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

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

• Basic plant functions (left)

• Disease interference with those functions (right)

Damage by Pathogens

* from Agrios, 1997* 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:

PATHOGEN (disease causing agent)

ENV

IRO

NM

ENTH

OST

PLA

NTDISEASE

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

Disease

CONCEPT #2: Disease Cycle

Overwintering

Management

Management

Management

Infection (Invasion , Growth, and Reproduction)

Host (Inoculation, Penetration)

Dissemination and Reinfection

Management

Management

Early Blight

Disease Cycle: Peach Brown Rot

Overseasoning

Host Infection

Reinfe

ction D

isease

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

TeliosporesTeliospores

Symptomology• Anthracnose– Describes an appearance as

dark, sunken leaf, stem, or fruit lesions.

– Typically describe symptoms made by fungi

Ivy- Colletotrichum

euonymus

Ash- Gnomoniella

SymptomologyBlight– Characterized by general

and rapid killing of leaves, flowers, or stems.

Rose-Botrytis

Red cedar -Phomopsis Oleander - Rhizoctonia

Forsythia-Sclerotium

Symptomology• Canker– A necrotic, often

sunken lesion on a stem, branch, or twig of a plant.

Sycamore-DisculaHolly

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

SymptomologyGall– A swelling or

overgrowth produced on a plant.

– Gall is plant tissue

Rose-AgrobacteriumAzalea-Exobasidium

Pine-Cronartium

SymptomologyGall– Root-knot

nematode causes galls on roots

– Meloidogyne spp.

Tomato-Meloidogyne

Symptomology• Leaf spot– A self-limiting lesion

on a leaf.

Photinia-Entomosporium

Rose-Diplocarpon

Ivy-Xanthomonas

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

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.

Azalea-Cylindrocladium

Rose-Verticillium

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.

FungiDamping-off Pythium sp.

Fungi

Alternaria Petal Blight of Rose

Fungi Dollar spot of turfSclerotinia homoeocarpa

Fungi

Blackspot of roses

Fungi• Entomosporium leaf

spots

Fungi

• Powdery mildew

Rose

Lilac Crape myrtle

Crape myrtle

Fungi

• Rust

Apple

Cedar

Chrysanthemum

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.

Bacteria• Bacterial scorch

Bacteria• Galls

Bacteria• Blights

Lilacs - Pseudomonas syringae

Bacteria• Spots

Pittosporum

Tomato

Ranunculus

Bacteria• Wilts

Diagnostic

Crassula

Zucchini

Bacteria

• Soft rot

Orchid

Onion

Potato

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

Plant Parasitic Nematodes• Root knot

Plant Parasitic Nematodes

• Lesion

NematodesNematodes

Lesion Nematode damage on

Rose

No nematode nematode

Plant Parasitic Nematodes

• Cyst

SoybeanCactus

Plant Parasitic Nematodes• Foliar

Lantana

African Violet

Chrysanthemum

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 problemsPhysical agents

Temperaturemoisturelight extremes

Chemical agentssoil pH or nutrient imbalancespollutantspesticides

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

Abiotic problems• More chemicals

Copper on areca palm

Glyphosate on tomato

Abiotic problem• Urban blight