James L. DoddCargill, Inc., Aurora, IL

The Role of Plait Stresses inIDevelopmellt of Coril Stalk Rots

Stalk rots of corn cause problems forfarmers somewhere every year. Potentialgrain yields are lost because kernels onrotted plants are lightweight and ears aremissed during harvest. Furthermore,lodging resulting from corn stalk rotcauses difficult, slow machine harvestingin the U.S. corn belt at a time whenfarmers are concerned about finishingbefore winter.

The distribution of stalk rot of maize isfrequently difficult to explain or predict.A cultivar may have acceptable stalkquality one year and be badly lodged inthe same field the next year. Performancein adjacent fields may differ, with nearlyall plants lodging in one area but standingin another. Often, a stalk-rotted plant isnext to a genetically identical but healthyplant.

A concept unifying host, pathogen,and environment interactions is presentedto explain the distribution of maize stalkrot. This discussion of the role of plantstresses is intended not only to lead toimproved stalk rot control but also tostimulate investigation of the mechanismsof resistance to weak, facultativeparasites.

The Stalk Rot SyndromeThis discussion concerns the deterio-

ration of lower stalk tissue after floweringand includes Fusarium, Diplodia,Gibberella, anthracnose, and charcoalstalk rots; bacterial and Pythium stalkrots are excluded. Disease pattern is thesame for each stalk rot, although specificenvironments apparently favor onefungus over others.

Typically, the first sign of stalk rot ispermanent wilting of the plant (Fig. 1).Within a day, all leaves become gray andthe leaves and ear droop. The outer rindof the lower stalk is yellow-green, turningyellow-brown within a week. At thispoint, pith tissue in the lowest internodeis rotted and pulls away from the rind,greatly weakening the stalk, whosestructure is changing from a solid rod to atube.

Stalk rot develops in every plant withwilt symptoms. Wilting also stops all

0191-291 7/80/06053305/$03.00/0©1980 American Phytopathological Society

translocation to grain, and kernels arelighter if normal grain filling is notcomplete. Symptoms do not occuruniformly within a field but are scattered,with wilted plants adjacent to healthyones (Fig. 2). Rot generally begins at thelowest internodes, and developmentprobably increases with rising tempera-tures. Affected plants usually have rottedroots and are easily pulled from theground. Whether an affected plant willlodge is determined by strength dynamics,including weight and height of the ear,amount of stalk deterioration, rindstrength, and wind force.

Dynamics of the Disease TriangleThe interactions of host, pathogen, and

environment greatly influence corn stalkrots. Fortunately, the many investi-gations of each part of the disease triangleallow development of a general concept

elucidating the dynamics of theirinteractions.

The pathogen. All fungi associatedwith corn stalk rots are ubiquitous,although some apparently are favored bycertain temperature, humidity, and soilmoisture regimens. With the exception ofColletotrichum graminicola (Ces.) Wils.on a few genotypes, no fungus appearsable to damage healthy, vigorous celltissue. In other words, as long as cellsremain vigorous, most host genotypeshave genetic components for resistance topotential pathogens.

Rarely is only a single fungal speciespresent in a rotted stalk. AlthoughGibberella zeae (Schw.) Petch peritheciaare visible on the outer rind, species ofFusarium, Penicillium, Aspergillus, andTrichoderma may be isolated from thesame rotted tissue. Obviously, manyfungal species are able to receive

Plant Disease/June 1980 533

nourishment from cellulose and lignin indead stalk tissue. Identifying the fungirarely contributes to controlling thedisease; the host-environment inter-actions are the pertinent variables. Forexample, inoculating three pathogensinto the lower stalk of five genotypes didnot increase the number of plants thatdeveloped stalk rot (Table 1).

The host. Genotypes (hybrids orcultivars) vary in tendency toward stalkrot but all probably are predisposed tothe disease. Cultivars that develop stalkrot most often in a particular environmentand individual plants that becomeaffected have several characteristics incommon. One of these is cellularsenescence. Senescence or cell death ofpith tissue precedes stalk rot, apparentlybeginning at the top of the plant andprogressing downward. Living pith tissue

resists pathogens, but dead tissue doesnot. Apparently, synthesis of cellularresistance substances decreases withsenescence (2). The ability of several fungito rot pith tissue correlates positively withthe percentage of cell death in that tissue.

Although no nutritional basis for pithcell death has been established and theresults of some studies are contradictory,lowered concentrations of reducing andtotal sugars are associated with increasesin stalk rot development (6). Undernormal culture conditions, the sugar

content is higher in the lower stalks of"resistant" hybrids than of "susceptible"hybrids. Treatments that predisposeplants to stalk rot, such as high plantdensity and leaf removal, reduce sugarcontent in lower stalk tissue. Preventionof kernel development and low plantdensity are associated with higher stalk

sugar content and increased stalk rotresistance (6).

That carbohydrate nutrition influencesresistance is supported by findings that

stalk-rotted plants have 10-19% morekernels than adjacent healthy, geneticallyidentical plants with no obvious

differences in stresses (Table 2) (5).Apparently, translocation of a dispro-

portionate amount of available sugar tothe kernels in the rotted plants results instarvation of other tissues.

Root rot generally precedes stalk rot incorn (9). Roots are in a milieu of highfungal populations for much of theseason and some tissues are invaded byFusarium spp., but only the roots ofstalk-rotted plants are completely rottedby harvest time. Several factors point tolack of soluble carbohydrate as thereason for lowered resistance in theseroot tissues.

The environment. Several fieldenvironmental stresses have beenassociated with occurrence of stalk rots(3). Stress influences the ability of plantsto produce carbohydrates. When disease,

artificial cutting, or second-brood cornborers destroy leaf area, critical photo-synthetic capabilities are removed.Photosynthesis is also reduced when soilmoisture is below the minimum level andwhen light intensity is decreased bycloudy weather or high plant density.

Environment can favor one potentialpathogen. Exceptionally dry soil favorsMacrophominaphaseoli (Maubl.) Ashby,the incitant for charcoal rot, over

Fusarium spp. In the U.S. corn belt, G.zeae is more prevalent in the humideastern areas than in the drier westernregions. Regardless of the pathogen,however, stalk rot does not occur unlessthe host is adversely affected. Theenvironment-host interaction is mostimportant in determining whether stalkrot develops in a plant or becomesprevalent in a field.

The PS-TB ConceptA concrete explanation of the myriad

factors in each component of the stalk rot

disease triangle seems to be lacking atfirst. The confusing differences in diseaseoccurrence in adjacent plants, areas in afield, or fields can be frustrating to thefield pathologist, the agronomist, and thefarmer. However, a central theme called

the photosynthetic stress-translocationbalance (PS-TB) concept of predisposi-tion of corn to stalk rot has beenproposed to explain the field distributionof the disease and the host-pathogen-environment interactions (3). Accordingto the concept, root and lower stalktissues are decayed by several micro-organisms as the tissues lose theirmetabolically dependent defense systembecause of an increase in cellularsenescence caused by carbohydratedeficiency. This deficiency is due to the

Fig. 1. Wilting of plant Is first sign of stalk rot; plant roots are deteriorated. Adjacent healthyplants are genetically Identical to wilted plant.

534 Plant Disease/Vol. 64 No. 6

Fig. 2. Brown stalk of rotted plant next to green stalk of healthy, genetically identical plant. Stalks turn brown within 1-3 weeks after plantwilts.

Table 3. Effect of northern leaf blight on stalk rot"

Percent plants with: Kernels/plant Kernel weight Bushels/ Quintals/GIenotype' Phenotype Treatment' Stalk rot Stalk lodging (no.) (gin) acre hectare

HtHt Resistant Inoculated 3.3 0.0 425.3 0.365 149.7 94.0Htht Resistant Inoculated 0.0 0.0 462.7 0.335 146.1 91.7htht Susceptible Inoculated 93.2 38.3 496.1 0.273 127.7 80.2htht Susceptible Not inoculated 0.0 0.0 472.3 0,359 159.9 100.4

Fromt Dodd (111published)."Isogenic types of Cargill hybrid 19379.Spore suispension of Exserohihum turcicum placed in whorls at sev en-leaf stage.

combination of insufficient photo-synthesis and competition with trans-location to grain. Root tissue sufferswhen the plant becomes more"committed" to grain fill than photo-synthesis can provide. Root rot thendevelops, the plant wilts because ofinsufficient water uptake, and stalk-rotting organisms decay stalk tissue.Although some organisms, eg, Fusariumspp., may already be in these stalks, activedecay of tissue apparently does not beginuntil cellular senescence.

Every plant requires a specific amountof carbohydrate to fill kernels andmaintain root growth. These requirementsare not static, however. Genetic and

preflowering environment interactionsdetermine the number of kernels a plantestablishes. The major influences onkernel establishment, which begins about25 days after germination, are moisture,fertilizer, and temperature. The rate thatcarbohydrate moves to each kernel, onthe other hand, is fairly constant for10-50 days after pollination. The dailyrate per kernel varies with the genotype,but environment, with the possibleexception of nitrogen deficiency (8), haslittle effect. Kernel number, therefore, isthe main variable among geneticallyidentical plants that affects totaltranslocation to grain.

Not much is known of the energy

requirements for cellular maintenance inroots. The requirements probablyincrease with heat, as they do with thewhole plant (7).

Environmental Influenceson Photosynthesis Rate

Photosynthesis before and after flower-ing provides carbohydrate for storage ingrain and cellular maintenance in plants.Although much preflowering energy isspent on growth and cellular maintenance,up to 20% of grain weight comes fromcarbohydrate stored in stalks. Becausephotosynthesis after pollination providesmost of the ability to meet grain and plant

Plant Disease/June 1980 535

energy requirements, environment duringthis period is of major importance. Lightintensity, effective leaf area, moistureavailability, and minerals interact withinherited characteristics of the plant to

determine the rate of photosynthesis.Light intensity. In maize, the rate of

photosynthesis increases with lightintensity. The rate in a top leaf exposed tofull sunlight (10,000 ft-c of light intensity)is 20 times that in the same leaf on a dark,cloudy (500 ft-c) day. The rate in leaftissue shaded by another leaf is one-tenththat of a fully exposed leaf. Thus, cloudyweather and plant density greatly affectthe rate a plant synthesizes carbohydrate.A plant density resulting in good yieldand acceptable stands for a hybrid in a

field one season may be too high for thesame hybrid in the same field during acloudy season.

Effective leaf area. Although genotypesdiffer in basic leaf area, effective leaf areacan be reduced by certain environmentalstresses. For example, artificiallyremoving the upper leaves increases theprobability of stalk rot, as shown in anexperiment in which stalks were cut at

different nodes below and above the ear.Stalk rot developed in plants cut onenode above the ear but not in those cut

immediately below the ear (Fig. 3). Thefewer leaves removed, the lower theincidence of stalk rot. Plants whose leafarea is removed by hail, corn borers, ordisease produce less carbohydrate. Haildamage after pollination usually resultsin an increased incidence of stalk rot.Second-brood corn borers that causeplants to break above the ear are alsooften associated with stalk rot; stalksbroken below the ear remain green, eventhough the ear may be on the ground.

Such leaf diseases as northern leafblight [Exserohilum turcicum (Pass.)Leonard and Suggs (= Helminthosporiumturcicum)] and southern corn leaf blight(Helminthosporium maydis Nisi. andMiy.) can be major contributors to stalkrot. With these blights, yield reductionsare associated more closely with kernelweight loss due to premature wilting thanwith gradual loss of leaves. A plant that

stays alive until the black layer formssustains little yield loss. The incidence of

stalk rot was studied in a hybrid verysusceptible to E. turcicum; the incidencewas 93% in plants inoculated with E.turcicum, 3.3% in inoculated plantsprotected with the Ht gene, and 0% innoninoculated plants. A yield loss of

15-21% in the unprotected inoculatedplants was mostly attributable to reducedkernel weight (Table 3).

Moisture availability. Photosynthesisrates in maize are not directly correlatedwith moisture content after a certainminimum turgidity is maintained. Therate drops drastically when the leaf waterpotential is less than-11 bars (1). Theamount of rainfall and/or irrigationneeded to maintain optimal photo-synthesis depends greatly on transpirationrates and soil types. Plant density also isan important determinant of waterrequirements.

Abundant moisture before floweringpromotes good ear development and highkernel numbers; if the postfloweringenvironment is dry, stalk rot is likely tooccur. If, on the other hand, an earlydrought inhibits pollination and laterrains promote photosynthesis, stalk rot isless likely to occur. The timing ofirrigations can influence the incidence ofstalk rot by affecting the availability of

water before and for 55 days afterpollination.

Minerals. Fertilizers high in nitrogenand low in potassium often are associatedwith an increased incidence of stalk rot.

The role of these minerals in the PS-TBconcept is not as clear as that of the otherplant stress components. Nitrogenpromotes growth of leaf and stalk tissue,helps establish high kernel numbers, andinteracts in the translocation of carbo-hydrate to grain. Potassium is involved inmany metabolic processes, includingstomatal function. The photosynthesisrate is lower in plants deficient inpotassium than in those high inpotassium. Perhaps high nitrogenpromotes a large grain "sink" and moreenergy-expensive plant tissue to maintain,whereas low potassium reduces the

plant's ability to meet these energydemands.

Efforts to Control Stalk RotThe PS-TB concept apparently applies

to all maize genotypes. If a grain sink isestablished, sufficient stress can beapplied to predispose the plant to stalkrot. Aspects of the concept varyaccording to genotype, however. Inheriteddifferences include size of the grain sink,utilization of minerals, resistance to leafdiseases, photosynthesis rate per unitarea of leaf, total leaf area, resistance to

drought, and efficiency of energy use.That the inheritance of stalk rotresistance is regarded as complex istherefore not surprising. The environmentin which a hybrid is grown must beconsidered when characterizing agenotype as susceptible or resistant. Amore accurate way to predict a genotype'svulnerability to stalk rot is to expose thehybrid to specific stresses. Breedingefforts must concentrate on selectinggenotypes with consistently high grainyields and low percentages of stalk-rottedplants in field environments.

Certain cultural methods help reducevulnerability to stalk rot. Fertilizerbalance, with equal amounts of nitrogenand potassium, is important. Mineralanalysis of leaf tissue at pollination is thebest way to assure adequate potash

application; several tests have shown

proper amounts of potash decrease the

incidence of stalk rot. Because of light

and water stresses, plant density is very

influential. Optimal density is determined

by weather conditions, soil type, kind and

amount of fertilizer, and genotype.Unfortunately, what is optimal density

during a sunny season with adequate

moisture will not be satisfactory during a

cloudy or dry season.Timely irrigations control water

stresses somewhat. The amount of

moisture available before pollinationgreatly influences grain sink size, yieldpotential, and vulnerability to stalk rot.Avoiding water stress during the grainfilling period (about 55 days) after

100

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-3 -2 - 1 1 2 3 4 5 6NODE C UT BELOW OR ABOVE EAR NODE

CUT

Fig. 3. Relationship between artificial removal of leaves and development of stalk rot.

Stalks were cut at various nodes (nodes below ear node are indicated by minus signs)

approximately 3 weeks after silking, and stalk condition was recorded 8 weeks later.

Brown stalks began rotting before yellow ones, but both were regarded as rotten.

Unshaded areas represent green, healthy stalks.

536 Plant Disease/Vol. 64 No. 6