INTRO TO PLANT PATHOLOGY - University of Florida

INTRO TO
PLANT

PATHOLOGY

A Course for
Master Gardeners

Developed by:

Sydney Park Brown1, Robert J.
McGovern2, and Ken Pernezny3

University of Florida-IFAS

1Hillsborough County Coop.
Extension, Seffner , FL

2Gulf Coast Research and
Education Ctr., Bradenton, FL

3Everglades Research and
Education Ctr., Belle Glade, FL

Plant
Pathology
is the study
of the suffering
or dis-ease of

plants.

Plant pathologists focus on
problems caused by parasitic
(biotic) organisms.

Non-parasitic disorders
(abiotic causes) can also
cause disease in plants.

NON-PARASITIC

DISORDERS

Ø Nutrient imbalances
Ù Temperature extremes
Ú Toxic chemicals
Û Mechanical injury
Ü Water imbalances
Ý Air pollution
Þ Genetic defects



PLANT
PATHOGENS

9 Fungi
9 Bacteria
9 Viruses

FUNGI

85% of all plant diseases are
caused by fungi.

You are more likely to
encounter fungal diseases
than any other types of
diseases.

Fungi includes molds,
mildews, and mushrooms.

Sometimes fungal growth is so
profuse that it can be seen with the
naked eye.
But, usually, a microscope is
needed to see and identify fungi.

Target Spot on Tomato

Most fungi reproduce by seed-
like spores.

The peculiar size,
shape, and color of
spores are most
useful in ID of fungi.

Spores can be
spread by air and
water splash.

Fungi may also spread through infected seed
and plants, contaminated soil and tools, and by
insects and man.

Spores germinate in the presence of
high moisture and humidity...

...and penetrate and infect
plant tissue.

BACTERIA

9One-celled microorganisms
9Reproduce through cell division

R. H. Brlansky

9 Spread by Bacterial streaming
splashing water,
man, seeds.

9 Enter plants
through wounds
or natural
openings.

9 Are extremely
contagious.

Points to Remember:

Ø Fungi cause more diseases

than bacteria.

Ù Bacterial diseases are
more difficult to control.

Ú Most fungal and bacterial
diseases promoted by high
temperature and humidity.

VIRUSES

Smallest of the plant pathogens
(a strand of DNA or RNA coated
in protein).

Bean Golden Mosaic Virus

9 Viruses must have a living host in order
to replicate themselves.

9 Viruses are "vectored" or carried from
infected plants by insects.

9 Aphids - and in Aphid
recent years, Whitefly
whiteflies - are the
most important
vectors of viruses
in Florida.

PLANT
DISEASE
SYMPTOMS

Symptoms
are abnormal
states that
indicate a
disorder

Plant Functions/Symptoms

Courtesy: G.N. Agrios,
“Plant Pathology”

Although fungi and
bacteria produce similar
symptoms, subtle differences
may occur that can aid plant
disease diagnosis.

Leaf spots:
Small, distinct
lesions with
definite borders

Bacterial leaf spots are sometimes
angular (but not always!).

Fungal leaf spots
are usually round, (but
not always!).

Bacterial leaf spots have a
water-soaked appearance.

Fruit Spots

Leaf Blight: Spots
grow and coalesce

Leaf Blight

Leaf Spot

Galls or tumors:
Masses of growth

Stem Cankers:
Sunken lesions
on stems and

branches
(Citrus Canker)

Wilt: Problem with water uptake;
diseased vascular system.

xylem

Vascular Wilt Disease:
Note discoloration of xylem
(water conducting) tissue.

Fruit Rot: Soft, almost
watery tissue

Stem Rot: "Damping Off" disease in
seedlings

Virus Symptom: Mosaic - pattern
of dark/light colors

Mosaic symptoms on squash

Virus symptom: Stunted and distorted
growth

Virus symptom: Ringspots

SIGNS
OF THE
PATHOGEN

Signs are physical evidence
of the pathogen in association with

unhealthy plant tissue:

1. Fungal mycelium (mildew or mold)
2. Fungal fruiting bodies

(Ex: Sclerotia, Rust spores, Conks and
mushrooms)
3. Bacterial ooze & odors
4. Virus diseases have no signs

Powdery
Mildew
fungus

Sign: White
“talcum-powder”
growth of fungal

mycelium.

Gray Mold fungus

Sign: Whiskery growth
of fungal mycelium

Leaf Mold fungus

Sign: Olive-green/gray growth
within a yellowish spot

SouSthouetrhnerBn lBiglighhttFFuunngguuss

Sign: Fungal mat with sclerotia

Sign: Hard seed-like
sclerotia within stem

Geranium rust

Sign: Rust colored spores

Corn
Smut
fungus

This was
an ear of

corn.

Sign: Grotesque growth

Fairy Ring Fungus

Sign: mushrooms

Bacterial Spot of malanga

Sign: Bacterial ooze

Bacterial decay is
associated with a bad

odoor dor

Sign: Bad smell

Bacterial Wilt of tomato

Sign: Bacterial streaming

REVIEW: Signs are physical
evidence of the pathogen

1. Fungal mycelium (mildew or mold)
2. Fungal fruiting bodies

(Ex: mycelium, sclerotia, rust spores,
mushrooms or conks)
3. Bacterial ooze and odors
4. Virus diseases have no easily visible
signs