MSU Extension Publication Archive Archive copy of publication, do not use for current recommendations. Up-to-date information about many topics can be obtained from your local Extension office. A Guide to Identifying Plant Disease Symptoms Michigan State University Extension Service Christine Stephens, Anne Hartung, and Patrick Hart, Department of Botany and Plant Pathology; and Michael Mispagel, Department of Entomology Issued March 1996 8 pages The PDF file was provided courtesy of the Michigan State University Library

Scroll down to view the publication.

Extension Bulletin E-1592, Rep. March 1996

By Christine Stephens,1 Anne Hartung,1 Patrick Hart2 and Michael Mispagel3

"•Formerly Dept. of Botany and Plant Pathology; 2Dept. of Botany and Plant Pathology; 3Dept. of Entomology

Introduction

A plant disease is the result of a seriesof physiological changes followed bydevelopmental or structural changes tothe plant over time. A plant disease canbe caused by a living organism, called aplant pathogen (bacteria, fungi and vi-ruses) and also by non-biological stress(herbicide injury, mineral deficienciesor toxicities, excessive water anddrought). Sometimes, both biologicaland nonbiological diseases are respon-sible for the plant disorder, or one maymake the plant more susceptible to theother.

Plant diseases can often be diagnosedby certain signs and symptoms. A sign isthe actual visible presence of the patho-gen that caused the problem. Examplesare rusts, powdery and downy mildews(see photos). A symptom is a plant ex-pression or response to the changeswithin the plant brought about by thepathogen or stress. It is often difficult todetermine the exact cause of a plant dis-order because several disorders cancause the same symptom. For example,wilting of the aboveground parts of aplant may be caused by 1) root rottingorganisms, 2) nematodes or 3) waterstress. Plant leaves may turn yellow be-

Michigan State University Extension

cause of nitrogen deficiency in the soil,or because the roots are rotted and are nolonger able to take up sufficient nutri-ents.

The following photographs illustratesymptoms and signs generally as-sociated with common plant disorders.These symptoms also occur on othercrop or ornamental plants and are notlimited to those shown. Even thoughthere are limitations to the use ofsymptoms for diagnosis, certaincategories of symptoms provide definiteclues to the cause of a plant disease.This guide should make it easier for youto discuss plant disorders with the

specialist. It should help you collectpertinent information and perhaps diag-nose the problem yourself. It should beof use to county extension agents,teachers of vocational agriculture, in-structors of plant science, personnel ofstate and local regulatory agencies andchemical companies, growers, person-nel in charge of parks and recreationalareas, gardeners, field scouts, privateconsultants and pest control operators.

Diagnosing Plant Problems

It is difficult to standardize diagnosis— diagnosis is often an art that is de-veloped from a great deal of practice andexperience. However, there are logicalsteps to follow. Be sure to identify theplant affected and, if possible, which va-riety. This allows the use of plant dis-ease checklists and may reduce thenumber of possible diseases to a realis-tic level. Note the distribution and loca-tion of diseased plants in a field. Thepattern of occurrence may help indicatecause. It is important to note if one ormany types of plants are affected. Ifmore than one type of plant is affected, itindicates that a parasitic disease is notinvolved and that weather conditions,chemical treatments or other causes areresponsible for the symptoms. Observewhat proportion of the whole plantpopulation is involved. Diseases aregenerally progressive and it is unlikelythat 100% of the host plants will be af-fected simultaneously. Obtain a historyof the problem including such informa-tion as date first noticed, previous cropsgrown, previous weather conditions,cultural practices, chemical treatmentsand soil conditions. The problem oftenresults from poor management or cul-tural practices.

It is necessary to know what a healthyplant looks like before deciding that it isdiseased. Occasionally, normal plantsare mistakenly assumed to be diseasedbecause of some oddity in one of theirparts. For example, variegation in tulipflowers can result from a viral disease ormay be a genetic attribute of the cultivar.Note which plant parts are affected byexamining roots, stem, leaves and fruityparts. Check for internal as well as exter-nal damage. Be sure to include severalplants in your survey and collect whatappears to be various stages of diseasedevelopment. Try to determine whichpart of the plant was first affected.

Symptoms are not fixed or rigid, theychange over time. A particular diseasemay initially cause brown lesions on theleaf surface. As time progresses, the area

around the lesions may yellow andeventually the leaf may drop off theplant. Thus, one disease may cause 3types of symptoms; leaf spotting, leafyellowing, and leaf defoliation.Symptoms may also change over timedue to 1) environmental factors thatfurther damage the plant, 2) attack frominsects and other organisms, 3) recoveryof the plant, and many other reasons.

Collecting the RightInformation

Accurate diagnosis depends on twofactors:

i) the rapid receipt of a fresh andrepresentative plant sample withthe symptoms of concern; and

ii) the completion of the DiagnosticSubmittal Form for each sample.This form is available at your localcounty extension office.

Answer all the questions on the form.Be sure to indicate all types of symp-toms which are of concern. When thewhole specimen can't be submitted(e.g., a tree) it is especially important toexamine the entire plant thoroughly andinclude those observations on the form.Although obvious symptoms may be onthe leaves, the primary problem couldbe elsewhere. Thus, it is helpful to notethe conditions of the roots, presence ofstem or trunk cankers, mechanical in-jury to the plant, etc. Provide as muchadditional information as you can in thespace provided or on a separate sheet.For example:

a) Description of all diseasesymptoms not included in the ma-terial submitted.

b) Age of plant and length of diseasecondition.

c) Cultural practices — spraying in-formation (dates, chemicals,amounts), fertilization.

d) Growing conditions — soil type,moisture, shading, when trans-planted, etc.

You can never provide too much in-formation, but too little informationmay hinder diagnosis or make it impos-sible.

Specimen Collectionand Submission

The arrival of dead plant material ordecomposed plant tissue is of little or novalue in diagnosis because many non-pathogens will develop on the dead tis-sue and overrun or hide the real patho-gen. Please collect the samples as de-scribed below.a) LEAF — Collect early and late stages

of infection. Press leaves between heavypaper or cardboard (magazines workwell).b) FLESHY PLANT PARTS — Do notsend samples of a rot disease in an ad-vanced stage of decay. Collect freshspecimens with early symptom de-velopment. Wrap individually in news-paper or dry paper toweling and pack ina crush-proof box. Do not add moistureto the samples.c) CANKERS — Select recently pro-duced cankers. Submit the whole can-kered portion where possible; prefera-bly with healthy wood above and/orbelow the canker. Wrap loosely in paperand ship in a crush-proof mailing tubeor box.d) WILT OR GENERAL DECLINE —Send the entire plants, with roots, iffeasible; submit several plants, fromhealthy to severely infected. Dig, do notpull, plants from the soil so diseasedroots will remain intact. If the wholeplant cannot be sent, select samplesfrom areas of active symptom develop-ment. Include the intact root system ifroot rot is suspected. If the whole plantis submitted, wrap the root ball tightlyin plastic; send the entire plant in acrush-proof container. Diseased areascut from the plant should be sent as forcankers (above).e) TURF — Submit several 6-inch plugsof grass cut as deeply as roots will holdin soil. Plugs should show gradationfrom healthy to severely diseased. Col-lect and send plugs quickly. If mailed,pack plugs tightly with newspaper in acrush-proof box or plastic bag. Do notadd moisture to the sample.

Always take samples to your localcounty extension office first. Extensionagents are familiar with many plant dis-eases and may diagnose the problem im-mediately. If they cannot diagnose theproblem, they will ship the material tothe Plant and Pest Diagnostic Clinic atMichigan State University. Take samplesearly in the week to avoid a weekend lay-over in the Post Office. Keep samplesrefrigerated if there is a lapse of timebetween collection and submission.

NOTE: Diagnosis and recommendedcontrols by the MSU Plant and PestDiagnostic Clinic are based solely onthe material and information submitted.The less representative the sample, andthe less complete the information pro-vided, the greater the chance for mis-diagnosis. Also bear in mind that somesamples may have to be cultured out inorder to diagnose. This takes time!Following the steps outlined in thisbulletin will result in a speedier andmore accurate diagnosis.

TYPES OF SYMPTOMS01. BLOTCH: Largeand irregular-shapedspots or patches onleaves, stems andshoots. Algal leaf spoton magnolia.

04. CANKER: Necrot-ic (blackened), oftensunken spot on a root,stem, branch, or twigof a plant. Usuallysharply defined withthe margins associ-ated with callus tis-sue. Bacterial cankeron cherry.

02. BRONZING: Copper or bronze color in leaves orneedles. Air pollution injury on beans caused by ozone.

03. CALLUS: Tissueovergrowth around awound canker. Devel-ops from actively di-viding plant cells.Nectria canker onmaple.

05. CHLOROSIS (yellowing): Yellowing of normallygreen tissues due to the destruction of the chlorophyllor the partial failure of the chlorophyll to develop. Ironchlorosis on shingle oak (right). Normal leaf on left.

\> •

06. CRACKING: Splitting of tissue usually associatedwith drying cells. May result from different growth ratesin adjoining tissue. Tomato crack.

07. CUPPING: Upor down-ward curlingof the entire leaf mar-gin forming a cup- orbowl-like shape. Gasinjury to begonia.

10. DAMPING OFF (Post-emergence): Rapid rottingof the stems of seedlings at the soil line or the rotting ofthe root system after their vegetative growth above-ground is established. Damping off of impatiens.

08. CURLING: Abnormal bending of leaves from theunequal development of its two sides. Peach leaf curl.

11. DIEBACK: Progressive death of branches orshoots beginning at the tips and moving toward themain stem or trunk. Usually associated with woodyplants. Fireblight on crabapple.

control 2i

09. DAMPING OFF (Pre-emergence): A rapid rottingof the base of the stems or seedlings after germinationbut before the seedling breaks the soil surface. Damp-ing off of impatiens.

12. DWARFING (stunting): Subnormal size of plant orsome of its organs. Dwarfing from excess fungicide oncoleus.

13. PROLIFERA-TION: Excessive de-velopment of secon-dary roots or stemsaround a main root orstem. Marked dwarf-ing may be as-sociated with shor-tened internodes ofstems. Fasciation ofgeranium.

16. HERBICIDE INJURY: Gross leaf distortion. 2, 4-Dinjury on grape.

14. GALL: A more or lessspherical overgrowth orswelling of unorganizedplant cells, usually the re-sult of attack by insects,bacteria, fungi, or nema-todes. Root crown gall oneuonymous, stem crowngall in rose.

17. HYDROSIS (water soaking): A water-soaked ap-pearance caused by the movement of water from cellsinto spaces between the cells; a common symptomduring early stages of many bacterial diseases. X. dief-fenbachia on dieffenbachia.

15. HALO: A spot on a leaf surrounded by a discol-ored (usually yellow) circle. Bean rust.

18. LEAF OR NEE-DLE DROP: An un-timely and uncom-monly large loss ofneedles or leaves.Anthracnose on syca-more.

22. NECROSIS:Dead tissue orplant parts. Alter-naria on cauliflow-er. Downy mildewon cabbage.

20. LODGING: Falling over of plants at the base, usu-ally associated with root rots. Wind damage on wheat.

23. MARGINAL NECROSIS (scorch): Burning ordying and browning of leaf margins. Usually resultsfrom unfavorable environmental conditions. Scorchsymptoms on maple.

21. MUMMIFICATION: Darkening, wrinkling, harden-ing of rotted fruit. Results from a rapid loss of moisturefrom the fruit. Brown rot of peach.

24. MOSAIC: Variegated patterns of shades of greensand yellows in normally green leaves. Also character-ized by intermingled patches of normal and light greenor yellowish color. A common symptom of many vi-ruses. Rose Mosaic Virus.

25. POWDERY MILDEW: Whitish growth on plant sur-faces, usually the leaf or stem. Cotton-like in appear-ance, initially occurs in spots. Powdery mildew onpansy.

28. RUGOSITY: Wrinkling, ridging or puckering ofnormally flat leaves. Virus symptoms on soybean.

26. REDDENING (purpling): Reddish or purplish dis-coloration of leaves or other organs which are normallygreen. May be a localized or general symptom. Asteryellows on carrot. 29. RUSSETTING: Brownish roughened areas on the

skin of a fruit or tuber as a result of cork formation. Rus-sett of Golden Delicious apple.

27. ROT: Disinte-gration, dis-coloration, anddecomposition ofplant tissue. Adry or hard rotif the decay is firm and dry; or a wet rot if soft, wateryand foul smelling. Soft rot of lettuce and blossom endrot in tomato.

30. RUST: A disease giving a "rusty" appearance to aplant and characterized by dense masses of reddish-brown to orange spots on leaves and other plant parts.Orange rust on brambles.

31. SCAB: A roughened, crust-like diseased area onthe surface of a plant organ. Potato scab, apple scab.

33. STREAK (stripe): Narrow, elongate areas onleaves characterized by yellowing or necrosis of the af-fected area. Wheat streak virus on wheat.

32. SMUT: A disease caused by smut fungi and char-acterized by masses of brown to black powderyspores. Loose smut in wheat, corn smut.

.34. WILTING: Loss of freshness and rigidity, anddrooping of plants due to insufficient water in the plant.Cucumber wilt.

COVER: Apple scab caused by venturia inaequalis. Photo credit: Sandy Perry, MSU.

MSU is an Affirmative-Action/Equal-Opportunity Institution. Extension programs and materials are available to all without regard to race,color, national origin, sex, disability, age or religion. • Issued in furtherance of Extension work in agriculture and home economics, actsof May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Gail L. Imig, extension director, Michigan StateUniversity, E. Lansing, Ml 48824. • This information is for educational purposes only. References to commercial products or tradenames does not imply endorsement by the MSU Extension or bias against those hot mentioned. This bulletin becomes public property

upon publication and may be printed verbatim with credit to MSU. Reprinting cannot be used to endorse or advertise a commercial product or company.Produced by Outreach Communications and printed on recycled paper using vegetable-based inks.

MICHIGAN STATEU N I V L R S I T Y

EXTENSION

Rep. 3:96 - 10M - KMF/CP, Price .75, For Sale Only. FILE:27.31 Pests-Diseases