Diagnosing Plant Problems-Plant Diseases

8/3/2019 Diagnosing Plant Problems-Plant Diseases

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National Plant Diagnostic Network

National Plant Diagnostic Network

Diagnosing PlantProblems: Plant

DiseasesNational Plant Diagnostic Network

First Detector Training

Williamson, Riley, and Maloy. December 2006.

Update, August 2008.NPDN Publication No. 0013


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MODULE 4Diagnosing plant problems: Plant Diseases

Adapted from Plant Disease

Diagnosis by M. B. Riley, M.

R. Williamson and O. Maloy.In: The Plant Health Instructor.DOI:10.1094/PHI-I-2002-1021-01

Used by permission of the American Phytopathological Society


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Plant Problem DiagnosisDiagnosisprocess used for the

identification of problems.Tools:

Plant sciences, especially plant

pathology and entomologyArts of investigation and detective

work


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Plant Problem Diagnosis:Preliminary Considerations

Know Normal Appearance of Plant

Proper plant identification is critical.

Scientific vs. common names

Variety or cultivar

Recognize healthy plant appearance. Whats normal for specific plant?

Compare with healthy plants of same genus,species and cultivar.


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Plant Problem Diagnosis Steps1. Consider the Possible Agents:

Abiotic non-living agent

Damage from chemicals, weather,mechanical

Nutritional problems

Biotic living agent Pathogens - parasitic microorganisms that

cause diseases

Pests insects or mammals feeding on or

damaging plants


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Plant Problem Diagnosis Steps2. Consult literature resources for

possible diseases and disorders.

Indexes listing hosts and theirpathogens

Websites providing information

Books with background info and

host/pathogen listsCompendia for information on specific

crops


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Plant Problem Diagnosis Steps3. Investigate symptom progression.

Biotic disease symptoms progressand nearby plants become infected.

Abiotic disease generally a lack ofsymptom progression. Does not

spread. Exception nutritional symptoms

progress slowly.


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Plant Problem Diagnosis Steps

4. Ask questions

When was the problem noticed? Was the damage sudden or gradual?

How old are affected plants?

Percentage of plants affected? What is the degree of injury?


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Plant Problem Diagnosis Steps5. Observe patterns. Determine prevalence of problem

Large area/All plants generallyabiotic

Scattered, localized generally biotic

Check for distribution of symptoms

Uniform generally abiotic

Random generally biotic


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Observation of Patterns

Scattered symptom

distribution

Random pattern ofdamage

Loblolly pine killed by southernpine beetle

Photo: R. Billings, Texas Forest Service,http://www.forestryimages.org


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Observation of Patterns

Symptoms distributed

in a large area.Damage pattern isuniform.

Cotton field with chemicaldamage

Used by permission of M. Williamson


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Plant Problem Diagnosis Steps6. Review cultural practices.

Proper planting technique

Fertilizer and pesticide application Irrigation frequency

7. Review environmental conditions. Temperature extremes

Drought or excess rain

Soil type and conditions

8. Check host specificity many hosts

may mean an abiotic problem


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9. Check for symptoms and signs.

Symptoms - plant reactions oralterations of a plants appearancedue to a disease or disorder

Signs - actual pathogen, parts or by-

products seen on a diseased hostplant


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Symptoms Identify and classify symptoms

Underdevelopment

Overdevelopment Necrosis or death

Alteration of normal appearance

Wilting

Identify plant parts affected

May need to identify source of symptomswithin plant system


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SymptomsUnderdevelopment Stunting of plants,

leaves Shortened

internodes

Inadequatechlorophyllproduction

Caused by manytypes of pathogens

Photo: J. Dunez, www.forestryimages.org

Chrysanthemum stunt viroid infection ofchrysanthemum showing stunting andearlier blooming of affected plants; healthy

plants in middle.


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Symptoms -

Overdevelopment

Galls form on all plantparts, caused by

many pathogens.

Oak leaf blister, caused by Taphrinacaerulescens.

Gall on stem of eastern redcedar

caused by Agrobacteriumtumefaciens.

Overgrowth of leaf tissuecauses thickening and

distortion.

Photo: Edward L. Barnard, FloridaDepartment of Agriculture and ConsumerServices, www.ipmimages.org

Photo: Joseph O'Brien, USDA Forest Service,www.ipmimages.org


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SymptomsOver-development - more biotic causes Root knot nematodes - root galls Callus formation around cankers

Tissue proliferation -some downy mildewsand phytoplasmas

Some insects and mites also cause galls.


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SymptomsOver-development

Abiotic causes

Oedema

Fasciation can bephysiological or due

to biotic causes.

Some tree burlsOedema of camellia, a physiologicaldisorder

Used by permission of M. Willliamson


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Symptoms and SignsTissue Necrosis

Fungal leaf spots

Usually round, not vein-limited

Elongated on narrowleaves or stems

May have alternatingzones of light and darktissue

Sporulation or myceliamay be evident

Target spot of tobacco, caused byThanatephorus cucumeris.

Photo: Paul Bertrand, University of Georgia,www.ipmimages.org


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Bacterial leaf spots

Often dark and water-soaked

Often vein-limited,giving angular shape

Bacterial flowobserved undermicroscope

Bacterial leaf spot of sweet peppercaused by Xanthomonas campestrispv. vesicatoria

Photo: Volcani Center Archives, AgriculturalResearch Organization, www.ipmimages.org


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Fruit rots

Firm or soft andwatery

Colors vary

Fungal sporulationmay be present

Fungal and bacterialcausesBrown rot of peach, caused by

Monilinia fruticola

Photo: Clemson University - USDA CooperativeExtension Slide Series, , www.ipmimages.org


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SymptomsTissue Necrosis

Cankers = localized

necrotic lesions Sunken or swollen or

both

Mainly caused byfungi and bacteria

Mechanical injurycan causeSugar maple canker caused by

Nectriaspp.

Photo: Joseph O'Brien, USDA Forest Service,www.ipmimages.org


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Blight = rapid death

or dieback. Also from coalescing

leaf spots, e.g. earlyblight of tomato

Mainly fungal andbacterial causes

Fire blight caused by Erwinia amylovora

Photo: Robert L. Anderson, USDA Forest Service,www.ipmimages.org


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Dieback - many causes

Girdling cankers. Followdieback to detect

Dieback pathogens w/outcankers

Root problems

Mechanical, chemical orcold damage

Pitch canker of Virginia pine, causedby Fusarium subglutinans

Photo: Edward L. Barnard, Florida Department ofAgriculture and Consumer Services, www.ipmimages.org

O i i i


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Root rots

Root lesions Darkening and

softening of roots

Sloughing off of outer

tissues Yellowing of foliage

and stunting of plants

Fungal and bacterial

causes

Root rot of boxwood, caused byPythiumsp.


MODULE 4 Di i l bl Pl Di


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Damping off

Rapid death ofseedlings and cuttings

Stem infected at soilline, seedling topples

Spreads rapidly undercrowded conditions

Mainly fungal agentsDamping off of tobacco, caused byRhizoctoniaspp. and Pythiumspp.

Photo: Clemson University - USDA Cooperative ExtensionSlide Series, www.ipmimages.org

MODULE 4 Di i l t bl Pl t Di


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SymptomsWilting

Biotic factors

Root, crown or stem rots

Vascular wilts

Root crown or stemdamage from insects oranimals

Mainly fungal andbacterial causes

Used by permission of M.Williamson

Stem wilt of Exacumfrom INSVinfection



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Symptoms

Wilting

Abiotic factors

Damage fromweather extremes

Dry or flooded soil

Mechanical damageto roots, crown orstemCold damage on azalea

stem. Shoots wilt above

damage.




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SymptomsAlteration of normal

appearance

Mosaic Irregular patches of

discolored tissue.

Often with distortion

Viruses mainly

Mosaic symptoms on cucumber due toa viral infection

Photo: David B. Langston, University of Georgia,www.ipmimages.org



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appearance

Ringspot On leaves and fruits.

Distinct ring shapedlesions, often in

concentric zones

VirusesTomato spotted wilt virus (TSWV) ongeranium leaf

Photo: Dr. Backhaus, Biologische Bundesanstalt frLand- und Forstwirtschaft, www.ipmimages.org



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appearance

Abiotic causes

Chemical damage

Nutritional deficiencies

Cotton damage by the herbicide 2,4-D


MODULE 4 Diagnosing plant problems: Plant Diseases


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Symptoms

Alteration of normalappearance

Yellowing

Often nutritional,cultural orenvironmental

Root malfunctionbiotic or abiotic

Nematode infestation Yellows phytoplasma

diseasesBurford hollies yellowing dueto root malfunction. Soil ispoorly drained.




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SymptomsBe aware of symptom variability.

May have more than one problem.

More than one pathogen may be involved.

Pathogens have varying levels of virulence.

Environmental conditions can affect symptom

expression.

Host genetics and physiology can effect symptomexpression.



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Symptom variability

due to multiple pathogens Peach seedling on left

infected with both

Prune dwarf virus(PDV) and Prunusnecrotic ringspot virus(PNRSV)

Seedling in middleinfected with PDValone

Seedling on rightinfected with PNRSV

alone

Peach seedlings infectedby single or multiple

viruses show symptomvariability.

Used by permission of S. Scott



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Signs

Fungal signs

Evidence of

pathogen on tissue. Spores, mycelia or

fruiting bodies.

Use hand lens or

knife for fielddetection.

White mycelia of Armillariasp. ontree trunk affected by Armillaria

root and stem rot.

Photo: USDA Forest Service Archives, USDAForest Service, www.ipmimages.org



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Signs

Fungal signs

Fruiting bodies

Shape of fruitingbody aids in fungalidentification.

Tissue location may

help differentiatebetween species.

White rust of chrysanthemum, fruitingbodies of Puccinia horiana

Photo: Central Science Laboratory, HarpendenArchives, British Crown, www.ipmimages.org



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Signs

Fungal signs

Powdery mildew

fungi form myceliaand spores on tissuesurface.

Powdery material

rubs off. Leaves often

distorted, discolored

Dark, round fruiting

bodies form in fall.

Oidiumsp. causing powderymildew on Viburnum

suspensum.




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Symptoms and Signs

Downy mildews oftenproduce angular leafspots.

Downy mildewsdevelop sporulation

on leaf undersidesonly.

Downy mildew of veronica, caused byPeronospora sordida


Downy mildew of Buddleiacaused by Peronospora harotii.





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Signs

Bacterial signs

Streaming from freshly

cut stem in water.

Stringing from cut

stems pushed together,then pulled apart.

Bacteria flow observed

from tissue mount on

compound microscope.

Bacterial ooze can beobserved on-site withsome diseases.

Bacterial streaming from tomatoinfected by Ralstonia

solanacearum.




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12. Laboratory Examination and Testing

Dissecting microscope Compound microscope

Electron microscope

Moist chamber incubation

Culturing Additional tests for biotic agents Tests for abiotic agents



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Laboratory Examination

Examination using dissectingmicroscope

Observe sporulation

thats invisible tonaked eye.

Closer view of

structures seen bynaked eye

Nematologist can IDnematodes to

genus.




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Laboratory Examination

Examination using microscopeof higher magnification

ID fungi to genus or to

species. View bacterial flow;

observe morphology athighest magnification.

View virus inclusionbodies.

Using electronmicroscope, view

viruses, phytoplasmas


Rust spores, Pucciniasp.



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Laboratory tests

Moist Incubation

Goal to induce

sporulation Important for

obligate fungalpathogens

Avoid overly moistconditions.

Can surface sterilize


Moist chamber



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Laboratory tests

Fungal isolation

Surface sterilize.

Use margin ofdiseased area.

Nutritive agar media,can be selective

Diagnostician canoften ID to genusfrom appearance ofculture


Fungal culture.



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Isolate identification

Fungal identification Other methods

Examination and comparison withdrawings in literature

Utilizing keys in literature

Literature sources for host diseasesand disorders

Serological, molecular, other tests



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g g p p

Laboratory tests

Bacterial isolations

Grind tissue in

sterile water. Streak suspension

on bacterialmedium.

Can use selectivemedia

Transfer single,

isolated colony

CulturesofRalstonia

solanacearum

Used by permission of K. Ong



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g g p p

Isolate identification

Bacterial identification

Biochemical tests

Carbohydrate utilization (BIOLOG) Fatty acid methyl ester analysis

(FAME)

Molecular methods Polymerase chain reaction (PCR)

Serological tests, e.g. ELISA



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g g p p

Identification of other pathogens

Non-culturable pathogens

Viruses, viroids

Phytoplasmas

Fastidious bacteria



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g g p p

Identification of non-culturable

pathogensMethods

Serological tests, e.g. ELISA

Electron microscopy

Staining for virus-induced inclusionbodies

Molecular tests, e.g. PCR

Enzyme activity testing

Host range testing



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g g p p

Identification of other pathogensELISA tests

Multi-well test formultiple samples

Individual ELISA kit

Usedby permission of M. Williamson



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g g p p

Identification of Nematodes

Extraction from Soil and Plant Material

Soil extraction using centrifugal flotation

Baerman funnel techniques for soil andtissue

Mist chamber extraction for nematodes inroot or stem tissue

Direct examination of nematodes intissue.



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Laboratory tests

Diagnostic tests for abiotic causesSoil, water

pH testingNutrient analysis

Soluble salts analysis

Analysis for chemicals

Plant tissue tests

Nutrient analysis

Analysis for chemicals



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Plant Disease Diagnosis Steps

13) Investigate pathogenicity.

Is identified organism a known pathogen

- of host?

- of hosts in same genus?

Consult literature to determine

If not a host, Kochs Postulates may be next step A lengthy process, rarely used in routine

diagnoses

Problem arises when organism cannot be

cultured



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Kochs Postulates

Robert Koch German physician in late1800s

Proved that a bacterium caused anthraxin cattle

Developed steps proving the bacterium

was causal

Same steps used to prove pathogenicityfor plant disease organisms



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Kochs Postulates

1. Note constant association oforganism with diseased plants and

consistent, observable symptoms.2. Isolate and characterize organism in

pure culture.

3. Inoculate healthy plant with organism,and observe the same disease andsymptoms.

4. Re-isolate same organism.



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Plant Disease Diagnosis Steps

14. Final Diagnosis

All information compiled and analyzed

Organism identified, found to be pathogen ofhost

OR

Lab tests, other information reveal an abioticcause

15. Develop control recommendations andpresent to grower.



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Meg Williamson, Clemson University

Melissa Riley, Clemson University

Otis Maloy, Washington State University

Authors



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This publication can be used for non-profit, educationaluse only purposes. Photographers retain copyright tophotographs or other images contained in this publicationas cited. This material was developed as a core trainingmodule for NPDN First Detector Training. The authors

and website should be properly cited. Images orphotographs should also be properly cited and credited tothe original source.

Publication Number: 0013 Publication Date: December 2006

Diagnosing Plant Problems-Plant Diseases

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