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Master Gardener TrainingPlant Pathology
Greg Church, Ph.D.County Extension Agent – Horticulture, Plant Pathologisthttp://collincountygardening.tamu.edu
Go to http://collin-tx.tamu.eduClick Publications, then Master Gardener TrainingDownload Plant Pathology Information
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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.
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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
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• Basic plant functions (left)
• Disease interference with those functions (right)
Damage by Pathogens
* from Agrios, 1997* from Agrios, 1997
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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.
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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)
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Module 1: Plant Disease Concept
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CONCEPT #1: Disease triangle• Three factors:
PATHOGEN (disease causing agent)
ENV
IRO
NM
ENTH
OST
PLA
NTDISEASE
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How do disease develop• Example:
Virulence of disease causing agent
Favorability of the environment
Susc
eptib
ility
of h
ost p
lant
SusceptibilityAggressiveness
Wet
Dry Disease Occurs
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Disease
CONCEPT #2: Disease Cycle
Overwintering
Management
Management
Management
Infection (Invasion , Growth, and Reproduction)
Host (Inoculation, Penetration)
Dissemination and Reinfection
Management
Management
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Early Blight
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Disease Cycle: Peach Brown Rot
Overseasoning
Host Infection
Reinfe
ction D
isease
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Sign and symptoms
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SIGN The pathogen (or parts of) can be visually
observed on the host plant.
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SYMPTOM External or internal reaction or alteration of a plant
due to the disease
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Sign or Symptom?
Urediospores
TeliosporesTeliospores
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Symptomology• Anthracnose– Describes an appearance as
dark, sunken leaf, stem, or fruit lesions.
– Typically describe symptoms made by fungi
Ivy- Colletotrichum
euonymus
Ash- Gnomoniella
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SymptomologyBlight– Characterized by general
and rapid killing of leaves, flowers, or stems.
Rose-Botrytis
Red cedar -Phomopsis Oleander - Rhizoctonia
Forsythia-Sclerotium
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Symptomology• Canker– A necrotic, often
sunken lesion on a stem, branch, or twig of a plant.
Sycamore-DisculaHolly
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Symptomology
• Chlorosis– Yellowing of
normally green tissue due to chlorophyll destruction or failure of chlorophyll formation
Rose-iron chlorosis
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Symptomology• Damping-off
– Destruction of seedlings near soil line, resulting in seedlings falling over on the ground.
– Usually used to describe a fungal disease.
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Symptomology• Dieback– Progressive death of shoots, branches, and roots generally
starting at the tip.
Rhododendron-Phytopthora Douglas Fir - Phytopthora
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SymptomologyGall– A swelling or
overgrowth produced on a plant.
– Gall is plant tissue
Rose-AgrobacteriumAzalea-Exobasidium
Pine-Cronartium
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SymptomologyGall– Root-knot
nematode causes galls on roots
– Meloidogyne spp.
Tomato-Meloidogyne
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Symptomology• Leaf spot– A self-limiting lesion
on a leaf.
Photinia-Entomosporium
Rose-Diplocarpon
Ivy-Xanthomonas
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Symptomology• Mildew– 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
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Symptomology• Ringspot– Circular area of
chlorosis with a green center.
INSV – Impatiens Necrotic Spot Virus
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Symptomology
• Rosette– Short, bunchy habit of plant growth.
Rose-unknown
Rose rosette (UT-Knoxville)
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Symptomology
• Scab– A roughen, crust-like
disease area on the surface of a plant organ.
Apricot-Cladosporium
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Symptomology
• Scorch– “Burning” pattern on leaf
margins.– Typically would indicate “water
deficiency” problems.
Sycamore-Xylella
Oak-Ceratocystis
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Symptomology
• Shot-hole– Holes on
leaves.
Cherry laurel-Xanthomonas (UT –Knoxville)
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Symptomology
• Stunting– Result of reduced
plant growth.
Cotton-Sting nematode
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Symptomology• Wilt– Loss of rigidity and drooping of plant
parts generally caused by insufficient water in the plant.
Azalea-Cylindrocladium
Rose-Verticillium
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Module 2:Causal Agents
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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.
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Causes of plant diseases
• BIOTIC– Fungi– Bacteria– Viruses– Nematodes– Parasitic
plants
• ABIOTIC– Temperature– Moisture– Light– Nutrition– Chemical
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Plant Pathogens (Biotic)• Majority microscopic• Fungi, bacteria,
viruses, nematodes, parasitic plants, spiroplasmas
* from Agrios, 1997
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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.
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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.
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FungiDamping-off Pythium sp.
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Fungi
Alternaria Petal Blight of Rose
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Fungi Dollar spot of turfSclerotinia homoeocarpa
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Fungi
Blackspot of roses
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Fungi• Entomosporium leaf
spots
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Fungi
• Powdery mildew
Rose
Lilac Crape myrtle
Crape myrtle
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Fungi
• Rust
Apple
Cedar
Chrysanthemum
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Fungi
• Brown patch-Rhizoctonia
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Fungi• Take-all- Gaeumannomyces
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Bacterial diseases
• Characteristics of bacteria– Absorb nutrients– One-celled
(prokaryote)– Reproduce by fission– Survive by dormancy
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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.
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Bacteria• Bacterial scorch
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Bacteria• Galls
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Bacteria• Blights
Lilacs - Pseudomonas syringae
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Bacteria• Spots
Pittosporum
Tomato
Ranunculus
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Bacteria• Wilts
Diagnostic
Crassula
Zucchini
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Bacteria
• Soft rot
Orchid
Onion
Potato
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Bacteria• Scabs and Cankers
Cherry
Peach
Watermelon Blotch
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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
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Viral symptoms• Mosaic
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Viral symptoms
Ring spot
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Viral symptoms
• Other symptoms
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Plant Parasitic Nematodes
• Characteristic of nematodes– Very small animals (microscopic
round worms)– Typically in the soil– Usually attack roots, sometimes
foliage– Reproduces with eggs
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Plant parasitic
Stylet
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Various shape and
sizes of nematodes
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Plant Parasitic Nematodes• Root knot
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Plant Parasitic Nematodes
• Lesion
NematodesNematodes
Lesion Nematode damage on
Rose
No nematode nematode
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Plant Parasitic Nematodes
• Cyst
SoybeanCactus
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Plant Parasitic Nematodes• Foliar
Lantana
African Violet
Chrysanthemum
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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.
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Parasitic plants
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ABIOTIC PROBLEMS
• Causes of abiotic problemsPhysical agents
Temperaturemoisturelight extremes
Chemical agentssoil pH or nutrient imbalancespollutantspesticides
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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.
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Abiotic problems
• Water
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Abiotic problems• Temperature
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Abiotic problems
• Other environmental conditions
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Abiotic problems• Soil pH and Nutrient
Imbalances
Boron Toxicity
Zinc deficiency Virus vs. Nitrogen Deficiency
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Sanjuan.wsu.edu
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Abiotic problems• Chemicals
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Abiotic problems• More chemicals
Copper on areca palm
Glyphosate on tomato
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Abiotic problem• Urban blight