Circular-1130

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Some Diseases of Corn in FloridaTom Kucharek and Richard Raid, Respectively Professor of Plant Pathology,Gainesville; Professor, Everglades Research and Education Center, BelleGlade.1994. Copied February 2000.

Design and Illustration: Katrina Vitkus

INTRODUCTION

Sweet corn and field corn are the principaltypes of corn grown in Florida. Field corn isgrown for animal feed and hybrid seed; sweetcorn is grown primarily for the fresh market.Numerous diseases of corn occur in Florida andfrequently have caused severe yield losses.Tentative field diagnoses should be confirmedwith laboratory tests.

SEEDLING BLIGHTS

Seedling blights are caused by fungi such asFusarium spp., Rhizoctonia spp., Pythium spp.,and Penicillium spp. It is not uncommon for twoor more of these fungi to be present in a seed-ling at the same time. These fungi infect manyplant species including grasses and broadleafplants. These fungi survive in the soil, someweeds, and old crop debris. Seedling blightstend to be most severe when the seedling isbeing stressed during emergence from factorssuch as cool soils or extremes in soil moisture.Seedlings may die before or after emergence.

Symptoms of seedling blights include dis-colorations of the roots and stems, stunting, andbrowning or yellowing of emerged seedlings.Infection from Pythium spp. typically results ina slightly greasy to watery rot of tissues, par-ticularly of the root tips, outer root tissues, andlower stems. Infection from Rhizoctonia spp.typically causes reddish brown-orange lesionsin stems and roots. Penicillium spp. secrete atoxin which translocates up the stalk and causesyellow streaks in, or a dull green appearanceof, the leaves.

Seedling blights are reduced by the treat-ment of seed with a labeled fungicide, crop ro-tation with non-grass crops, planting in warmsoils, and the avoidance of deep seeding. Also,debris from previous crops and weeds shouldbe allowed to decompose as long as possible

before planting. Every possible control measureshould be used with super-sweet types of sweetcorn because they are highly susceptible toseedling blights.

ROOT AND STALK ROTS CAUSED BYFUNGI

Root rots occur in sweet and field corn.Stalk rots that are caused by fungi are com-mon in field corn after tasseling but are typi-cally uncommon in sweet corn. The fungi thatcause root and stalk rots in Florida includethose that cause seedling blights, Exerohilumrostratum Helminthosporium rostratum), Diplodiamaydis, Macrophomina phaseolina, and others. Itis not unusual for more than one fungus tocause disease in stalks at the same time. Rootand stalk rots tend to be most severe whencrop rotation is not used (Fig. 1).

Root- and stalk-rotting fungi can survive inthe soil, particularly if old crop debris ispresent. In susceptible varieties, some stalk rotsprogress from infections that originated in theroots. However, stalk rots caused by E. rostratumand D. maydis begin in the stalks. Root and stalkrots are enhanced by injuries caused by culti-vation, excess fertilizer in the root zone, hail,nematodes, soil insects, dense stands, and ex-cessive damage to the foliage from leaf diseasesand foliar feeding insects.

Symptoms of root and stalk rots includestunting, off-colored plants, lodging (Fig. 1),discolored and abbreviated root systems (Fig.2), and internal stalk discoloration (Fig. 3). Dark,internal, stalk tissues are typical for infectionsfrom E. rostratum, D. maydis, and M. phaseolina.Stalk rots caused by Fusarium spp. typicallyhave reddish to pink, inner-stalk tissues.

Stalk rot can be reduced significantly by useof resistant varieties; crop rotation; control ofnematodes, insects, and foliar diseases; properuse of fertilizers; and avoidance of extremes in

soil moisture. Excessive rates of nitrogen andinsufficient rates of potassium are conducivefor development of root and stalk rots. The cropshould be harvested as early as possible.

BACTERIAL STALK ROT

Bacterial stalk rot (BSR) is caused by thebacterium Erwinia carotovora; it has occurred onoccasion on the muck soils in the Belle Gladearea. Warm soils that are excessively wet areconducive for BSR. Symptoms include lodging,shrunken or ropey stems (Fig. 4), watery or hol-low inner stalk tissues (Fig. 5), and foul odors.The main control is to avoid excessively wetsites and to be able to pump water out of theproduction area should excessive rains occur.

COMMON SMUT

Common smut, caused by the fungus Ustilagomaydis, is the only known smut of corn reportedto exist in Florida. Teosinte (an ancestor of corn)is also susceptible. Smut is more likely to oc-cur in field corn than in sweet corn. Some vari-eties of field corn are highly susceptible. Theblack spores (teliospores) produced in smutgalls can survive in the soil and serve as inocu-lum. Infection occurs when the teliospore ger-minates and forms a germ tube-like structure(promycelium) which can penetrate the tissueor by the formation of a similar structure formedby the mating of sporedia (spores formed onpromycelia) of two opposite mating types.Temperatures from 79 to 93° F are conducivefor the infection process. The wounding of corntissue from insects, hail, pesticides, fertilizers,cultivation, blowing sand, or other causes canincrease the severity of smut. Young and mer-istematic tissues are most susceptible to infec-tion.

Galls that are pea-sized to several inchesacross can be formed on any part of the plantabove the soil. Commonly, galls occur on stemsnear nodal tissues (Fig. 6), ears (Fig. 7), and tas-sels. Sometimes, galls form on leaves (Fig. 8),

leaf axils, and brace roots above the ground.Initially a gall is a compact mass of white, fun-gal hyphae that gradually becomes transformedinto a mass of dark teliospores which is envel-oped by a whitish-colored covering (peridium)of hyphae. Smut galls that have not yet formedthe teliospores have been eaten by humans.Recently, a limited fresh market has developedin the United States for these white, immature,smut galls.

Control of smut is best achieved by the useof resistant varieties. Ample resistance is avail-able for corn breeders to incorporate into theirvarieties. Physical damage to corn plantsshould be minimized to the extent possible.

RUSTSThe three rust diseases that have occurred

in corn in Florida are common rust (CR), causedby Puccinia sorghi; southern corn rust (SCR),caused by Puccinia polysora; and tropical cornrust (TCR), caused by Physopella zeae. The formertwo rusts continue to occur but the one timeoccurrence of TCR at Fairchild Gardens wasquickly followed by its eradication in 1974.Teosinte, a relative of corn, is also susceptibleto CR.

Both CR and SCR produce similar symptomswith the formation of spore-bearing, orange-reddish brown pustules (uredia) in leaves orhusks. However, CR typically produces itspustules without the peridium (covering overpustule) persisting (Fig. 9). The pustule of SCRmay also exist without a persistent peridium,but typically, it is persistent (Fig. 10). The colorof the spore mass of CR tends to be chocolatebrown and that of SCR tends to be orange.

The shape of the pustule varies for the tworusts, but CR tends to have elongated pustulesand SCR tends to have somewhat rounded pus-tules. Also, formation of the pustules on thelower surface of the leaf is delayed and com-monly absent with SCR. Identification of whichrust is present can be done quickly with a mi-

croscope. The somewhat round urediosporesof CR are nearly isodiametric whereas those ofSCR are oblong. Another spore stage, theteliospore, occurs in black telia after urediahave formed. Teliospores of CR and SCR canbe distinguished with microscopic examina-tion. The effects from rusts include lodging,reduced ear fill, and lower market grades.

Common rust is a cool weather disease. Insouth Florida, CR occurs routinely in the spring.Urediospores of CR germinate from 39-86° Fwith the optimum for spore germination andinfection being from 59-63° F. After infection,pustules form most rapidly from 59-68° F. Astemperatures deviate from the optimum, thetime for formation of pustules takes longer attemperatures such as 86° F or 50° F.

Oxalis spp. serve as alternate hosts for twoother spore stages (pycniospores and ae-ciospores) of CR. Infection of this alternate hostoccurs when teliospores germinate and formbasidiospores which infect Oxalis spp. Appar-ently, this alternate host is of importance inMexico.

Southern corn rust tends to occur morefrequently during the fall in south Florida andsummer months in north Florida, particularlywith late-planted or double-cropped corn. Theoptimum temperatures for SCR are higher thanfor CR. Near 82° F, germination of urediosporesand penetration of stomates of leaves is abun-dant for the fungus that causes SCR. New rustpustules will appear in nine days at 82° F butrequire about 14 days at 75°F. Development ofpustules ceases at 90°F. Leaf-wetness periodsof eight hours will support infection near opti-mal temperatures. As temperatures deviatefrom the optimum, leaf-wetness periods up to16 hours may be required for infection.

Control of corn rusts is done best with resis-tant varieties. Resistance to CR is available, butresistance to SCR has not generally been incor-porated into varieties. Any available resistance

is likely to be partial and subject to infectionby “new” or different races of the rust fungi.When sequential plantings are used, earlierplantings should be downwind in relation topredominant winds. For example, in southFlorida, east winds predominate during thesweet corn seasons, hence the corn should beplanted from west to east.

Spraying with fungicides is a major measureto control foliar diseases in sweet corn, particu-larly in peninsular Florida. Spray programsshould be initiated at the first sign of rust. Sev-eral sprays may be required. Super-sweet(shrunken-two types) are particularly suscep-tible. Spraying field corn for rust control maynot be economical. However, if a major epi-demic occurs and control is mandated, somefungicides may be labeled for field corn. How-ever, strict adherence to cessation of sprays inrelation to the days-to-harvest must be used toavoid residues in milk and meat products if thecorn is used for animal feed.

FUNGAL LEAF SPOTS AND BLIGHTS

Northern corn leaf blight (NCLB), is causedby the fungus Exerohilum turcicum(Helminthosporium turcicum). NCLB is a problemprimarily in sweet corn in Florida, particularlyin peninsular Florida, but it can be a seriousproblem in field corn if susceptible varieties aregrown.

Symptoms of NCLB vary depending on thegenes for resistance and races of the funguspresent. Individual leaf spots are typically 1 to6 inches long and 1/2 to 1 inch wide (Figs. 11& 12). Resistance is expressed as smaller le-sions, fewer lesions, or delayed appearance oflesions. Usually the lesions are widest in thecenter and tapered or serrated at the ends. Le-sions are typically tan to dark brown in color.When spores are produced in mass, the innerzones, sometimes concentric, have a greenish-black tinge. Sometimes the outer part of the le-

sion, particularly expanding lesions, will havewater-soaked tissue. Lesions tend to form firstin lower leaves and progress to higher leavesover time (Fig. 11). However, mid- or upper-stalk leaves may display lesions first, often in aband of lesions (Fig 12). This latter situation isthe result of spores germinating in the mois-ture within the leaf whorl during a specific pe-riod of time that was favorable for infection orfrom large “spore showers” from nearby fields.When the leaves unroll and expand, a line oflesions is evident.

Sources of spores are from lesions in old cropdebris, volunteer corn, and existing productionfields nearby, particularly if they are upwindfrom your field. Spores of E. turcicum are dis-seminated naturally by wind. However, sporescan be dislodged in mass by mechanical move-ment in nearby fields during cultivation, har-vesting, or other processes.

Temperatures between 59° F and 86°F are suit-able for production of spores and infection pro-vided leaf wetness periods of at least 7 to 8hours occur. With favorable weather, the timefrom infection to occurrence of symptoms canbe as short as 5 to 7 days. As temperatures de-viate from the optimum, fewer lesions form andthe time for lesions to form is increased.

Sorghum, Sudangrass, Johnsongrass,gamagrass, and teosinte are also susceptible toE. turcicum. Pathogenic races that infect sor-ghum and Sudangrass apparently do not infectcorn. However, some races that infect corn caninfect Sudangrass.

Control of NCLB for field corn is accom-plished primarily by the use of resistant variet-ies. When alternate rows of different varietiesare planted to enhance pollination, higher lev-els of NCLB have resulted if one of the variet-ies is susceptible. For sweet corn, resistancemay be available but sprays of fungicides arecommonly necessary in peninsular Florida.Crop rotation should be used and sequential

plantings should be made into the predominantwind patterns for your area as described ear-lier for the control of rusts.

Southern corn leaf blight (SCLB) is causedby the fungus Bipolaris maydis (Helminthosporiummaydis). In 1970, SCLB became epidemic onthose F1 hybrid varieties produced with Texasmale-sterile, cytoplasm (Tcms) primarily in thesoutheastern but also in other corn producingareas in the United States (Fig. 13). In Florida,the average statewide yield of field corn wasreduced by 50%.

Seedling blight can be caused by B. maydis.However, symptoms of SCLB occur typicallyin leaves (blades & sheaths). Mature foliar le-sions can be rounded on the sides but they tendto be parallel-sided, often restricted by theveins. Lesions are light tan in the center with adarker reddish- brown border (Fig. 14). A green-ish growth near the center of the lesion may beevident if abundant spores are present (Fig. 14).Mature lesions range from 1/4 to 1 1/2 inchesin length and may be tapered, flat or somewhatserrated on the ends. Typically, lower leavesare infected first (Fig. 15) with the epidemicprogressing upward to higher leaves over time.Occasionally, infections of the ear husk, silks,kernels, cob, and floral bracts in tassels occur.

Sources of spores for infection include vol-unteer corn (Fig. 16), old corn debris on the soilfrom previous crops, stored corn seed, fodder,and nearby corn plantings. Teosinte and a wildgrass (Rottboellia exaltata) are also susceptible.Other crop species are not susceptible to B.maydis in natural field situations.

With optimal weather conditions, the timefrom infection by germinating spores to lesionformation with new spores may be as short as 3to 5 days. Temperatures near 73° F are optimumfor production of spores, formation of germtubes, infection, and formation of lesions. Astemperatures deviate to 59° F and 86° F, thefungus is still active but progress of the dis-

ease will be delayed. Six hours of leaf wetnessis all that is needed for spore germination andinfection. Leaf wetness is not required for le-sion expansion.

Control of SCLB is best achieved with resis-tant varieties. However, where resistance islacking, spraying with fungicides may be nec-essary, particularly with sweet corn producedin peninsular Florida. Spray programs shouldbegin at the first sign of disease if favorableweather is likely in the future. Also, the use ofcrop rotation and deep plowing of old cropdebris will reduce inoculum for the next crop.

Two other leaf spots caused byHelminthosporium spp. occur occasionally inFlorida but have been minor problems.

Helminthosporium leaf spot (HLS) iscaused by the fungus Bipolaris zeicola(Helminthosporium carbonum). It can cause a leafspot that is intermediate in size to NCLB andSCLB and may be linear or slightly oval (Fig.17). Also, it causes an ear rot that usually ap-pears near the tip of the ear on the kernels. InFlorida, HLS has occurred in inbred lines withinseed production fields. Leaf spots associatedwith Helminthosporium setariae have been foundoccasionally in sweet corn in the Zellwood andEverglades areas.

DOWNY MILDEWS

Two downy mildew fungi occur in corn inFlorida. Sclerophthora macrospora causes crazytop and Peronosclerospora sorghi causes sorghumdowny mildew.

Crazy top (CT) has been diagnosed only inEscambia and Santa Rosa Counties. The fun-gus grows systemically in the plant. It has oc-curred in spring planted corn when the soilbecame saturated or flooded for 1 to 2 days af-ter planting but before the plants had 4 or 5leaves. Stunting, excessive tillering, excessivetwisting, and rolling of leaves, are commonsymptoms of plants prior to flowering. Exces-sive leafyness of ear husks and tassels (Fig. 18)

occur on older plants.

The fungus that causes CT produces restingspores (oospores) that can persist in the soil formany years. With water-saturated soils and cooltemperatures, the oospores germinate and pro-duce sporangia which, in turn, produce motilespores with tails (zoospores) that migrate in freewater. Zoospores produce germ tubes thatpenetrate corn tissue in contact with the moistsoil. Optimum temperatures for formation ofzoospores is between 53 and 61° F but the fun-gus is active beyond these temperatures. Some-times, sporangia produce germ tubes directly.Corn plants are susceptible until the fourth orfifth leaf stage. This fungus has been identifiedin more than 140 species of grasses includingoats, wheat, sorghum, rice, crabgrass, witch-grass, and foxtails.

The best controls for CT are to provide foradequate water drainage in fields and to breaksoil hard pans. Planting in areas that are proneto flooding should be avoided.

Sorghum downy mildew (SDM) was foundin the Zellwood and Leesburg areas in the early1980s in sweet corn, forage grasses in the sor-ghum group used as summer cover crops, andSorghum alum. Teosinte and Panicum spp. arealso susceptible. The fungus grows systemi-cally within the plant if infection occurs beforethe plants are about 4 weeks old. Symptomsinclude stunting, yellowing of leaves, light yel-low-white leaf striping, leafy proliferation oftassels, and the presence of a greyish-bluedowny, mold growth on the leaves (Fig. 19).

The downy growth is composed of sporan-gia (spore-like structures) and sporangiophores(stalks on which sporangia form). Sporangia aretransported by wind or water. Sporangia areformed, germinate, and infect leaf tissue be-tween 63° F and 84° F. Optimum temperaturesare near 70 to 77° F if high humidity or freemoisture is present. Some isolates have consid-erably lower optimal temperatures for germi-

nation (e.g., 59° F). Sporangia germinate byforming germ tubes that penetrate the leavesdirectly or through stomates (breathing pores).The fungus then grows systemically in theplant. After the plant becomes about four weeksold, infection of leaves results in the formationof distinct lesions.

Within the infected leaf, thick-walled, rest-ing spores (oospores) can be formed which al-low the fungus to survive in the soil for longperiods of time. Also, infected corn or sorghumdebris among seed or fodder could allow thefungus to be transported to new locations. Theoospores produce a germ tubes which penetrateroots of seedling or plant tissue in contact withthe soil and the fungus then grows systemically.

Control of SDM includes use of resistant va-rieties, if available; crop rotation, and plowingof land deep enough to place the oospores be-low the level of seed at planting. Susceptiblegrass weeds from the field area should be elimi-nated. Also, the use of select, systemic, fungi-cidal, seed treatments (e.g., those withmefenoxam) has effectively controlled SDM insweet corn in Florida. Cover crops of sorghum-type grasses should be avoided.

BROWN SPOT

(Chocolate Brown Spot or Physoderma BrownSpot)

Brown spot (BS) is caused by the fungusPhysoderma maydis. Teosinte is also susceptible.More than 50 years ago, this disease was re-ported to be a major problem in Florida. Overthe past 30 years, BS has been seen on occasionin field corn but has not been a major problem.Symptoms include clusters (blotches) or bandsof round to oblong yellow spots, each spot usu-ally being less than 1/2 inch across, in leafblades, leaf sheaths, ear husks, or tassels (Fig.20). Symptoms tend to be more abundant nearthe base of the leaf blades or other areas wherefree water accumulates. Later these symptom-

atic spots turn brown to red (Fig. 20) and mayform blister-like pustules. These pustules dis-integrate which then expose a mass of dustybrown, powdery resting spores. Lodging canoccur if severe infections are present.

The resting spores (sporangia) are the pri-mary means by which this fungus survives be-tween crops. Each sporangium can germinateby producing up to 50 spores (zoospores) withtails so they are motile in water. Zoospores ger-minate by producing small threads (hyphae)which penetrate corn tissue, particularly youngtissue. New sporangia can be produced within16 to 20 days after infection. Free water accom-panied by temperatures from 73 to 90° F aremost suitable for germination of spores andinfection. Thus BS is most likely to be a prob-lem in young field corn during warm and wetspring weather.

Control of BS includes crop rotation, tillageprograms that allow for decomposition of corntissue, and resistant varieties, if available.

BACTERIAL LEAF BLIGHT

Bacterial leaf blight (BLB) is caused by thebacterium Pseudomonas avenae (P. alboprecipitans).This disease has been an occasional problemon sweet corn in the central Florida area. Vaseygrass (foliage and seed), Johnsongrass, Setarialutescens (foxtail), Sudangrass, and goosegrassare also susceptible.

Symptoms of BLB include aggregations ofelongated lesions of more than several inchesin leaves (Fig. 21). When moist conditions ex-ist, the lesions will have a greasy, water-soakeddark appearance, particularly on the margins(Fig. 21). Older lesions will tend to be lightbrown or tan in the center and may be shred-ded. Other symptoms include a basal rot of theear, stalk rot, lodging, and a tassel rot.

Infections of leaves often occur when the

bacteria are in the moisture within the leafwhorl. The bacteria enter leaves throughstomates (breathing pores) or wounds in leaves.New lesions appear three to four days after in-fection. As leaves age, they become progres-sively more resistant to infection. Althoughtemperatures between 65 and 70° F are suitablefor development of BLB, temperatures from 85to 95° F are more conducive for developmentof this disease especially during wet weather.

Rain and overhead irrigation can spread thisbacterium. Tractors and other mechanicalmovement within fields also spread this bacte-rium along the rows.

Because this bacterium does not survive wellin soil, it is important to suppress the amountof vasey grass and other susceptible grassesaround plantings of sweet corn. Some varietiesare more resistant than others. Also, the move-ment of equipment should be minimizedwithin fields when plants are wet if BLB ispresent.

BACTERIAL SOFT ROTS

An extensive discussion of this disease is avail-able in Plant Pathology Fact Sheet No. 12.

FUNGAL EAR AND KERNAL ROTS OFFIELD CORN

Numerous fungi can cause ear and kernelrots of field corn. Infection can occur before orafter harvest. Of significant concern are Fusariumspp., Aspergillus spp., and Penicillium spp. be-cause many of these are capable of producingmycotoxins that are toxic to livestock and man.Some of the disorders in animals related tothese toxins are cancer, reduced growth, hem-orrhagic enteritis, abortion, tremors, staggering,convulsions, edema, toxicity to kidneys and liv-ers, vomiting, feed refusal, decreased egg pro-duction, diarrhea, infertility, reduced immune

responses, gastrointestinal inflammation, de-generation of bone marrow, hormonal imbal-ances, and death. Because other factors cancause these symptoms in animals, consultationswith veterinarians are advised.

Mold growth of Fusarium spp. tends to bewhite-pink-red. A brief treatise on Fusarium-infected corn and its relationship to toxic ani-mal feed is presented in Plant Pathology Cir-cular 1025.

Aspergillus flavus and A. parasiticus produceaflatoxins and grow rapidly above 75° F. Thepresence of greenish-yellow molds that fluo-resce when exposed to a “black light” on ker-nels and silks are suggestive (but not conclu-sive evidence) of A. flavus. Many biologicalmaterials fluoresce and not all strains of A. flavusproduce aflatoxins. It is imperative to have feedsamples analyzed in a laboratory for the pres-ence of any kind of mycotoxin. Mold growth ofPenicillium spp. is blue-green-grey.

Ear and kernel rots are often associated withcorn that is or was stressed from drought orpoor production practices or damaged frominsects (weevils, worms, thrips, or others). Somevarieties are more susceptible, particularlythose that do not have an adequate husk coverover the kernels. Harvesting time (22-30% mois-ture content of grain) and equipment (slowercylinder speeds) should be adjusted to mini-mize damage to grain. Corn grain should al-ways be clean and dried to moisture contentbelow 14% as soon as possible before storage.Storage facilities should be cleaned before use.Fans should be used routinely during dryweather to displace moisture that builds up onthe storage structure or the grain. Sources ofmoisture during storage are condensation(warm grain with cool ambient temperatures),grain, and storage molds. Moldy grains gener-ate more moisture than healthy grain. ConsultAgricultural Engineering Circular 1104 for de-tails on maintaining storage facilities. Certainchemical treatments (e.g., ammoniation, propi-

onic acid) are labeled to prolong storage by re-ducing damage from fungi. Some of them arecorrosive to equipment, explosive, and harm-ful to humans. Some modern conveying equip-ment is coated with protective materials. Sup-pression of ear- and kernel-rotting fungi by non-chemical methods is best. Chemically pre-served grain is not accepted in the grain trade.

DISEASES CAUSED BY VIRUSES ANDMYCOPLASMA

Viral diseases of corn have been erraticallyoccurring problems in Florida. Maize dwarfmosaic virus (MDMV) has occurred in sweetand field corn in Alachua County near densestands of Johnsongrass. Aphids transmit thesubmicroscopic viral particles from infectedcorn or Johnsongrass on their mouth parts whenfeeding. Symptoms from nutrient imbalancescan appear similar to those caused by virusesand mycoplasma. Symptoms of MDMV in corn

include stunting and somewhat linear, diffuseshades of greens, yellows, reds, and purples(Fig. 22). The elimination of Johnsongrass in andaround corn fields is an effective control forMDMV.

In south Florida, particularly Dade County,several viral diseases have occurred in seedproduction fields for field corn. Maize stripevirus (MSV), maize rayado fino virus (MRFV),and maize mosaic virus (MMV) have been iden-tified. In addition, maize bushy stunt myco-plasma and corn stunt spiroplasma (CSS) havebeen identified. Symptoms of these diseases canbe similar to those of MDMV and display chlo-rotic spots or stripes (Fig. 23). Stunting and leaftwisting may be present (Fig. 23). While MSVand MMV are spread by planthoppers, MRFVand CSS are spread by leafhoppers. Resistantinbreds and varieties should be used. Also,weed grasses (e.g., itch grass) in the vicinityshould be minimized as they may serve as hostsfor these organisms. Little is known about vi-rus-corn interactions in Florida at this time.

Figure 1. Most severe lodging where cornfollowed corn vs. corn followed by soy-beans.

Figure 2. Most severe root rot where cornfollowed corn.

Figure 3. Internal stalk discoloration fromfungal stalk rot.

Figure 4. Bacterial stalk rot.

Figure 5. Bacterial stalk rot in corn. Figure 6. Corn smut on stalk.

Figure 7. Corn smut on ears. Figure 8. Corn smut on leaf.

Figure 9. Common corn rust. Figure 10. Southern corn rust.

Figure 11. Northern corn leaf blight. Figure 12. Northern corn leaf blight inbands.

Figure 13. Susceptible vs resistant varieties tosouthern corn leaf blight.

Figure 14. Southern corn leaf blight.

Figure 15. Southern corn leaf blight. Figure 16. Southern corn leaf blight onvolunteer corn.

Figure 17. Leaf spot caused by Bipolariszeicola.

Figure 18. Crazy top (downy mildew) incorn.

Figure 19. Sorghum downy mildew in corn. Figure 20. Brown spot in corn

Figure 21. Bacterial leaf blight in corn. Figure 22. Maize dwarf mosaic virus infec-tions in corn.

Figure 23. Complex of mixed viral infections in corn.