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Plant Pathology

Brian A. Charlton

OSU Research & Extension Agent

Objectives

Understand the concepts of disease, injury, and the signs and symptoms of plant pathogenic agents

Learn the basic biology of major disease agents of plants

Understand the interaction of pathogens, their host plants, and environmental factors on plant disease

Disease vs. Health in Plants

What constitutes disease in plants? Debate among scientists

Variability in ‘normal’ physiology difficult to define

A plant is considered healthy when: Able to carry out physiological functions to the

best of their genetic potential

If this does not occur because of interactions with pathogens or adverse environmental factors then disease may be present

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What is Disease?

Any malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms.

Referred to as: Disease Agents

To determine the presence of a disease agent you need to look for: Signs and Symptoms

Disease vs. Injury

Is there a difference between disease and injury?

Yes . . .

Disease is a process that occurs over time.

Usually referred to as chronic

Injury is a more instantaneous form of damage.

Usually referred to as acute

Can be caused by insects, animals, physical damage, frost, chemical, etc.

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Signs vs. Symptoms

Signs are physical indications of the presence of a disease agent The pathogen itself or products and parts of the

disease agent

Symptoms are the internal or external effects of alterations exhibited by a plant suffering from disease

Common Symptoms

Abscission, blight

Blotch, canker

Chlorosis, defoliation

Desiccation, dieback

Dwarfing, enation

Epinasty, etiolation

Fasciation, flagging

Gall, mosaic

Mottle, necrosis

Rot, rugose

Russet, scab

Stippling

Water-soaked

Wilt

Witches’ Broom

Common Signs

Pest itself – e.g. aphid, nematode, spores

Bacterial Slime or gumosis

Conk, cyst

Frass, Fruiting body

Girdling, honeydew

Mole mounds, mycelia

Weather records, webbing

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Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

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Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

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Sign or Symptom?

Sign or Symptom?

Disease Triangle

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Disease Triangle (cont.)

Host relates to factors such as genetic susceptibility, plant age, growth rate, and plant stress.

Pathogen relates to factors such as virulence, amount of inoculum, pathogen density, fitness, etc.

Environment relates to physical parameters such as temperature, moisture, wind, light, etc.

Disease Triangle (cont.)

Disease results when the following occurs: Host is susceptible

Environment favors the pathogen

Pathogen is present, living, and active

Disease Agents

Separated into two major categories:

Non-infectious

Infectious

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Non-infectious Disease Agents

Can not be spread from one plant to another

Abiotic is another term used to describe non-infectious disease agents

Non-biological entities

Result of adverse interactions between the plant and its environment

Most Common Factors – Abiotic Disease

Nutrient Deficiencies and Toxicity

Water Stress

Temperature Stress

Chemical Injury

Mechanical - Cultural

Symptoms of Water Stress

Insufficient Moisture Wilting, Stunting

Leaf Scorch, Defoliation

Reduced fruit set

Excess Moisture Similar Symptoms + seed & root rot disease

Hypoxia: oxygen-starved roots Increased Respiration Rates

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Symptoms of Deficiency & Toxicity

Deficiency Necrosis

Chlorosis

Stunting

Rosettes at buds

Anthocyanin build-up

Toxicity Necrosis and Chlorosis most prevalent

Symptoms of Temperature Stress

Too Cold Frost or freeze damage – kills cells

Interferes with cellular processes

Inactivates crucial enzymes

Too Hot Necrotic – kills or ‘cooks’ cells

Cellular processes break down

Inactivates biochemical reactions

Types of Chemical Injury

Air pollution

Pesticides

Herbicides Plant growth regulators

Cell division inhibitors

Photosynthesis inhibitors

Cell membrane disrupters

Inhibitors of cell metabolism

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Types of Cultural Injury

Examples???

Infectious Disease Agents

Can be spread from one plant to another

Biotic is another term used to describe infectious disease agents

Biological entities

Known biotic pathogens: Fungi, bacteria, mollicutes

Nematodes, viruses, protozoa

Parasitic plants

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Common Symptoms of Disease

Leaf spots and blights

Ringspots

Mottling and mosaics

Root rot & Stem rot

Wilt

Galls & Cankers

Abnormal Growth – dwarfing, witch’s broom

Decay of flowering structures & seeds

Leaf spots and blights

Most common foliar symptoms

Leaf spots Necrotic lesions – delimited border

Does not continue to spread across leaf surface

Blights Severe withering

Rapid killing of leaves, flowers, shoots, fruit, entire plant

Cucurbit – Leaf Spot (Cercospora)

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Rose – Black Spot (Diplocarpon)

Late blight lesions

Early blight lesion Gray mold lesion

Sporulation on underside of potato leaf

fluffy white sporulation on lesion’s edge

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Rose - Botrytis blight

Leaflet collapsing from late blight infection

Ringspots

Chlorotic rings on leaf surface

Contain green tissue within the ring

Associated with some viral diseases

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Rhododendron Necrotic Ringspot

Mottling and Mosaics

Disruptions in plant tissue pigments

Mottling Irregular pattern of indistinct light & dark pigment

Mosaics Intermingled patches of normal and light green or

yellowish color

Most often associated with viral disease

Potato Virus Y

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Root Rot & Stem Rot

Root Rot Decay of living root tissue (root hairs, cortex, etc)

Discolored, softened, or disintegrated tissue

Fungi and Bacteria most common causal agents

Stem Rot Essentially the same as root rot

Tissues affected are cambium, pith, and structural tissues

Bean – Root Rot (Fusarium)

Corn –Crown Rot (Fusarium)

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Wilt

Loss of rigidity or drooping plant parts

Insufficient water in plant

Diseases can impact the following: Water transport tissues – xylem

Water retention in plants

Tomato – Fusarium Wilt

Tomato – Fusarium Wilt

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Cucurbit – Bacterial Wilt (Erwinia)

Galls

Abnormal swellings or overgrowths (tumors)

Can occur on roots, stems, leaves, etc.

Composed of unorganized tissue resulting from stimulus provided by infective organism

Forsythia -- Stem Gall (Phomopsis)

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Rose – Crown Gall (Agrobacterium)

Cankers

Localized sunken necrotic lesions on stems, branches or twigs.

Surrounded by living tissues and grow concentrically

Colonize new tissue at their margins

Apple – Perennial Canker (Cryptosporiopsis)

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Fir, Douglas – Stem Canker (several fungi)

Abnormal Growth

Stunting

Dwarfing

Epinasty

Witch’s Broom

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Decay of Flowering Structures & Seeds

Bunts

Smuts

Scabs

Ergots

Blasts

Seed Rot

Smuts and Ergot

Scab

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Bean – Seed Rot (Rhizoctonia, Pythium, etc.)

Brief Symptom Schematic

Symptoms Bacteria Fungi Viruses Nematodes PhytoplasmaWilts X X X XLeaf Spots & Blights X X XFruit Rots X XRoot Rots X X XDamping Off XDistorted Growth X X X X X

Fungi

Non-photosynthesizing eukaryote

Produces enzymes to absorb food

Responsible for 85% of all plant disease

Live on organic matter

Reproduce primarily by spores

Perennial inhabitants of all ecosystems

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Fungi Terminology

Hypha – tubular thread of connected single cells (hyphae = plural)

Mycelium – hyphal threads collectively growing together

Thallus – interwoven mass of mycelia

Stroma – dense organized thallus tissue

Sclerotia – dense masses of stroma tissue Overwintering structure

Fungi Terminology (cont.)

Spores – sexual reproductive structure Essentially like seeds

Fruiting Body – structure that produces spores

How do Fungi Enter Plants

Through Natural Openings between Plant Cells (stomates)

Penetrate directly into the plant cell by digesting the cell wall

Wounds are an ideal port of entry

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Role of Moisture in Fungal Disease

Favors Growth

Favors Spore Production

Favors Spore Germination

Favors the Spread of Spores

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Where Might Control Tactics be Useful?

Bacteria

Single-celled microscopic organism

Multiples by cell division

Composed of individual cells

Nearly 200 species cause disease in plants

Identified by physiology

Perennial inhabitants of all ecosystems

Bacteria Terminology

Flagella – tails that aid in mobility (water)

Gram stain – method of identification Gram Positive and Gram Negative

Based on staining process

Most plant pathogens are G Negative

Can be vectored by insect activity

Composed of a cell wall and membrane

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How do Bacteria Enter Plants?

Need a wound or natural plant opening Leaf stomata or certain flower parts

Spread of Bacteria Occurs via Water (rain, irrigation, plant liquids)

Plant propagation (cuttings)

Dirty tools (pruning)

Main Genera of Plant Pathogenic Bacteria

Agrobacterium - tumors

Clavibacter – ringrot, wilt, canker

Erwinia – fireblight, softrot

Pseudomonas – leaf spots, wilts

Streptomycetes - scab

Xanthomonas – black rot

Xyella – obstructs xylem vessels (vectored)

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Viruses

Small particle consisting of genetic material (DNA, RNA, etc) encased in a protein capsule

Can only reproduce within living organism

Become systemic in their host plants

Are some of smallest known pathogens

Pesticides not effective against specific virus

Viruses (cont.)

Use a plant’s cellular machinery and chemical processes to replicate

Particle accumulation in plant cells alters cell physiology and obstructs cellular processes

Three main morphologies Rod-shaped

Sphere-shaped

Cylindrical rod-shaped

How do Viruses Enter Plants?

Via Insects (aphids, leafhoppers) Sucking mouth parts

Mechanical Spread Sap transmitted

Machinery, rough handling

Propagating Materials Vegetative organs (tubers, bulbs)

Occasionally seed-borne

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Management of Viral Diseases

Avoid wounding plants – avoid mechanical

Border Plants, Trap Crops, Buffer Strips

Rogue Infected Plants

Resistant Varieties

Use clean propagating material

Control Vectors Themselves

Alternate Habitats

Nematodes

Microscopic, worm-like animals

Live saprophytically in water and soil

Live as parasites on plants

Are True Animals

Reproduce Sexually

Parthenogenetically – NO MALES! Essentially clones

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Nematodes (cont.)

Life Cycle – egg, juvenile, molting, adult

Eggs – can remain dormant for long periods Suitable host - root exudates trigger hatch

Juvenile – look similar to adults but smaller 4 stages – determine sex

Feeding causes damage

Adults

Four Categories of Nematodes

Migratory Ectoparasites Never enter plant tissue, feeding and moving

Migratory Endoparasites Enter plant tissue and move from cell to cell

Sedentary Ectoparasites Remain outside plant tissue w/ long-term feed site

Sedentary Endoparasites Permanent feeding site inside plant tissue

Are all Nematodes Bad?

Most are Beneficial

Several are Plant Pathogenic

Some Vector Viruses Stubby-root

Yield Reducing at High Populations

Serve as Infection Courts for other Disease

Quality Issues with Viruses CRS

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Corky Ringspot

Are Nematodes Found Above Ground?

Some infect above ground portions of plants Stem nematodes in Alfalfa Leaf deformations Loss of photosynthetic surface area

Most are found below ground Root and Rhizome Infections Bulb and Tuber Infections

Mostly reside in top foot of soil Migratory Ectoparasites – depths of 5 ft.

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Management of Nematodes

Exclusion – avoid introduction

Resistant Varieties

Crop Rotation Non-host

Green Manure

Soil Fumigants - Nematicides

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Plant Parasitic Plants

Derive Nutrition and in some cases Physical support from host plants

Haustoria – similar to a root

Haustoria – penetrates host plant Absorb water

Absorb Nutrients

Absorb Light

Symptoms of Plant Parasitism

Wilting

Stunting

Chlorosis

Deformation of Plant Tissue

Examples Dodder

Witchweed – Broomrapes

Mistletoes

Dodder

Lacks chlorophyll

Relies on host for organic nutrients, minerals, and water

Most common in alfalfa, potatoes, onions, beets Common pest in Treasure Valley

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Cucurbit – Dodder

Mistletoes

Parasitize woody perennials

Have chloroplasts but rely on host for water and mineral nutrition

Can cause substantial loss to tree industries

Common on Oak Trees – Hwy 140 to Medford

Oak – Mistletoe

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Witchweed and Broomrape

Witchweed Root parasitizer

Does produce some chlorophyll

Limited to SE USA

Broomrapes Mostly lack chlorphyll

Of little economic importance to temperate regions

Management of Parasitic Plants

Exclusion

Prevent Spread

Crop Rotation – non-hosts

Biological Control

Resistant Varieties

Chemicals

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Disease Cycles

Pathogen Survival

Inoculum Production

Pathogen Dispersal

Deposition

Germination

Host Penetration

Intrahost Spread

Secondary Inoculum

Pathogen Survival

Overwinter from one growing season to another via five protective environs:

Crop Residue Provide nutrients

Physical buffer from the elements

Seed Transported by man or animals

Pathogen may be on surface or inside seed

Pathogen Survival (cont.)

Soil Dormant structure - sclerotia

Host Plants Perennial Plants – dormant or semi-dormant state

Insects Dormancy

Vectoring – Persistent & Non-persistent manner

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Inoculum Production

Inoculum – Established pathogens can produce material that incites new infections Fungal sporulation

Bacterial cell division

Nematode - eggs

Pathogen Dispersal

Wind

Water

Insects

Soil Movement

Propagative Plant Materials

Human Interactions

Deposition

Once pathogens are dispersed into the environment they must be able to come into contact with their new host

Often referred to as juxtaposition

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Germination

Favorable Environmental Conditions Optimal Moisture

Optimal Temperature

Host Penetration

Suitable Infection Court Natural plant openings

Stomates, lenticels, inflorescences, wounds

Direct Penetration

Intrahost Spread & 2o Inoculum

Intrahost Spread Movement within plant

Local

Systemic

Secondary Inoculum Survival Strategy to reproduce

Polycyclic – production of secondary inoculum

Monocyclic – one sporulation event per season

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Disease Triangle

Disease Triangle (cont.)

Host relates to factors such as genetic susceptibility, plant age, growth rate, and plant stress.

Pathogen relates to factors such as virulence, amount of inoculum, pathogen density, fitness, etc.

Environment relates to physical parameters such as temperature, moisture, wind, light, etc.

Disease Triangle (cont.)

Disease results when the following occurs: Host is susceptible

Environment favors the pathogen

Pathogen is present, living, and active

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Control of Infectious Plant Diseases

Exclusion

Avoidance

Eradication

Protection

Resistance

Tolerance for Disease Seed Certification

Disease Diagnosis

Define the Real Problem

Look for Patterns – uniform vs non-uniform

Determine Time Development of Damage

Look for Signs and/or Symptoms Root, storage organs (tubers, bulbs)

Seedling Disease, Leaf, Stem, Branch, Trunk

Flower, Fruit

Determine Cause of Plant Disease

Disease Diagnosis (cont.)

ASK QUESTIONS Varieties

Cultural Practices

Irrigation Practices

Seed Source

Patterns, Affected Plant Parts

When did Symptoms/Signs Develop

Take Representative Samples

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ASK MORE QUESTIONS

Consult All Resources Possible Publications

Reference Books

OSU Research & Extension Centers

Private Consultants

Web-based References – BE CAREFUL

Experience Best Teacher

References

OSU Plant Disease Clinic

Iowa State University – MS Agronomy Agron 514 and 533

Cornell Cooperative Extension

Additional Slides by Dr. Cindy Ocamb

Cornell Cooperative Extension

ISU Agronomy

Plant Pathology – Cram Session!! Any Questions?

Brian A. Charlton

OSU Research & Extension Agent