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

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)

• Example: How do disease develop

Disease Occurs

Susceptibility Virulence of disease causing agent

CONCEPT #2: Disease Cycle

Management Management

Overwintering Dissemination and Reinfection

Management

Host (Inoculation, Penetration)

Management

Disease Infection (Invasion , Growth, and Reproduction)

Management

Early Blight

Disease Cycle: Peach Brown Rot Host Infection i D 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

Urediospores Teliospores

Sign or Symptom?

• Symptomology Anthracnose – Describes an appearance as dark, sunken leaf, stem, or fruit lesions.

euonymus – Typically describe symptoms made by fungi Ivy- Colletotrichum Ash- Gnomoniella

Symptomology Blight – Characterized by general and rapid killing of leaves, flowers, or stems.

Forsythia-Sclerotium Rose-Botrytis Red cedar -Phomopsis Oleander - Rhizoctonia

• Canker – A necrotic, often sunken lesion on a stem, branch, or twig of a plant.

Symptomology Sycamore-Discula Holly

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 Azalea-Exobasidium Rose-Agrobacterium

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

• 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

• Ringspot – Circular area of chlorosis with a green center.

Symptomology INSV – Impatiens Necrotic Spot Virus

Symptomology • Rosette – Short, bunchy habit of plant growth.

Rose rosette (UT-Knoxville) Rose-unknown

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

• Shot-hole – Holes on leaves. Symptomology Cherry laurel-Xanthomonas (UT –Knoxville)

• Stunting – Result of reduced plant growth.

Symptomology 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.

Damping-off

Pythium sp.

Fungi

Alternaria Petal Blight of Rose Fungi

Fungi Dollar spot of turf

Sclerotinia homoeocarpa

Fungi Blackspot of roses

• Entomosporium leaf spots Fungi

Fungi • Powdery mildew Crape myrtle Lilac Crape myrtle Rose

Apple • Rust Fungi Cedar Chrysanthemum

• Brown patch-Rhizoctonia Fungi

• 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

• Blights Bacteria

Lilacs - Pseudomonas syringae

• Spots Pittosporum Tomato Bacteria Ranunculus

• Wilts Diagnostic Bacteria Crassula Zucchini

• Soft rot Potato Bacteria Orchid Onion

Bacteria • Scabs and Cankers Cherry Watermelon Blotch Peach

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

• Mosaic Viral symptoms

Ring spot Viral symptoms

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

Stylet Plant parasitic

Various shape and sizes of nematodes

• Root knot Plant Parasitic Nematodes

Plant Parasitic Nematodes • Lesion Lesion Nematode damage on Rose

Nematodes

No nematode nematode

Soybean • Cyst Plant Parasitic Nematodes Cactus

• Foliar Plant Parasitic Nematodes 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.

• Water Abiotic problems

• Temperature Abiotic problems

Abiotic problems • Other environmental conditions

Abiotic problems • Soil pH and Nutrient Imbalances Boron Toxicity Virus vs. Nitrogen Deficiency Zinc deficiency

Sanjuan.wsu.edu

Abiotic problems • Chemicals

• More chemicals Abiotic problems Glyphosate on tomato Copper on areca palm

• Urban blight Abiotic problem