Master Gardener Training
Plant Pathology
Greg Church, Ph.D.
County Extension Agent – Horticulture, Plant Pathologist
http://collincountygardening.tamu.edu
Go to http://collin-tx.tamu.edu
Click Publications, then Master Gardener Training
Download Plant Pathology Information
Outline of training
• UNIT 1 : History of Plant Diseases &
Plant Disease Concept.
• UNIT 2: Causal Agents.
• UNIT 3: The art of diagnosis.
• UNIT 4: Decisions on solutions.
What is a Plant Disease?
• Plant Disease: Injurious physiological processes induced
by a continuous irritation of a primary causal agent
resulting in characteristic symptoms.
• Plant pathogen: any organism that cause a plant disease
• Host: plant under pathogen attack
Damage by
Pathogens
• Basic plant functions
(left)
• Disease interference
with those functions
(right)
* from Agrios, 1997
Brief history of Plant Diseases
• Plant disease (blight and mildew) is mentioned
in the bible: Deuteronomy 28:22 (approx.
2000BC) and Amos 4:9 (approx. 750BC).
• Romans created 2 gods for rust disease on
grain: “Robigo” and “Robigus”.
• Albertus Magnus (1200AD) conceived that
mistletoe is a parasite.
• Robert Hooke (1667) first to associate a fungus
with a disease [teliospores of a rust fungus].
• M. Tillet (1755) proved that bunt disease is
contagious.
More recent
History of Plant Pathology
• 1855 – Bordeaux Mixture (Millardet)
• 1845/1846 – Irish potato famine
• Early 1900s – USA: Chestnut Blight
(Endothia) and Dutch Elm Disease
(Ceratocystis).
• 2003 – Homeland security takes over
APHIS (Animal Plant Health
Inspection Service)
Module 1: Plant Disease Concept
CONCEPT #1: Disease triangle
• Three factors:
DISEASE
PATHOGEN (disease causing agent)
How do disease develop
Disease Occurs
• Example:
Susceptibility
Virulence of disease causing agent
CONCEPT #2: Disease Cycle
Management
Management
Overwintering
Host
(Inoculation, Penetration)
Management
Dissemination
and Reinfection
Management
Disease
Management
Infection
(Invasion , Growth, and Reproduction)
Early Blight
Disease Cycle: Peach Brown Rot
Host
Infection
D
i
s
e
a
s
e
Overseasoning
Sign and symptoms
SIGN
The pathogen (or parts of) can be visually
observed on the host plant.
SYMPTOM
External or internal reaction or alteration of a plant
due to the disease
Sign or Symptom?
Urediospores
Teliospores
• Anthracnose
Symptomology
– Describes an appearance as
dark, sunken leaf, stem, or fruit
lesions.
– Typically describe symptoms
made by fungi
euonymus
Ivy- Colletotrichum
Ash- Gnomoniella
Symptomology
Forsythia-Sclerotium
Blight
– Characterized by general
and rapid killing of leaves,
flowers, or stems.
Rose-Botrytis
Red cedar -Phomopsis
Oleander - Rhizoctonia
• Canker
Symptomology
– A necrotic, often
sunken lesion on a
stem, branch, or
twig of a plant.
Holly
Sycamore-Discula
Symptomology
• Chlorosis
– Yellowing of
normally green
tissue due to
chlorophyll
destruction or
failure of
chlorophyll
formation
Rose-iron chlorosis
Symptomology
• Damping-off
– Destruction of seedlings near soil line,
resulting in seedlings falling over on the
ground.
– Usually used to describe a fungal disease.
Symptomology
• Dieback
– Progressive death of shoots, branches, and roots generally
starting at the tip.
Rhododendron-Phytopthora
Douglas Fir - Phytopthora
Symptomology
Gall
– A swelling or
overgrowth
produced on a
plant.
– Gall is plant
tissue
Pine-Cronartium
Rose-Agrobacterium
Azalea-Exobasidium
Symptomology
Gall
– Root-knot
nematode causes
galls on roots
– Meloidogyne spp.
Tomato-Meloidogyne
Symptomology
• Leaf spot
– A self-limiting lesion
on a leaf.
Photinia-Entomosporium
Ivy-Xanthomonas
Rose-Diplocarpon
• Mildew
Symptomology
– Used to describe fuzzy growth on
plants.
– Typically refers to mycelium and
spores of fungi that are observed.
Euonymous-Oodium
Crape myrtle-Erysiphe
Rose-Peronospora
Symptomology
• Ringspot
– Circular area of
chlorosis with a
green center.
INSV – Impatiens Necrotic Spot Virus
Symptomology
• Rosette
– Short, bunchy habit of plant growth.
Rose-unknown
Rose rosette (UT-Knoxville)
Symptomology
• Scab
– A roughen, crust-like
disease area on the
surface of a plant organ.
Apricot-Cladosporium
Symptomology
• Scorch
– “Burning” pattern on leaf
margins.
– Typically would indicate “water
deficiency” problems.
Sycamore-Xylella
Oak-Ceratocystis
Symptomology
• Shot-hole
– Holes on
leaves.
Cherry laurel-Xanthomonas (UT –Knoxville)
Symptomology
• Stunting
– Result of reduced
plant growth.
Cotton-Sting nematode
Symptomology
• Wilt
– Loss of rigidity and drooping of plant
parts generally caused by insufficient
water in the plant.
Rose-Verticillium
Azalea-Cylindrocladium
Module 2:Causal Agents
Definitions:
Biotic vs Abiotic
• Biotic – having a mode of life, caused or
produced by living beings.
• Abiotic – non living agent, factors of the
physical environment.
Causes of plant diseases
• BIOTIC
– Fungi
– Bacteria
– Viruses
– Nematodes
– Parasitic
plants
• ABIOTIC
– Temperature
– Moisture
– Light
– Nutrition
– Chemical
Plant Pathogens (Biotic)
• Majority microscopic
• Fungi, bacteria,
viruses, nematodes,
parasitic plants,
spiroplasmas
* from Agrios, 1997
Fungal Diseases
• Most of the common diseases occurring on landscapes
are caused by fungi.
• 85% of plant diseases caused by fungi.
• Majority of fungi are saprophytic.
• Characteristics of fungi
–
–
–
–
–
Absorb nutrients
Multi-celled
Radial growth as tubular filaments
Reproduce and survive as spores
May “overwinter” as sclerotia, rhizomorph or spores.
Images from the Plant Management
Network Image Collections.
Diagnosis of fungal diseases
• Presence of visible fungal structures
– May be observed unaided or with
magnification.
• Can usually be cultured on artificial
media for identification
– Exceptions: obligate parasite such as rust
and mildew fungi.
Fungi
Damping-off
Pythium sp.
Fungi
Alternaria Petal Blight of Rose
Fungi
Dollar spot of turf
Sclerotinia homoeocarpa
Fungi
Blackspot
of roses
• Entomosporium leaf
spots
Fungi
Fungi
Rose
• Powdery mildew
Crape myrtle
Lilac
Crape myrtle
Fungi
• Rust
Apple
Chrysanthemum
Cedar
Fungi
• Brown patch-Rhizoctonia
Fungi
• Take-all- Gaeumannomyces
Bacterial diseases
• Characteristics of
bacteria
– Absorb nutrients
– One-celled
(prokaryote)
– Reproduce by fission
– Survive by dormancy
Diagnosis of bacterial disease
• Leaf lesions sometimes limited by
veins (angular)
• Ooze or streaming from cut tissue
• Soft rot (fruit) has foul odor
• Can be culture on media
– Use of selective media for identification
of pathogen.
• Bacterial scorch
Bacteria
• Galls
Bacteria
Bacteria
• Blights
Lilacs - Pseudomonas syringae
Bacteria
• Spots
Pittosporum
Tomato
Ranunculus
Bacteria
• Wilts
Diagnostic
Crassula
Zucchini
Bacteria
• Soft rot
Potato
Orchid
Onion
Bacteria
• Scabs and Cankers
Cherry
Peach
Watermelon Blotch
Viral diseases
• Characteristic of viruses
– Sub-cellular, composed of
DNA or RNA surrounded by
protein coat
– Replicate by “hijacking” plant
DNA
– Require wound to enter plant
cell
– Require living host
– Usually transmitted by a
vector
Viral symptoms
• Mosaic
Viral symptoms
Ring spot
Viral symptoms
• Other symptoms
Plant Parasitic Nematodes
• Characteristic of nematodes
– Very small animals
(microscopic
round worms)
– Typically in the soil
– Usually attack roots, sometimes
foliage
– Reproduces with eggs
Plant parasitic
Stylet
Various
shape and
sizes of
nematodes
• Root knot
Plant Parasitic Nematodes
Plant Parasitic Nematodes
• Lesion
Lesion
Nematode
damage on Rose
Nematodes
No
nematode
nematode
Plant Parasitic Nematodes
• Cyst
Cactus
Soybean
Plant Parasitic Nematodes
• Foliar
African Violet
Chrysanthemum
Lantana
Parasitic plants
• Characteristic of parasitic plants
– Obtain all or some of their needed nutrient from other
plants.
– Many has little or no chlorophyll.
– Cause relatively few problems when compared to other
disease problems.
Parasitic plants
ABIOTIC PROBLEMS
• Causes of abiotic problems
Physical agents
Temperature
moisture
light extremes
Chemical agents
soil pH or nutrient imbalances
pollutants
pesticides
Diagnosis of Abiotic Problems
•
•
•
•
Eliminate all possibilities of pathogenic attack.
Know history of the plant and its culture.
Know the range of growing conditions for the plant
Consider symptom patterns, site characteristics,
involvement of other, non-related plants.
Abiotic problems
• Water
Abiotic problems
• Temperature
Abiotic problems
• Other environmental
conditions
Abiotic problems
• Soil pH and Nutrient
Imbalances
Boron Toxicity
Zinc deficiency
Virus vs. Nitrogen Deficiency
Sanjuan.wsu.edu
Abiotic problems
• Chemicals
• More chemicals
Abiotic problems
Glyphosate on tomato
Copper on areca palm
Abiotic problem
• Urban blight