Plant Pathology

Brian A. CharltonOSU 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

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.

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

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

Sign or Symptom?

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

Disease Agents

Separated into two major categories:

Non-infectious

Infectious

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

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

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

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)

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

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

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

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)

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

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)

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)

Fir, Douglas – Stem Canker (several fungi)

Abnormal Growth

Stunting

Dwarfing

Epinasty

Witch’s Broom

Decay of Flowering Structures & Seeds

Bunts Smuts Scabs Ergots Blasts Seed Rot

Smuts and Ergot

Scab

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

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

Role of Moisture in Fungal Disease

Favors Growth

Favors Spore Production

Favors Spore Germination

Favors the Spread of Spores

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

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)

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

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

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

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.

Management of Nematodes

Exclusion – avoid introduction Resistant Varieties Crop Rotation Non-host Green Manure

Soil Fumigants - Nematicides

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

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

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

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

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

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

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

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

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. CharltonOSU Research & Extension Agent