious to Physical (Environmental) Factors
live plants.
Primary sources of diagnostic informa-
Small mammals—Chewed bark and tion for damage caused by physical factors
cambium tissue on small trees and shrubs are damage patterns and weather records.
most frequently is caused by mice, rabbits, Temperature extremes
squirrels, or possibly beavers. Look for teeth
marks. Extremely hot or cold temperatures can
damage plants. In both cases, recognizable
Large mammals—Cattle, goats, deer, and damage patterns are likely to occur. Heat
horses tear or cut branches. damage is most likely to occur in the early
afternoon when the sun is in the southwest
Birds—Missing flower petals or punc- quadrant of the sky. Therefore, damage
tured bark may be caused by yellowbellied occurs primarily on outer, unshaded leaves
sapsuckers. They make even rows of holes on a plant’s southwest side. Leaves shaded
in tree trunks. by other leaves or those on the northeast
side may be undamaged. The most severe
damage occurs on plant parts farthest from
water-carrying roots, stems, and leaf veins.

106 • Diagnosing Plant Problems Chapter 7

Thus, leaves on the outer perimeter of the Aboveground symptoms of cold damage
plant, leaf tips, and interveinal areas are to roots generally do not become evident
most commonly damaged. This damage until shoots begin to grow in the spring. At
pattern is likely to occur uniformly over all that time, leaf expansion may be incomplete
plants in an area. Cold damage occurs on because of restricted water and nutrient
the least hardy plants and is most severe on uptake by the damaged root system. The
the least hardy tissues of those plants. Plants result is smaller-than-normal leaves. As
vary in their hardiness (cold tolerance). By air temperature increases, water loss from
checking how low the temperature dropped shoots and leaves may exceed the roots’
and knowing the indicator plants for vari- ability to take up water. As a result, the plant
ous USDA cold hardiness zones, you can loses leaves or wilts, and it may die.
begin to determine whether plant damage
was caused by cold. Light extremes
Plants can acclimate to various light
A plant gains cold hardiness gradually
beginning at its terminal (tip) buds, and conditions but need time to do so. They
regains it gradually (also beginning at the tip respond adversely to rapid light change. A
buds), so the location of cold damage on a sudden shift from low to high light destroys
plant will tell you approximately when the a plant’s chlorophyll and leads to yellow-
damage occurred. Damage to buds occurs ing and necrosis. Rapid change from high
most often in late winter or early spring, to low light, on the other hand, results in
while damage to lower plant parts may reduced growth and leaf drop. Under low
occur in the fall or early winter. light, new leaves are larger, thinner, and
darker green than normal, while flowering
On a given structure (such as a leaf or is reduced, delayed, or absent.
bud), exposed, nonhardy tissues are dam-
aged in a recognizable pattern. For example: Oxygen and moisture extremes
• Spring frost damage uniformly kills new, Oxygen and moisture extremes primar-

succulent growth. New growth emerging ily affect the root environment, where
after the frost will be healthy. the quantities of oxygen and moisture are
• Frost cracks are lengthwise separations inversely related. That means a waterlogged
of bark and wood that generally occur on (saturated) root environment lacks suf-
the southwest side of a trunk. They are ficient oxygen for root metabolism and
the result of wide day/night temperature growth. Consequently, less water and nutri-
fluctuations in winter. ents are taken up by plants.
• The dividing cells on outer portions of
leaves may freeze while still inside the Drought and waterlogging produce
bud. As a result, distorted or lacelike leaf many of the same aboveground symptoms.
blades develop. The first symptoms to appear are wilt-
Generally, root systems cannot survive ing, chlorosis, and abscission (dropping)
at as low a temperature as can aboveground of older leaves. Under severe, continuing
plant parts. Fortunately, soil temperatures in moisture stress, wilting and necrosis occurs
winter usually are warmer and more stable on tips and interveinal regions of recently
than air temperatures. Thus, cold damage to expanded leaves and new growth.
roots primarily is a concern with container-
grown plants, where soil temperatures Chemical Toxicities
fluctuate more than in ground soil. Examine
the root system to detect damage in Patterns of chemical injury on indi-
container-grown plants. It generally occurs vidual plants differ, depending primarily on
on the periphery of the root ball (near the whether a chemical causes damage directly
container edge). Evidence includes black or on contact or is absorbed and moves
spongy roots that lack new growth or new throughout the plant.
root hairs.

Chapter 7 Diagnosing Plant Problems • 107

Direct-contact damage Figure 11. Foliar chemical spray injury.
Direct-contact damage can occur on
same result.
both foliage and roots. Keep in mind that many other factors

Shoot/foliage contact also injure roots or inhibit their growth,
Symptoms from shoot-contact chemicals including nematodes and other pathogens,
soil compaction, cold weather, salinity, and
occur over the general plant canopy. The nutritional deficiencies or excesses.
injury does not spread with time or move to Translocated chemical damage
previously undamaged plants.
Some chemicals can move throughout
Injury is typified by chlorotic or necrotic a plant after being absorbed. The effects of
spotting. Spots usually are uniformly and these mobile chemicals depend on whether
evenly distributed over the leaf surface and they are transported in the xylem or the
generally are the same size. Color usually phloem.
is uniform across the spot, and the margin
between affected and healthy tissue usually If transported solely in the xylem, the
is sharp (Figure 11). chemical moves upward through the
plant, primarily causing symptoms in older
If a chemical is applied directly to foliage. Examples of xylem-transported
aboveground parts, you may be able to see chemicals include urea fertilizer and the
the application pattern. For example, the agricultural herbicides triazine, alachlor,
pattern of spray droplets may be visible or and metolachlor.
areas where spray accumulated along leaf
edges may show the most damage. In the If transported in the phloem, a chemi-
case of a toxic gas (volatile chemical), areas cal may move in many directions from
between leaf veins and along leaf margins— the point of absorption; for example, it
where water concentration is lowest—show may move from the shoots to the roots or
damage first. vice versa. Symptoms caused by phloem-
transported toxic chemicals occur primarily
Examples of shoot/foliage contact in the plant’s new growth and meristematic
chemicals are foliar-applied fertilizers; the regions. Affected young tissue is discol-
agricultural herbicides paraquat, acifluor- ored or deformed, and injury may persist
fen, and dinoseb; and herbicidal oils. (Very for several sets of new leaves. Examples
few, if any, contact herbicides are available of phloem-transported toxic chemicals
to home gardeners.) include the common garden herbicides
2,4D, dicamba, and glyphosate.
Root contact
Toxic contact chemicals in the root zone,

including excess fertilizer, result in poor
root development. Roots are injured and
root tips may be killed.

Aboveground shoots may show water
and nutrient stress symptoms—for example,
reduced growth, wilting, or chlorosis—
because the roots are unable to obtain
water. In severe cases, wilting can occur
even when the soil is wet. Lower leaves
generally wilt first, followed by drying of
leaf margins.

Herbicides that inhibit root growth
include the agricultural herbicides dini-
troanilines, DCPA (Dacthal), and diphena-
mid. Excess nitrogen fertilizer can have the

108 • Diagnosing Plant Problems Chapter 7

Nutrient Deficiencies Calendar for Common Kentucky Shade
Tree and Woody Ornamental Pests.
Like the effects of toxic chemicals, the http://www.ca.uky.edu/entomology/ent-
effects of nutrient deficiencies depend on facts/ef447.asp
whether the chemical is transported only in
the xylem or also in the phloem. Woody Plant Disease Control Guide for
Kentucky (ID-88). http://www.ca.uky.
Nutrients that are transported solely in edu/agc/pubs/id/id88/id88.pdf
the xylem are immobile in a plant once they
have been moved upward from the roots. Guide for Control of Annual and Perennial
They cannot be moved through the phloem Flower and Ground Cover Diseases in
to new growth if a soil deficiency develops, the Landscape (ID-87). http://www.
so symptoms of these chemicals typically ca.uky.edu/agc/pubs/id/id87/id87.pdf
develop on new growth. Phloem-immobile
chemicals include boron, calcium, iron, Other publications
manganese, zinc, copper, molybdenum, Bennett, W.F., ed. Nutrient Deficiencies and
and, in some plant species, sulfur.
Toxicities in Crop Plants (APS Press, St.
In contrast, a phloem-mobile nutrient Paul, MN, 1993). 202 pp.
can be withdrawn from older leaves and Borror, D.J., and R.E. White. A Field Guide
moved to growing root and shoot tips if a to the Insects of America North of
soil deficiency develops, so that deficien- Mexico (Houghton Mifflin Company,
cies of these nutrients will first be visible on Boston, 1970).
older leaves. Nitrogen, phosphorus, potas- Flint, M.L. Pests of the Garden and Small
sium, magnesium, chlorine, and, in some Farm: A Grower’s Guide to Using Less
plant species, sulfur, are phloem-mobile. Pesticide, Publication 3332 (University of
California, Los Angeles, 1999).
For More Information Johnson, W.T., and H.H. Lyon. Insects That
Feed on Trees and Shrubs, 2nd ed.,
Many other publications on specific rev. (Comstock Publishing Associates,
insect and disease problems are listed in Cornell University Press, Ithaca, NY,
other chapters. 1991).
University of Kentucky Cooperative Extension The Ortho Problem Solver, 7th ed.
publications (Chevron Chemical Co., San Francisco,
Timing Control Actions for Landscape 2008).
Pirone, P.P. Diseases and Pests of
Insect Pests Using Flowering Plants as Ornamental Plants, 5th ed. (John Wiley
Indicators. http://www.ca.uky.edu/ento- & Sons, New York, 1978).
mology/entfacts/ent66.asp Pirone, P.P. Tree Maintenance, 7th ed.
(Oxford University Press, New York,
2000).
Sherf, A.F., and A.A. Macnab. Vegetable
Diseases and Their Control, 2nd ed.
(John Wiley & Sons, New York, 1986).
Sinclair, W.A., H.H. Lyon, and W.T Johnson.
Diseases of Trees and Shrubs, 2nd ed.
(Cornell University Press, Ithaca, NY,
2005).

Chapter 7 Diagnosing Plant Problems • 109

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls

General Water during dry periods.
Soil test; supply nutrients based on results.
Poor fruit yield; fruit may be Uneven moisture Thin plants; do not plant in shade.
small and have poor taste Poor soil fertility Growth will improve when weather warms.
Soil test; supply nutrients based on results.
Plants grow slowly; leaves light Insufficient light Soil test; adjust pH as needed.
green. Do not overwater; improve drainage.
Cool weather Water.
Use sterile pots and planting media; do not overwater;
Poor soil fertility allow soil to dry slightly between waterings; treat seed
with registered fungicide before planting.
Improper pH Plant at correct depth.
Wait for appropriate weather conditions before planting.
Excess water Use floating row cover; apply registered soil insecticide.
Use sterile pots and planting media; do not overwater;
Seedlings don’t emerge Dry soil allow soil to dry slightly between waterings; treat seed
with registered fungicide before planting.
Damping-off (fungal disease) Use cutworm collars, biological control agent; apply regis-
tered soil insecticide.
Wilted seedlings; seedlings Incorrect planting depth Use floating row cover for seed maggots only; apply regis-
fall over Slow germination due to weather tered soil insecticide.
Root maggots Place fence around garden.
Damping-off (fungal disease) Use slug barrier or bait.
Identify insect; use appropriate nonchemical or chemical
Cutworms control.
Water.
Root maggots, wireworms Use sterile pots and planting media; do not overwater;
allow soil to dry slightly between waterings; treat seed
Chewed seedlings Rodents, rabbits, or birds with registered fungicide before planting.
Slugs Plant resistant varieties; rotate.
Various insects
Plant resistant varieties; rotate with nematodes that attack
Wilted plants; bottom leaves Dry soil insects; remove diseased plants; fumigate soil.
may turn yellow Submit soil sample for nematode analysis; plant resistant
Root rot (fungal disease) varieties; rotate; remove diseased plants; fumigate soil.
Improve drainage; do not overwater.
Vascular wilt (fungal disease, mainly Soil test; supply nutrients based on results.
affecting tomatoes, potatoes, egg- Thin plants; do not plant in shade.
plants, and peppers) Use predatory mites; apply insecticidal soap or registered
Root-knot nematodes, white grubs miticide.
Water.
Various root-feeding nematodes Test soil for soluble salts; do not overfertilize; flush soil
with water.
General leaf yellowing; no Waterlogged soil Soil test; supply nutrients based on results.
wilting Nutrient deficiency Do not plant too early.
Insufficient light See specific plant listings below.
Leaves stippled with tiny, Spider mites Do not apply chemicals that are not registered for use on
white spots the plant; apply chemicals at recommended rates and in
Leaf margins turn brown and Dry soil the right environmental conditions.
shrivel Fertilizer burn Use resistant varieties; space plants adequately for better
air circulation; control weeds; apply registered fungicide.
Discrete, brown spots on Potassium deficiency
leaves; some spots may Cold injury
coalesce Fungal or bacterial leaf spot disease
Chemical injury

White, powdery growth on Powdery mildew (fungal disease)
upper leaf surfaces

110 • Diagnosing Plant Problems Chapter 7

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls
Leaves shredded or stripped —
from plant Hail damage Place fence around garden; use traps.
Rodents Use slug barrier or bait.
Slugs Avoid wetting foliage; space plants adequately for better
Dead tissue drops out after fungal air circulation; apply registered fungicide before problem
infection reaches this stage.
Identify insect; use appropriate nonchemical or chemical
Various insects control.
Plant resistant varieties if available; plant certified,
Leaves with yellow and green Viral disease disease-free seeds or seedlings; control weeds; remove
mosaic or mottle pattern and destroy affected plants.
Be careful when using herbicides.
Leaves curled, puckered, or Herbicide injury (common on toma-
distorted toes and cucumbers) Plant resistant varieties if available; plant certified,
Viral disease disease-free seeds or seedlings; control weeds; remove
and destroy affected plants.
Aphids Pick off, mash, or wash away insects; use biological control
agent; treat plant (especially underside of leaves) with
Tops turn yellow, brown, and Rust (fungal disease) insecticidal soap or registered insecticide.
die back; reddish-brown, Plant resistant varieties; cut tops close to ground in fall
orange, or black pustules Fusarium wilt (fungal disease) and destroy; remove and destroy affected leaves; improve
appear on stems and leaves Verticillium wilt (fungal disease) air circulation; avoid wetting foliage; apply registered
Shoots wilt, turn yellow, then fungicide.
brown; vascular tissue in Plant resistant varieties; destroy affected plants; rotate for
crown area is reddish-brown 2 to 4 years; fumigate soil.

Asparagus

Small spears; immature plants Asparagus produces small spears for 2 or 3 years after planting.

Plants overharvested during previous Do not harvest late into the season: plants can’t store
year enough food for following season.

Poor fertility Soil test; supply nutrients based on results.

Poor drainage Do not overwater; plant in well-drained area.

Spears crooked Mechanical injury from windblown Be careful not to damage emerging spears when
sand or mishandling harvesting.

Asparagus beetles, Japanese beetles Beat foliage over a tray and discard the insects; apply
registered insecticide.

Spears turn brown and soft Frost injury Protect spears with mulch.

Root rot (fungal disease) Rotate; plant in well-drained area; do not overwater;
remove plant debris.

Leaves chewed; slime may be Slugs (emerge at night and hide dur- Use slug barrier or bait.
present on leaves; no evidence ing the day)
of insects.

Spears and leaves chewed or Asparagus, Japanese beetles Beat foliage over a tray and discard the insects; apply
scarred registered insecticide.

Beans

Skeletonized leaves Mexican bean beetle larvae and adults Apply registered insecticide.

Plants wilt/are stunted; leaves, Dry soil Water.
may turn yellow Rotate; plant in well-drained area; do not overwater;
Root rot (fungal disease) remove plant debris.

Root-knot nematodes Rotate; remove diseased plants; fumigate soil.

Poor fertility Soil test; supply nutrients based on results.

Chapter 7 Diagnosing Plant Problems • 111

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls

Failure to set pods High temperature causes blossoms to Wait for cooler weather.
drop

Dry soil Water.

Wet soil causes lack of oxygen to roots Do not overwater; plant in well-drained area.

Mature pods left on vines cause seed Pick pods regularly.
production rather than pod set

Rust-colored powdery spots Rust (fungal disease) Plant resistant varieties; remove plant debris and pods;
surrounded by yellow halos on remove and destroy affected leaves; apply registered
leaves and stems fungicide.

Soft, watery spots or white, White mold (fungal disease) Rotate; remove plant debris; improve air circulation; use
moldy growth on leaves, registered fungicide.
stems, and pods; plants wilt
and die

Water-soaked spots followed Halo blight (bacterial disease) Delay planting until warm weather; plant disease-free
by irregular brown spots on seed; rotate; remove plant debris.
underside of leaves

Mottled patterns on leaves; Mosaic (viral disease) Use resistant varieties; avoid planting beans near sweet,
leaves often curl downward, red, or crimson clover or gladioli, which may harbor the
may become chlorotic, soon disease.
die

Leaves lose color; underside Spider mites Use predatory mites, insecticidal soap, or registered
appears dusty and webbed miticide.

Young leaves curled, distorted, Aphids Pick off, mash, or wash away insects; use biological control
and yellow; clusters of tiny agent; apply insecticidal soap or registered insecticide.
insects on leaves and stems

Beets Cercospora leaf spot (fungal disease) Rotate; pick off and destroy affected leaves; thin planting;
avoid wetting foliage.
Small, circular spots with light Boron deficiency
centers and dark borders on Maintain soil pH between 6 and 7; soil test; supply boron
leaves Overcrowding based on results.
Roots cracked; black areas on Cloddy soil
surface and inside of roots; Flea beetles Thin beets early.
plants stunted Prepare soil properly.
Deformed roots Use floating row cover or biological control agent; apply
registered insecticide.
Leaves with many small holes Use floating row cover; remove and destroy infested
leaves.
Irregular, tan blotches in leaves Leafminers

Carrots Aster yellows (phytoplasma disease) Remove and destroy affected plants; control weeds; con-
trol leafhoppers with registered insecticide.
Inner leaves yellow; outer Roots exposed to sunlight
leaves reddish-purple; roots Overcrowding Cover exposed roots with soil or mulch.
stunted and bitter Cloddy soil Thin carrots early.
Root tops green Root-knot nematodes Prepare soil properly.
Roots deformed Submit soil sample for nematode analysis; rotate; remove
Excess nitrogen diseased plants; fumigate soil.
Small maggots in roots Carrot rust fly larvae Do not overfertilize.
Rotate; plant resistant varieties; use floating row cover;
apply registered insecticide.

112 • Diagnosing Plant Problems Chapter 7

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls

Cole Crops (cabbage, broccoli, turnips, cauliflower, Brussels sprouts)

Cabbage heads crack Plant takes up excess water, causing Harvest heads as soon as mature.
head to burst

Poor heading Overcrowding Thin plants early.

Dry soil Water.

High temperature Wait for cooler weather.

Poor soil fertility Soil test; supply nutrients based on results.

Club root (fungal disease) Check roots for large swellings; rotate out of affected area
for 7 years; lime soil to raise pH above 7.0.

Root rot (fungal disease) Rotate; plant in well-drained area; do not overwater;
remove plant debris.

Discolored cauliflower heads Exposure to sun Tie leaves over heads early.

Plants wilt and turn yellow; Club root (fungal disease) Check roots for large swellings; rotate out of affected area
roots have large swellings for 7 years; lime soil to raise pH above 7.0.
(not to be confused with
smaller root knots caused by
nematodes)

Plants stunted and yellow Dry soil Water.
(especially cabbage); roots not Poor soil fertility Soil test; supply nutrients based on results.
discolored Use collars, floating row cover, or biological control agent;
Cabbage maggots work in a soil insecticide at planting time.

Heads soft and rotted Soft rot of broccoli (bacterial disease) Grow broccoli varieties that shed water (conical head);
provide good air circulation; avoid wetting heads.

Rough, brown, raised areas on Oedema (physiological problem due Water during dry periods; avoid overwatering in cool
underside of leaves to uneven water supply) conditions.

Leaves riddled with shot holes Flea beetles Use floating row cover or biological control agent; apply
registered insecticide.

Leaves chewed Imported cabbage worm, cabbage Identify insect; use floating row cover or biological control
looper, diamondback moth agent; apply registered insecticide.

Some leaves curled, yellow; Aphids Use floating row cover or biological control agent; apply
clusters of tiny gray or green insecticidal soap or registered insecticide.
insects

Corn

Stalks broken, pith tunnelled European corn borer Apply registered insecticide.
Plant in blocks of three or four short rows rather than
Ears not completely filled with Poor pollination single long one.
kernels Rotate; plant only 1 inch deep to encourage rapid germi-
nation; keep soil moist for first 4 weeks after germination;
White (smooth) or black (pow- Smut (fungal disease) cut off galls before they turn black; remove plant debris.
dery) galls on stalks, leaves, Control weeds, especially wild grasses; control aphids;
ears, or tassels destroy affected plants.

Plants stunted; yellow and Maize dwarf mosaic (viral disease) Soil test; supply nutrients based on results; avoid planting
green striped mosaic pattern in very cool soil.
on leaves; older leaves pale Be careful when applying herbicides.
yellow
Apply mineral oil or registered insecticide during silking
Leaves reddish on margins Phosphorus deficiency to prevent infestation.
Use cutworm collars or biological control agent; apply
Distorted leaves or stalks; Herbicide injury registered insecticide.
leaves may fail to unfurl or Use floating row cover or screen.
stalk may be bent Corn earworms
Caterpillars feeding on tips of Cutworms
ears
Young plants disappear

Birds

Chapter 7 Diagnosing Plant Problems • 113

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls

Cucurbits cucumbers, cantaloupes, pumpkins, squash, watermelons)

No fruit produced Poor pollination Be patient—at first, male and female flowers not pro-
duced at the same time; bee activity may be low due to
cool weather; protect bees when using insecticides.

Misshapen or bitter fruit Poor pollination Be patient; male and female flowers are not produced at
the same time at first; bee activity may be low due to cool
weather; protect bees when using insecticides.

Dry soil Water.

Poor soil fertility Soil test; supply nutrients based on results.

Water-soaked, sunken, brown, Calcium deficiency, usually caused by Water during dry periods; supply calcium with foliar spray.
or black spot on end of fruit uneven soil moisture during blossom-

ing and poor supply of calcium to fruit
during early development

Wilted plants Dry soil Water.

Bacterial wilt Control cucumber beetles.

Root rot (fungal disease) Remove old plant debris; rotate; plant in well-drained
area; do not overwater.

Fusarium wilt (fungal disease) Plant tolerant varieties if available; rotate.

Circular or irregular spots on Fungal or bacterial disease (any of Plant resistant varieties; space plants properly; control
leaves and/or fruit several) moisture and humidity.

White, powdery growth on Powdery mildew (fungal disease) Plant resistant varieties; rotate; provide air circulation;
leaves; may be on both leaf control weeds; apply registered fungicide.
surfaces

Yellow and green mottle pat- Viral disease Control weeds before plants emerge; control insects;
tern on leaves remove affected plants.

Leaves have strapped appear- Herbicide injury Be careful when applying herbicides.
ance: abnormally narrow with
leaf veins stretched out at
leaf margins so leaves appear
feathery

Holes chewed in leaves and Cucumber beetles Use floating row cover or biological control agent; apply
stalks; yellow-green beetles registered insecticide.
with black stripes or spots

Squash and pumpkin leaves Squash bugs Hand pick and destroy; use floating row cover; apply
wilt, eventually become black registered insecticide.
and crisp; dark gray 1/2-inch-
long bugs present

Lettuce

Bolting; may taste bitter Weather too hot Lettuce is a cool-season crop; plant early or late.
Rotate; plant in well-drained area; remove plant debris.
Sunken, water-soaked spots Rhizoctonia bottom rot (fungal Avoid crowding; plant in well-drained area; remove plant
appear on lower leaves that disease) debris.
turn brown and slimy; head Sclerotinia (fungal disease) Rotate; plant in well-drained area; remove plant debris.
turns brown
Rotate; control weeds; apply registered fungicide.
Stem and lower leaves rotted; Botrytis gray mold (fungal disease)
dense, fuzzy, gray mold on Remove and destroy affected plants; control weeds and
affected areas insects.
Remove and destroy affected plants; control weeds and
Yellow or light green blotches Downy mildew (fungal disease) insects.
on upper leaf surfaces; white, Soil test; supply nutrients based on results.
fuzzy mold on underside of
blotches; spots eventually turn
brown

Plants stunted, yellow; young- Aster yellows (phytoplasma disease)
est leaves curled; head soft

Mosaic virus

Nutrient deficiency

114 • Diagnosing Plant Problems Chapter 7

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls
Rotate; plant in well-drained area; remove and destroy
Leaf veins and area adjacent to Big vein (viroid disease) affected plants.
veins turn light yellow, causing
a “big vein” effect

Onions

White flecks form on leaves Downy mildew (fungal disease) Rotate; plant in well-drained area; eradicate wild onions;
and expand into elongated avoid wetting foliage; apply registered fungicide.
leaf lesions; white-to-purplish
mold develops on spots dur- Rotate; plant in well-drained area; remove plant debris;
ing moist weather; bulb qual- allow tops to die before harvesting; cure bulbs before
ity poor and often spongy storing; avoid bruising bulbs.
Rotate; plant in well-drained area; plant resistant varieties;
Leaves yellow and die back Fungal or bacterial bulb rot fertilize and irrigate properly for optimal growth; remove
from tips; bulbs soft and rotted plant debris.
Use floating row cover; destroy infested onions; work
Tops stunted; roots yellow and Pink root (fungal disease) registered insecticide into soil.
eventually become pinkish Use floating row cover or biological control agent; apply
insecticidal soap or registered insecticide.
Plants grow slowly, wilt, and Onion maggots
die; white maggots inside bulb
White streaks or blotches on Onion thrips
leaves.

Peas Hot weather Peas are cool-season vegetables; plant early in spring;
Root rot (several fungi) plant heat-resistant varieties.
Plants stop producing pods; Rotate; plant in well-drained area; do not overwater;
leaves turn yellow, then remove plant debris.
brown, and die. Rotate; plant resistant varieties; plant when soil tempera-
ture is below 65°F; remove plant debris.
Plants stunted; lower leaves Fusarium wilt (fungal disease)
yellow; internal stem tissue Rotate; plant resistant varieties; plant early; control weeds;
brown remove plant debris.

White, powdery mold on Powdery mildew (fungal disease) Plant resistant varieties; plant early before insect vectors
upper, then lower, surface of are active; control weeds and insect vectors; remove and
leaves; leaves and pods may destroy affected plants.
be distorted Plant resistant varieties; control weeds and insect vectors.

Yellowish areas on leaves; Pea enation mosaic (viral disease)
blister-like ridges on underside
of leaves and on pods

Yellow and green mottle or Viral disease (any of several)
mosaic pattern on leaves;
plants stunted

Peppers

Wilted leaves, shiny with Aphids Apply registered insecticide.
honeydew

Large, sunken, tan, water- Blossom-end rot, caused by calcium Apply calcium during soil preparation; mulch to conserve
soaked spot develops on deficiency to developing fruits. Occurs water; water during dry periods; use calcium foliar sprays.
blossom end of fruit; spot when young fruits receive uneven
turns black,; mold may grow moisture.
on surface.

Thin, wrinkled, tan areas Sunscald Provide shade if hot, sunny weather persists when heavy
develop on fruit and become fruit crop is on plants.
white and papery

Growth stunted; leaves turn Verticillium wilt (fungal disease) Remove old crop debris; rotate, avoiding tomatoes and
yellow, roll inward, and die potatoes in rotation.

Plants stunted; leaves curled Viral disease (any of several) Plant resistant varieties if available; control weeds and
with yellow and green mottle; insect vectors; remove plant debris; wash hands often
fruit misshapen with brown when working among plants; don’t smoke around plants.
streaks, rings, or yellow, green,
and red mottle

Chapter 7 Diagnosing Plant Problems • 115

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls
Plants wilt; lower leaves may Fungal or bacterial wilt disease
turn yellow Rotate, avoiding tomatoes and potatoes in rotation;
Dry soil remove plant debris.
Waterlogged soil Water.
Root rot (fungal disease) Improve drainage; do not overwater.
Rotate; plant in well-drained area; do not overwater;
Hole in fruit near cap, pink European corn borer remove plant debris.
caterpillar inside Apply registered insecticide.

Potatoes Exposure to sun Mound soil up around plants; do not eat green parts of
potatoes.
Potato tubers are green Water.
Rotate; remove plant debris.
Plants wilt; bottom leaves may Dry soil Rotate; plant in well-drained area; do not overwater;
turn yellow remove plant debris.
Vascular wilt (fungal disease) Improve drainage; do not overwater.
Use certified seed potatoes; use tolerant varieties; avoid
Root rot (fungal disease) using limestone or wood ashes where potatoes will be
grown.
Brown, corky scabs or pits on Waterlogged soil Do not overfertilize; maintain uniform soil moisture.
tubers; plants do not wilt Scab (fungal disease)
Watch for holes in leaves; use floating row cover or bio-
Tubers show irregular white or Hollow heart, caused by plants’ grow- logical control agent; apply registered insecticide.
brown cavities when cut open ing rapidly
Hand pick beetles; use floating row cover or biological
Tunnels bored just under the Flea beetle adults and larvae control agent; apply registered insecticide.
skin of tubers; small holes in
leaves

Leaves chewed; fat, red, Colorado potato beetles
humpbacked grubs or orange
beetles with black stripes
present

Radishes Plant in well-drained area; rotate; plant resistant varieties;
do not overwater; remove plant debris.
Purple-to-black spots develop Black root (fungal disease)
on root surface; black discolor- Use floating row cover or biological control agent; apply
ation extends inward in radial registered insecticide.
streaks; roots remain firm Use floating row cover or biological control agent; apply
Leaves riddled with tiny holes Flea beetles registered insecticide.

Roots infested with legless Cabbage maggots
white worms

Spinach Hot weather and long days Spinach is a cool-season crop; plant in early spring.
Downy mildew (fungal disease) Rotate; plant resistant varieties; remove infested plants.
Plants bolt
Pale yellow spots appear on Leafminers Use floating row cover; remove and destroy infested
upper leaf surfaces; grayish- leaves.
purple mold develops on
underside of spots; whole
leaves may wither
Irregular, tan blotches or tun-
nels appear on leaves

Tomatoes Early blight (fungal disease) Rotate; plant resistant varieties; improve air circulation;
avoid wetting leaves; remove plant debris; apply regis-
Dark brown, irregular spots tered fungicide.
with target rings, yellow
halos develop on fruit; spots
often are at stem end and are
sunken

116 • Diagnosing Plant Problems Chapter 7

Table 2. A diagnostic key to vegetables.

Symptoms Possible Causes Controls

Dark brown, leathery spot on Blossom-end rot, caused by calcium Apply calcium during soil preparation; mulch to conserve
blossom end of fruit; mold deficiency to developing fruits. Occurs water; water during dry periods; use calcium foliar sprays.
may grow on spot when fruits have uneven moisture sup-
ply during development

Extreme malformation and Catfacing, caused by cool weather or Protect young plants from cool temperatures; avoid using
scarring of fruit herbicide injury during fruit formation herbicides nearby.

Yellow-orange blotches that Sunscald Prevent foliar diseases that cause leaf drop and expose
do not ripen at stem end of fruits to sun; use cages to confine plants so they will shade
fruit, or white, papery spot on themselves better.
side of fruit facing sun

Leaves distorted with Herbicide injury Be careful when applying herbicides.
“strapped” or feathery look
(leaves narrower than normal, Tobacco mosaic (viral disease) Note: Plant resistant varieties; wash hands and disinfect tools
tips stretched out into thin It may be difficult to distinguish between plants; don’t smoke around plants; remove plant
projections, veins very close between herbicide injury and tobacco debris.
together) mosaic based on symptoms alone;
however, during the spring lawn weed
control season, strongly suspect her-
bicide injury. Leaves roll upward, feel
leathery, but remain green; plants are
not stunted.

Excess water Common physiological disorder after wet periods; do not
overwater.

Young plants cut off at ground Cutworms Use cutworm collars or biological control agent; apply
level registered insecticide.

Young plants have many tiny Flea beetles Healthy tomatoes tolerate a lot of flea beetle damage;
holes in leaves if needed, use biological control agent or registered
insecticide.

Tiny, white-winged insects on Whiteflies Use yellow sticky boards (smeared with grease) to attract
underside of leaves and trap adults; apply insecticidal soap or registered
insecticide.

White stippled areas in fruit Stink bug feeding Apply registered insecticides.

Note: This key is not a comprehensive list of plant symptoms, causes of damage, or controls. Refer to this chapter’s general discussion of problem
diagnosis and to other references for more information. Under controls, cultural, mechanical, and biological methods are listed first, chemical meth-
ods are listed last.

Table 3. A diagnostic key to tree fruits and nuts

Symptoms Possible Causes Controls

General Natural thinning Many trees produce more fruit than they
need and thin themselves naturally.
Premature fruit drop Frost often kills developing flower buds or
fruits.
Spring frost Tree may require another tree nearby
for pollination; be careful not to kill bees
Poor pollination when you use pesticides.
Drought, cold, and heat can cause fruit
Environmental stress drop.
Various diseases See controls in Midwest Plant Disease
Various insects Management Handbook.
Identify insects; see controls in Midwest
Insect Management Handbook.

Chapter 7 Diagnosing Plant Problems • 117

Table 3. A diagnostic key to tree fruits and nuts

Symptoms Possible Causes Controls
Few fruits on tree Poor pollination
Tree may require another tree nearby
Biennial bearing for pollination; be careful not to kill bees
when you use insecticides.
Fruits too small Improper pruning Apples and pears naturally bear a heavy
crop one year and few fruits the next year.
Frost injury Thin fruits to counteract this tendency.
Failure to thin Do not cut off fruit-bearing wood when
pruning.
Poor soil fertility —
Peaches, nectarines, and apples tend to
Gray-white, powdery growth on leaves; Powdery mildew (fungal disease) produce many small fruits. Thin fruits to
leaves and fruit may be distorted increase size of those remaining.
Soil test; supply nutrients based on results.
Black, sooty growth on leaves, stems, and/ Sooty mold (fungus that grows on hon- Improve air circulation; apply registered
or fruit eydew substance secreted by aphids and fungicide.
other insects) Identify insects; if aphids, hose down
tree with a powerful spray of water; use
Young leaves curled and distorted; clusters Aphids biological control agent or registered
of insects on underside of leaves insecticide.
Encourage predators; hose down tree
Leaves with tiny, white spots, often dirty Spider mites with a powerful spray of water to remove
with webbing aphids; use biological control agent or
registered insecticide.
Encourage predators; use predatory mites;
apply registered miticide.

Apples and Pears

Olive-brown, puckered spots on leaves Scab (fungal disease) Plant resistant varieties; rake and destroy
and young fruit; fruit spots develop into fallen leaves; prune for better air circula-
brown, corky lesions; mature fruit is tion; avoid wetting leaves; apply registered
distorted fungicide.
Apply lime to soil; use calcium foliar sprays
Sunken, light brown circular spots on out- Bitter pit (physiological disease) as fruit nears mature size.
side of apples, usually near blossom end;
small, brown spots in edible part of fruit Use traps to monitor pest population;
apply registered insecticide.
Pink-white worms bore into fruit and feed Codling moths Use traps to monitor pest population;
near core apply registered insecticide.
Prune out dead shoots and branches in
Apples have faint brown streaks in the Apple maggots winter.
flesh

Shoots die back with tips bent over like a Fire blight
shepherd’s crook

Stone fruits (apricots, cherries, peaches, plums, and nectarines)

Purple spots appear on upper surfaces of Cherry leaf spot (fungal disease) Rake and destroy fallen leaves and fruits;
cherry leaves; leaves become shot-holed apply registered fungicide.
and turn yellow; fruit also may be affected

Peach or nectarine leaves puckered, Peach leaf curl (fungal disease on peaches Pick off and destroy affected leaves; fertil-
thickened, and curled from time they first and nectarines) ize the tree; apply registered dormant-sea-
appear in spring; leaves red or orange at son fungicide before buds begin to swell.
first but turn yellow; shoots swollen and
stunted

Blossoms and young twigs wilt and decay Brown rot (fungal disease common on all Prune away badly affected twigs; follow
during bloom; sunken cankers with stone fruits) stone fruit fungicidal spray program.
gummy ooze develop on twigs; circular,
brown spots on fruit develop tufts of gray
spores during moist weather

Cherry fruits infested with small, white Cherry fruit flies Use traps to monitor presence of adult
worms flies; apply registered insecticide.

118 • Diagnosing Plant Problems Chapter 7

Table 3. A diagnostic key to tree fruits and nuts

Symptoms Possible Causes Controls

Walnuts Use traps to monitor presence of adult
flies; apply registered insecticide.
White maggots feed in husks of nuts, caus- Walnut husk flies Apply registered bactericide at early pre-
ing nutshell staining and poor nut quality bloom stage.
Reddish-brown spots on leaves followed Walnut blight (bacterial disease)
by black, slimy spots on husks and in nuts

Note: This key is not a comprehensive list of plant symptoms, causes of damage, or controls. Refer to this chapter’s general discussion of problem
diagnosis and to other references for more information. Under controls, cultural, mechanical, and biological methods are listed first, chemical meth-
ods are listed last.

Table 4. A diagnostic key to berries and grapes.

Symptoms Possible Causes Controls

General Powdery mildew (fungal disease) Provide better air circulation and drier
conditions; control weeds; apply regis-
Grayish-white moldy growth on leaves tered fungicide.
Water.
Plants wilt; leaves may turn yellow Dry soil Plant in well-drained area; do not
Waterlogged soil overwater.
Rotate; plant resistant varieties; fumigate
Verticillium wilt (fungal disease) soil.
Rotate; plant in well-drained area; do not
Root rot (fungal disease) overwater; remove plant debris; apply
registered fungicide.
Green and yellow mosaic or mottle pattern Viral disease (any of several) Purchase certified, virus-free plants;
on leaves; plants may be stunted remove and destroy affected plants; con-
Leaves rolled or tied together; small cater- Leafrollers, leaftiers trol insects that spread virus.
pillars feeding inside Use biological control agent or registered
insecticide.

Blueberries

Plants stunted and discolored Soil pH too high Blueberries require acid pH: soil test;
acidify soil as needed.

Nutrient deficiency Soil test; supply nutrients based on results.

Environmental stress (drought, soil insects, Identify problem; take corrective
moles, poor cultural practices) measures.

Berries turn reddish or tan as they ripen Mummyberry (fungal disease) Use shallow cultivation to bury mummies
and become shriveled and hard; blossoms and destroy spore cups; control weeds;
turn brown and wither; new leaves’ centers apply registered fungicide.
are black

Chapter 7 Diagnosing Plant Problems • 119

Table 4. A diagnostic key to berries and grapes.

Symptoms Possible Causes Controls

Caneberries (blackberries, boysenberries, loganberries, raspberries)

Plants wilt; leaves turn yellow at bottom Verticillium wilt (fungal disease) Rotate; use certified, disease-free plants,
of plant first; stems turn dark blue at base; resistant varieties; plant in well-drained
internal stem tissue may be discolored area; remove and destroy affected plants.
Water.
Plants wilt with symptoms as above, but Dry soil Plant in well-drained area; do not
stem discoloration is not evident overwater.
Waterlogged soil Rotate; use certified, disease-free plants,
resistant varieties; plant in well-drained
Verticillium wilt (fungal disease) area; remove and destroy affected plants.
Rotate; plant in well-drained area; do not
Root rot (fungal disease) overwater; remove plant debris; apply
registered fungicide.
Ripening berries covered with tufts of Fungal fruit rot (any of several) Pick berries regularly and cool imme-
gray, green, white, or black moldy growth diately; prune for better air circulation;
avoid wetting leaves; control weeds; apply
White or tan spots with purple borders on Anthracnose (fungal disease) registered fungicide.
canes; canes die back Prune out old canes immediately after
harvest; thin out weak and unproductive
canes; improve air circulation.

Grapes Powdery mildew (fungal disease) Train and prune for better air circulation;
control suckers and weeds; apply regis-
Whitish or gray fungus patches on leaves tered fungicide.
and later on fruit Remove fruit mummies from the vines
in winter; apply registered fungicide in
Brown angular spots on leaves; fruits Black rot (fungal disease) spring and summer.
decayed and dried up Purchase certified, virus-free stock; do
not replant in same location for 10 years
Leaf resembles a fan; main veins drawn Fan leaf (viral disease) unless soil is fumigated; remove and
together and teeth along margins elon- destroy affected plants.
gated; plants stunted Be careful when applying herbicides.

Herbicide injury

Strawberries Botrytis (fungal disease) Space plants for better air circulation;
provide drier growing conditions; do not
Gray, fuzzy mold on ripening fruit. ovefertilize; remove and destroy affected
fruits; apply registered fungicide.
Plants wilt; leaves may turn brown at Root rot (fungal disease) Rotate; use disease-free plants; plant
margins; roots and crowns look discolored in well-drained area; do not overwater;
when cut open remove plant debris.
Common in certain varieties—may be
Malformed berries; looks like several ber- Fasciation, a response to environmental caused by insect activity.
ries have grown together conditions

Note: This key is not a comprehensive list of plant symptoms, causes of damage, or controls. Refer to this chapter’s general discussion of problem
diagnosis and to other references for more information. Under controls, cultural, mechanical, and biological methods are listed first, chemical meth-
ods are listed last.

120 • Diagnosing Plant Problems Chapter 7

Table 5. A diagnostic key to ornamental trees and shrubs.

Symptoms Possible Causes Controls

General

Many small twigs broken off Squirrel damage Squirrels prune twigs for nest-building and
sometimes prune more than they need.

Wind breakage —

Large areas of split bark; no decay evident Frost cracks Use tree wrap to protect bark from winter
sun and temperature extremes.

Sunscald Thin-barked trees, such as young ones,
split when exposed to intense sunlight;
use tree wrap or block sun on bright days;
avoid heavy fertilization in late summer
and fall.

Mechanical injury (such as lawn mower Remove grass from around base of trunk
injury) to avoid mowing too closely.

Large areas of split bark; decay evident in Secondary decay of any of the wounds Remove loose bark back to live cambium
wood described above so the tree can heal itself.

Fungal or bacterial canker Prune away affected parts; apply regis-
tered fungicide.

Gray-white, powdery growth on leaves; Powdery mildew (fungal disease) Improve air circulation; control weeds;
leaves may be distorted apply registered fungicide.

Black, sooty growth on leaves and/or Sooty mold (fungus that grows on hon- Identify insects; if aphids, spray plant with
stems eydew substance secreted by aphids and a powerful stream of water to wash off
other insects). insects and mold; apply insecticidal soap
or registered insecticide.

Brown, dead areas on leaf margins Leaf scorch, caused by insufficient trans- Scorch usually is caused by weather, but
port of water to leaves root rots or other root damage also can be
involved. Bacterial leaf scorch is a major
disease of some species. Water tree deeply
during dry periods.

Cold injury —

Chemical injury Chemical injury to trees is not common,
but does occur. Be careful when using
herbicides.

Interveinal yellowing of leaves; no wilting Nutrient or mineral deficiency Soil test; supply nutrients based on results.

Iron tied up in soil because of high soil pH Soil test; acidify soil; supply iron if needed.

Waterlogged soil results in poor transport Improve drainage; do not overwater.
of nutrients to leaves

Large, corky galls at base of tree and on Crown gall (bacterial disease) Treat preventively with biological control
roots agent; minimize wounds when pruning;
disinfect pruning equipment between
trees; improve drainage; consult an arbor-
ist about pruning out galls. Trees may live
for many years in spite of galls.

Brown, gray, green, or yellow, crusty, leaf- Lichens Lichens are a combination of algae and
like growths on trunk and branches fungi; they grow in moist, shady areas and
seldom cause damage.

Early leaf drop Environmental stress such as drought, Provide better growing conditions.
compacted soil, or transplant shock

Various insects or diseases Look for signs of the causal agent; control
as needed.

Chapter 7 Diagnosing Plant Problems • 121

Table 5. A diagnostic key to ornamental trees and shrubs.

Symptoms Possible Causes Controls
General browning of conifer needles Water deeply during dry weather.
Drought Water regularly after transplanting.
Transplant shock Be sure main roots are not curled around
Girdled roots themselves when transplanted; remove all
strings and wraps before planting.
Plant is root-bound Cut container root ball in several places
before transplanting.
Dog urine injury —
Fungal disease Check Midwest Plant Disease
Management Handbook.
Yellow and green mottle or mosaic pattern Viral disease No controls; may need to remove plant if
on leaves; leaves may be distorted virus spreads easily.
Prune away affected limb or bark areas.
Sunken cankers on trunk or branches; Fungal or bacterial canker
plant may wilt or show poor growth Check Midwest Insect Management
Handbook.
Oozing sap on trunk Insect borers Prevent lawn mower or weed trimmer
injury.
Mechanical injury Identify pest; use biological control agent;
apply registered insecticide while insects
Leaves chewed or completely eaten Various caterpillars, sawflies, leaf beetles, are young and before damage is extensive.
etc. Check Midwest Insect Management
Handbook for types of scale that might be
Scurfy, scalelike structures tightly attached Various scale insects on the particular plant; use biological con-
to leaves, twigs, or branches trol agent; spray with dormant oil during
winter to destroy eggs.
Young leaves puckered, curled, or dis- Aphids Check Midwest Insect Management
torted; clear, sticky substance on leaves; Spider mites Handbook; remove insects with powerful
clusters of small insects on underside of water spray; use biological control agent.
leaves
Leaves off-color with tiny white or yellow Check Midwest Insect Management
spots; may appear dirty due to fine web- Handbook; use predatory mites.
bing and collected dust
Apply a registered insecticide between
Birches Bronze birch borers late May and early June.
Birch leafminers Treat with registered insecticide when first
Leaves sparse, especially at top of tree; leaves are fully formed.
swollen ridges in bark
Leaves with pale blotches of varying size Prune and destroy affected twigs; rake and
and shape burn fallen leaves; avoid wetting canopy;
apply registered fungicide.
Dogwoods Anthracnose (fungal disease) Avoid damaging trunks with lawn mowers,
Collar rot (fungal disease) etc.; remove loose bark; avoid wetting
Large, brown, irregularly shaped blotches trunk.
on leaves; dead leaves hang on through Dogwood leaves may curl from lawn
winter herbicides, too much water, or many other
Native trees die, bark sloughs away stress conditions.

Leaf petiole curls; leaf curls upward, some- Environmental stress Remove plant debris; rotate; plant in well-
times discolors drained area; do not overwater.
Prune and burn affected twigs; improve air
Junipers Root rot, probably Phytophthora circulation; avoid wounding twigs or wet-
Juniper twig blight (fungal disease) ting foliage; apply registered fungicide.
Entire plant dies

Twigs die

122 • Diagnosing Plant Problems Chapter 7

Table 5. A diagnostic key to ornamental trees and shrubs.

Symptoms Possible Causes Controls
Juniper webworm Apply registered insecticide in early
Twigs webbed together tightly; affected spring.
twigs turn brown in warm weather Juniper tip midge Apply registered insecticide.
Twigs appear clubby and gall-like; entire
plant appears more compact than normal Bagworms Apply registered insecticide.
Defoliation

Maples

Irregular, brown spots on leaves (on Anthracnose (fungal disease) Rake and destroy fallen leaves; destroy
Norway maple, brown areas follow leaf Scorch, caused by hot, dry weather cankered twigs; apply registered fungicide.
veins); tree otherwise healthy Note: Anthracnose can be confused with Water tree deeply.
scorch if leaf spots have enlarged and
run together. In early stages, it should be Water deeply to reduce disease impact.
possible to distinguish between the two; There is no cure for bacterial leaf scorch.
scorch is mainly at leaf margins. Avoid heavy fertilizing, which produces
soft, succulent growth; prune away dead
Brown, dry areas on margins of leaves in Scorch, caused by bacterial leaf scorch. branches; don’t replant the site with a
late summer. Tree may decline susceptible species; disinfect tools when
pruning.
Leaves on tree suddenly wilt, may turn Verticillium wilt (fungal disease) Water deeply.
yellow and drop off; wilt may occur on one No control; harmless in most cases.
side of tree only; tree may die suddenly or
decline over period of years; no external
trunk or branch damage evident; some
branches may have brown streaks in wood Drought

Small, red, green, or black globular Gall mites
growths on upper leaf surfaces

Oaks Bacterial leaf scorch Water trees as needed to reduce disease
impact. There is no cure for bacterial leaf
Brown, dry areas on leaf margins in late scorch.
summer. Trees decline over several years. Stressed trees may benefit from applica-
tion of a registered insecticide.
Defoliation Various caterpillars, some beetles No control needed

Galls: irregular growths on leaves or twigs Various wasps, mites, gnats

Pines

Rough, elongated, swollen areas develop White pine blister rust (fungal disease) Prune away affected branches; rate plant-
on trunks and branches; orange-colored ing sites for blister rust hazard; plant resis-
spore masses burst through the cankered tant varieties in high-hazard sites; don’t
bark plant near currants or gooseberries.
Remove galls where practical.
Rough, globular galls on trunk and Eastern gall rust (fungal disease)
branches covered with orange spores in Prune out dead and dying branches;
late spring clean up plant debris under tree; provide
good drainage and air circulation; control
Chlorotic spots appear on affected needles Red band needle blight (fungal disease) weeds.
in fall and winter; far end of needle turns Provide good air circulation; control
reddish-brown, while base remains green weeds; clean up plant debris under tree;
apply registered fungicide.
During spring, last year’s needles turn Lophodermium needle cast (fungal
yellow, then brown, and drop; football- disease) Preventive insecticide applications.
shaped fruiting bodies form on affected Promote tree health.
needles Various borers Apply registered insecticide.
Holes in bark, sawdust on trunk or at base Eriophyid mites
of tree Various sawflies Apply registered insecticide.
Needles at tips of twigs turn yellow,
become deformed and stunted
Defoliation, often from top down

Chapter 7 Diagnosing Plant Problems • 123

Table 5. A diagnostic key to ornamental trees and shrubs.

Symptoms Possible Causes Controls

Rhododendrons and Azaleas Azalea leaf and petal gall (fungal disease) Pick off and destroy galls; apply registered
fungicide.
Fleshy, thick, white galls form on leaves Nutrient deficiency Possible lack of nitrogen; review fertilizing
and/or flowers program.
General leaf yellowing on all or part of Nutrient deficiency (possibly iron defi- Use sulfur to lower pH; use appropriate
plant ciency due to high soil pH) iron compounds to supply iron.
Chlorosis of newest leaves and terminal Nutrient deficiency (possibly magnesium Soil test; supply magnesium based on
growth deficiency) results.
Chlorosis of second-year and older leaves. Winter injury Very common on rhododendrons in early
spring.
Leaf browning, especially on leaves Scorch, caused by hot, dry weather Supply water and/or shade.
exposed to wind or strong sunlight Phytophthora root rot Rotate; plant in well-drained area; do not
overwater; remove plant debris.
Leaves with yellow specks on upper sur- Lace bugs Remove by hand if population is low;
face; black, shiny spots underneath apply registered insecticide.
Leaves notched Root weevils Use predators, baits, or beneficial nema-
todes; apply registered insecticide.

Roses Black spot (fungal disease) Avoid wetting foliage; rake and destroy
affected leaves; prune affected canes back
Black, circular spots with feathery edges to two buds; improve air circulation; apply
surrounded by yellow halo on leaves; registered fungicide.
leaves drop Remove and destroy affected leaves;
remove plant debris; apply registered
Pustules containing orange or brown, Rust (fungal disease) fungicide.
powdery spore masses appear on lower Powdery mildew (fungal disease)
leaf surfaces first and then upper leaf Improve air circulation; control weeds;
surfaces remove plant debris; use registered
White, powdery fungal growth on young fungicide.
leaves; distortion is common. Common on roses; these viral diseases
generally enter through grafts and are
Various patterns of yellow and green on Viral disease not transmitted from plant to plant; buy
leaves, including streaks, rings, vein-clear- healthy plants; maintain vigor by proper
ing (yellow veins), or blotches care of shrub.
Soil test; supply nutrients based on results.
Nutrient deficiency

Spruce Spruce aphids Apply registered insecticide.
Rhizosphaera needle blight (fungal Space plants adequately for better air
Needles turn yellow in late winter and disease) circulation; remove plant debris; avoid
brown as weather warms; drop off in late wetting foliage; do not prune when foliage
spring, leaving bare branches is wet; apply registered fungicide.
Older, inner needles of branches appear
speckled with dull yellowish blotches;
later, needles turn brown or purple from
tips back and drop; rows of tiny, black
specks on needles are visible with a mag-
nifying glass

Note: This key is not a comprehensive list of plant symptoms, causes of damage, or controls. Refer to this chapter’s general discussion of problem
diagnosis and to other references for more information. Under controls, cultural, mechanical, and biological methods are listed first, chemical meth-
ods are listed last. Most problems of ornamental trees and shrubs can be diagnosed from the list of general problems; only very common problems
specific to certain plants are listed under specific plants.

124 • Diagnosing Plant Problems Chapter 7

Table 6. A diagnostic key to annual and perennial flowers.

Symptoms Possible Causes Controls

General Dry soil Water.
Waterlogged soil Improve drainage; do not overwater.
Plants wilt; flowers may drop; leaves may Transplant shock Do not transplant during hot, sunny weather;
turn yellow water regularly after transplanting.
Plant in well-drained area; do not overwater;
Root, stem, or corm rot (fungal or destroy affected plants.
bacterial disease) Look for and identify problem insects; check
Soil-inhabiting insect pests Midwest Insect Management Handbook for
controls.
Seedlings wilt; stems turn brown and soft Damping-off (fungal disease) Plant in well-drained area; improve air circula-
and may be constricted at the soil line tion; allow soil to dry slightly between water-
ings; apply registered fungicide in early stages.
Plants fail to flower; foliage looks healthy Wrong season Plants have specific day-length requirements
for flowering.
Cool weather Wait for warmer weather.
Insufficient light Do not plant sun-loving plants in shade.
Too much nitrogen Do not overfertilize; nitrogen stimulates
foliage growth, sometimes at the expense of
Immature plants flower production.
Biennials and young perennials often do not
Tall, “leggy” plant; stem and foliage pale or Insufficient light flower the first year.
yellow Do not plant sun-loving plants in shade.
General yellowing of leaves; yellowing may Nutrient deficiency
be interveinal; plant may be stunted; no Viral disease (any of several) Soil test; supply nutrients based on results.
wilting Check Midwest Plant Disease Management
Handbook for specific crop susceptibility to
Grayish-white, powdery growth on leaves Powdery mildew (fungal disease) viruses.
Plant resistant varieties; improve air cir-
Pustules containing orange, yellow, or Rust (fungal disease) culation; control weeds; apply registered
brown, powdery substance on leaves fungicide.
Plant resistant varieties if available; remove
Flowers wilt or fail to open; grayish mold Botrytis gray mold (fungal disease) and destroy affected leaves; avoid wetting
appears on flowers in moist weather foliage; remove debris; apply registered
fungicide.
Yellow and green mottle or mosaic pattern Viral disease (any of several) Remove and destroy affected flowers; improve
on leaves air circulation; apply registered fungicide.
Use resistant varieties; control insect vectors;
Tiny, white dots (stippling) or white, inter- Spider mites remove and destroy affected plants.
veinal areas on leaves Use predatory mites; apply insecticidal soap or
registered miticide.
Clusters of insects on stems or underside of Aphids Pick off, mash, or wash away insects; encour-
leaves; leaves may be curled or distorted age insect predators; apply insecticidal soap
or registered insecticide.
Leaves chewed or completely eaten Various insects Check Midwest Insect Management
Handbook for specific plant and controls.
Slugs Look for slime trails; use slug barrier or bait.
Light-colored tunnels or blotches in leaves Leafminers Remove and destroy affected leaves. Check
Midwest Insect Management Handbook for
Tiny, white-winged insects on underside of Whiteflies specific plant and controls.
leaves Use yellow sticky traps; apply insecticidal soap
White, cottony masses on leaves or stems Mealybugs or registered insecticide.
Use biological control agent; apply registered
insecticide.

Chapter 7 Diagnosing Plant Problems • 125

Table 6. A diagnostic key to annual and perennial flowers.

Symptoms Possible Causes Controls

Chrysanthemums Aster yellows (phytoplasma disease) Control insect vectors; destroy affected plants.

Flowers greenish instead of normal color; Verticillium wilt (fungal disease) Use resistant varieties if available; obtain cut-
upper branches of flowering stem are yel- tings only from healthy plants; rotate; remove
lowish and upright and destroy affected plants early.
Yellowing and wilting of foliage; leaves
die from base of plant upward; discolored
brown areas in stems

Geraniums Oedema, a physiological problem Do not overwater.
associated with overwatering Plant in well-drained area; do not overwater;
Corky, raised spots on lower leaf surfaces. Fungal or bacterial root or stem rot remove dead plants.
(any of several)
Plants wilt; brown or black rotted area evi-
dent at base of stem; brown spots may be
present on leaves.

Iris Dispose of affected plants and rhizomes;
divide plants frequently to avoid overcrowd-
Leaves turn yellow and wilt; if pulled gently, Bacterial soft rot (bacterial disease) ing; avoid wounding rhizomes; dry rhizomes
leaves detach from plant; soft, slimy, smelly in sun before replanting.
rot at base of plant Improve air circulation; lime soil to raise pH
above 6.0; remove and destroy affected plant
Small, brown spots with water-soaked Leaf spot (fungal disease) parts; apply registered fungicide.
edges; spots may run together; leaves may
die

Marigolds Fusarium wilt (fungal disease) Improve soil drainage and air circulation;
rotate; destroy affected plants; drench beds
Leaves yellow; plants wilt and die. with registered fungicide.
Delay planting until conditions are right.
Wet, cold soils Wash off foliage with water frequently; use
Leaves discolored and look dusty with tiny Spider mites predatory mites; apply insecticidal soap or
webbing. registered miticide.
Provide nitrogen fertilizer.
Leaves reddish, smaller than normal. Lack of nutrients

Narcissus (daffodils) Rotate out of area for 2 years; improve air
circulation; remove and destroy affected parts;
Water-soaked areas that enlarge and wither Fire (Botrytis) (fungal disease) apply registered fungicide.
on flower edges; later, small, elongated
spots that turn brown on leaves; leaves may Rotate to new planting site each year; apply
die registered fungicide.
Spots appear on top 2 to 3 inches of leaves Leaf scorch (fungal disease) Control insect vectors; remove and destroy
as they emerge; leaves die affected plants.
Leaves have light green, grayish-green, or Yellow stripe (viral disease) Control insect vectors; remove and destroy
yellow stripes or mottles affected plants.
Flowers smaller than normal; white streaks White break (viral disease) Dig and inspect bulbs at least every 2 years;
or blotches may appear discard those that are infested.
Plants smaller than normal; fewer flowers Narcissus bulb flies
than normal; plants die

Peonies Botrytis blight (fungal disease) Prune out affected parts; improve air circula-
Frost damage tion; remove plant debris; do not use mulch
New shoots wilt and turn black; flowers, in spring when plants are emerging; apply
buds, leaves, and stems turn brown and registered fungicide.
leathery; gray, fuzzy mold may appear in Provide cover.
wet weather

126 • Diagnosing Plant Problems Chapter 7

Table 6. A diagnostic key to annual and perennial flowers.

Symptoms Possible Causes Controls

Tulips

Stems are very short and flowers bloom at Warm spring and/or inadequate After digging bulbs, chill them in refrigerator
ground level winter cooling before replanting.
Rotate; improve air circulation; destroy
Light or dark spots on leaves and flowers; Botrytis blight (fungal disease) affected plants; apply registered fungicide.
spots enlarge to form large, gray blotches;
fuzzy, brown or gray growth appears on Many of the stripes in tulips are caused by viral
spots during wet weather; leaves and stems diseases, but some may be desirable; either
distorted destroy the affected plants or put them far
enough away from others to prevent spread of
Flowers streaked, spotted, or mottled in Viral disease (any of several) disease; control insect vectors.
an irregular pattern; leaves also may be Use biological control agent, insecticidal soap,
streaked or mottled or registered insecticide.

Thrips (tiny insects that feed on
emerging flowers)

Zinnias

Off-color lesions near base of plant followed Stem rot (fungal disease) Space plants for better air circulation; remove
by white, cottony growth on stems; plant and destroy affected plants; apply registered
wilts and dies fungicide.
Supply moisture.
Plants wilt; no sign of root rots or stem Dry soil Improve drainage; do not overwater.
disorders Soil too wet

Note: This key is not a comprehensive list of plant symptoms, causes of damage, or controls. Refer to this chapter’s general discussion of problem
diagnosis and to other references for more information. Under controls, cultural, mechanical, and biological methods are listed first, chemical meth-
ods are listed last. Most problems of annual and perennial flowers can be diagnosed from the list of general problems; only very common problems
specific to certain plants are listed under specific plants.

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Programs, University of Kentucky College of Agriculture, Lexington, and Kentucky State University, Frankfort. Copyright © 2011 for materials developed by University of
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Issued 10-2011