Plant Pathology 101
Master Gardener Training
Carrie Harmon, Phil Harmon
Plant Pathology Dept., UF Gainesville and the
Southern Plant Diagnostic Network
UNIVERSITY OF
FLORIDA
IFAS EXTENSION
Disease
Environment
Pyramid
Disease
Time
Pathogen
Host
Diseases and disorders
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Disease occurs when
– Pathogen
– Host
– Environment
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Disorders lack pathogens
– Plant
– Environment
Put it in human terms… radiation, salmonella
Diagnosis is critical
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Incorrect changes to cultural
management can make problems
worse.
Fungicides only work on fungal
diseases.
The applicator needs to know if plant
damage is disease, if the disease is
caused by a fungus, and which fungus
is involved.
Disease diagnosis
Diagnosis: changes in time/space
Questions to ask:
 What pathogen structures (signs) occur
when and where?
 What symptoms develop over time?
 What is the pattern of dispersal?
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–  field signature or distribution
– abiotic vs. biotic
(images helpful!)
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What are the known diseases associated
with this host?
Disease diagnosis

Injury and disorders often:
– occur “suddenly”
– may affect all or many plant species
– may have regular, uniform pattern
 follow equipment patterns, boundaries
 look at pattern of problem in relation
to other items in the area – driveways,
construction activities, etc.
Symptoms and Signs
Symptoms: Changes in growth
or appearance of a plant in
response to a damaging factor.
Sign: Evidence of the damaging
factor.
Main types of pathogens
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Fungi
Bacteria
Viruses
Others
Armillaria produces white fungal growth under the
bark of diseased plants This sign is diagnostic.
Removing the bark allows you to see the fungus.
Sclerotinia sclerotiorum produces two distinct signs
The black bumps in the blighted area
of this holly stem contain small black
fruiting bodies of the pathogen,
Sphaeropsis tumefaciens.
Spores are usually too small to see. In this case
many spores are released from the “puffball”
mushroom together and they look like smoke rising
from the fruiting body of this fairy ring causing
fungus.
In this case, many spores of
Colletoctrichum form on this rotting
blueberry and appear orange in color.
Main types of pathogens
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Fungi
Bacteria
Viruses
Others
Photo by Mark Longstroth
Photo by Hank Dankers
Main types of pathogens
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Fungi
Bacteria
Viruses
Others
Micrograph by Carl Wetter
http://mmtsb.scripps.edu/viper/1f15.html
Main types of pathogens
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Fungi
Bacteria
Viruses
Others
Mycoplasma-Like-Organism (MLO)
Ex. Coconut Palm Lethal Yellowing MLO
?
http://www.inbar.int/publication/txt/tr10/little.htm
Types of disease symptoms
What does the disease do to the plant?
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Underdevelopment of tissues or organs. Examples include such
symptoms as stunting of plants, shortened internodes, inadequate
development of roots, malformation of leaves, inadequate production of
chlorophyll and other pigments, and failure of fruits and flowers to
develop.
Overdevelopment of tissues or organs. Examples include: galls on
roots, stems, or leaves, witches' brooms, and profuse flowering.
Necrosis or death of plant parts. These may be some of the most
noticeable symptoms, especially when they affect the entire plant, such as
wilts or diebacks. Other examples include shoot or leaf blights, leaf spots,
and fruit rots.
Alteration of normal appearance. Examples include mosaic patterns of
light and dark green on leaves, and altered coloration in leaves and
flowers.
From: Riley, M.B., M.R. Williamson, and O. Maloy. 2002. Plant disease diagnosis. The Plant Health
Instructor. DOI: 10.1094/PHI-I-2002-1021-01
Incidence VS. Severity
Incidence: Percent of
the crop affected
Severity: a measure of
impact on a plant or the crop
Common disease symptoms
Courtesy R.L. Forester
Courtesy J.W. Pscheidt
Peach leaf curl, caused by
Taphrina deformans.
Overgrowth of leaf tissue
causes thickening and
distortion.
Crown gall of uonymous by
Agrobacterium tumefaciens.
Galls form on all plant
parts, caused by
many pathogens.
Common disease symptoms
Used by permission of M.Williamson
Wilting
Biotic factors
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Stem wilt of Exacum from INSV
infection
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Root, crown or stem
rots
Vascular wilts
Root crown or stem
damage from insects or
animals
Mainly fungal and
bacterial causes
Common disease symptoms
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Leaf spots
Cankers
Foliar blights
Root rots
Cercospora leaf spot of crinum lily (symptoms no signs)
Common disease symptoms
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Leaf spots
Cankers and
diebacks
Foliar blights
Root rots
Photo by A. L. Jones
Photo by Dean Gabriel
ere dieback resulting from stem and bole cankers
sed by Botryosphaeria dothidea
Stem canker caused by Botryosphaeria dothide
Needle blight on
Leyland cypress
Entomosporium leaf spot on
Dwarf Indian Hawthorne
There are resistant cultivars
Image credit: Celeste White
Sphaeropsis tumefaciens
Main types of diseases
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Leaf spots
Cankers
Foliar blights
Root rots
Main types of diseases
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Leaf spots
Cankers
Vascular wilts
Root rots
Soil borne organisms, usually fungi cuase root rot. Abiotic factors also cause root rot—
water logged soil.
Main types of diseases
Photo by R.O. Hampton
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Leaf spots
Cankers
Vascular wilts
Root rots
Viral diseases
Photo by W. Witcher
Unknown ringspot virus symptoms on phalenopsis orchids
How do we approach a plant problem?
A Five Step Process…
1. Determine that a ‘REAL’ problem exists.
2. Look for PATTERNS, in the community, on
an individual plant and on an individual plant
part.
3. Determine the TIME development of the
damage pattern.
4. Ask QUESTIONS.
5. SYNTHESIZE the information.
1. Determine that a ‘REAL’ problem
exists…

Identify the plant.
• Learn about it’s normal characteristics.
• Determine normal vs abnormal characteristics.
• Look for symptoms and signs.
– Symptoms: Changes in growth or appearance of a
plant in response to a damaging factor.
– Sign: Evidence of the damaging factor.
2. Look for Patterns…

Look for patterns in the plant community.
– Is the damage on more than one plant?
– Is the damage on more than one plant
species?
• Look for patterns on an individual plant.
– Is the damage on the entire plant or certain
parts?
– Is the damage on certain age of growth?
• Look for patterns on an individual plant
part.
Patterns of damage…
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Non-uniform, expanding damage patterns are
usually caused by living factors, because of
movement of feeding sites, life cycles, and
population increases and decreases.
• Uniform, non-expanding damage patterns are
usually caused by non-living factors such as
chemical injuries, temperature changes, and
mechanical damage.
Damage patterns in the plant
community…
Used by permission of R. Billings
Loblolly pine killed by southern pine beetle
Used by permission of M. Williamson
Cotton field with chemical damage
Rhododendron with a Phytophthora sp. infection.
Woody container plants with Chemical injury .
Turfgrass displaying Fairy Ring symptom.
Damage patterns on an individual
plant…
Used by permission of W. Sinclair
American Elm displaying symptoms of Dutch Elm Disease.
Rhododendron displaying symptoms of Nitrogen Deficiency.
Damage patterns on an individual plant
part…
Douglas Fir with Rhabdocline Needlecast.
Fir with Freeze Injury.
Used by permission of M. Williamson
Downy mildew of Buddleia caused by Peronospora harotii.
Kalmia with a viral infection.
3. Determine the TIME development of
the damage pattern…
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Progressive spread with time to other areas
is characteristic of living factors.
• Intensification of symptoms where damage
first occurred but no spread to new sites is
characteristic of non-living factors.
4. Ask QUESTIONS…
• Get a history of the problem.
• Get a history of all pesticides and fertilizers that
have been applied.
• Find out the history of the site.
• Could environmental conditions explain the
problem?
• Look for obvious symptoms and signs…
• Don’t ignore the roots…
• Beware of secondary insects and pathogens…
• Be patient and avoid jumping to conclusions…
5. SYNTHESIZE the information…
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Refer to literature…
Send a sample to the
Plant Disease Clinic
UF Plant Disease Clinic
Building 78 Mowry Road
University of Florida
Gainesville, FL 32611
352-392-1795
Samples must contain the right material:
an entire plant or several plants if practical.
Foliage diseases
Keep most roots and
soil intact if possible
Diseases may
show up on any
part of the
plant.
Check for injuries, disease
on the main stem/trunk
Dead Plants Tell no Tales
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Avoid dead plants
Choose plants which show
a range of symptoms:
moderate to severe
Sample Quality: Packaging
& Shipping
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Keep soil on roots
No extra water
Wrap in dry paper then double bag in
plastic
Disinfest exterior of bags
Strong crush-proof box, tape all seams
Packaging & Shipping
Good
Intentions
Actual Results
Packaging and Shipping
blunders
Soil on foliage during
shipping creates
“diseases” that were
not there when the
sample was collected.
Packaging and shipping blunders
Sample
Soup
Don’t add water or wrap in wet paper towels
Good Packaging
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Plastic bag to keep soil on roots
Dry paper towels to protect
leaves from contact with plastic
bag
Disease management
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Watch for problems
– Rule out abiotic factors
– Look for symptoms and signs
– Consider the recent weather
– Consult references, agent
– Send a sample to the disease clinic
– Follow progress
– Keep records