Plant Pathology Fact Sheet PP-38

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

Diseases of Small Grains in North andCentral FloridaTom Kucharek, Professor and Extension Plant Pathologist, Department of PlantPathology, University of Florida, Gainesville, F L 32611. 1988; Revised 2001.

Diseases of small grains (wheat, oats,rye, triticale, barley) are some of the most im-portant plant diseases on a worldwide basis.Because of small profit margins for these crops,the primary control techniques have been re-sistant varieties, basic tillage techniques, andrelatively inexpensive seed treatments. In re-cent years, emphasis on high yields coupledwith new chemical control strategies offer po-tential for less traditional approaches. Effectiveuse of the new wave of plant disease controltechniques requires accurate diagnosis of thecausal agent (fungus, virus, bacterium, non-parasite) of disease. Fungal and viral diseaseshave been the primary limiting factors for yieldand quality of small grains grown in Florida.

DISEASES CAUSED BY VIRUSES

Soilborne wheat mosaic virus (SBWMV)was first found in Florida in 1970 in EscambiaCounty. Since then, it has been found in SantaRosa, Okaloosa and Madison Counties. Al-though the virus can be spread mechanically(by hand), natural spread occurs by moving soilthat contains the soilborne fungus vector,Polymyxa graminis. Wheat that is infected by thisfungus but not the virus sustains no knowndamage. Because the virus is associated with asoilborne organism, the initial distribution ofthe virus in a field is usually patchy or in streaksdue to soil tillage operations (Fig. 1). SBWMVis sometimes found where the tillage equip-

ment is first used in the field. In 2000, SBWMVor a closely related virus was found in rye inan experimental rye planting in Gadsen County.

Wheat roots are infected by the zoospore(swimming) stage of the fungus. After enteringthe root, the fungus proceeds with a complexlife cycle, simultaneously releasing the virusinto the root. Virus multiplication and spreadin the plant follows. Later, the fungus formsthick-walled spores for survival in soil. Thesespores are capable of carrying the virus duringthe survival period. The fungus will becomeactive again during cool, wet weather. Trans-mission of the virus through seed has not beendemonstrated.

Patches of infected plants show variousdegrees of symptom severity depending ontime of infection, temperature, and the amountof virus within a plant (Fig. 1). Symptoms in-clude stunting of plants, reduced tillering, de-layed heading, shriveled seed, seed abortion,and vein-restricted, yellow to brown to whitestreaks or mosaic patterns on leaf blades andsheaths (Figs. 1 & 2). Some varieties may dis-play oval-like leaf scalding symptoms. Also,root systems are reduced. In Florida, leaf symp-toms are most vivid during late February toearly April. Rosetting (severe stunting withabnormally high amount of leaf area to stemlengths) prior to stem jointing may occur insome susceptible varieties.

The most effective control is the use ofresistant varieties. Equipment (tractors, imple-ments, soil tube samplers, etc.) should bewashed to remove clinging soil after its use inan infested field. Crop rotation with oats willoffset damage because wheat, rye, barley andtriticale are hosts of this virus. The fungus vec-tor with the virus can live in the soil for years.

Oat Soilborne Mosaic Virus (OSBMV)is thought to occur in Florida but such has notbeen ascertained. Symptoms, affects on yield,and control measures would be expected to besimilar to that of WSBMV.

Barley Yellow Dwarf Virus (BYDV) oc-curs in Florida and has been severe in someyears. This virus is spread by many species ofaphids and is capable of causing disease inwheat, barley, oats, and many grasses. Aphidsacquire BYDV after feeding on infected plantsfor 30 minutes to 30 hours. One to four dayslater, they can transmit BYDV and some aphidsmay be able to transmit the virus for the rest oftheir lives. As the acquisition feeding time isincreased the latent period of one to four daysmay be reduced. BYDV is not passed to the eggor nymph stages of the aphid. BYDV is notseedborne.

Symptoms of BYDV vary from none tobrilliant leaf yellowing in wheat and barley orred to purple coloring of oat leaves (Fig. 3). Suchoff-colors tend to be associated with leaf mar-gins at first, but later envelops the entire leaves.Leaves tend to curl or twist. Infected plants havereduced root systems, stiffer leaves, and possi-bly, darkened phloem within infected tissue.

Control BYDV with resistant varieties.In addition, control of aphids with insecticideshas reduced this viral disease.

Wheat Spindle Streak Virus (WSSV)has been identified in Georgia and Alabama butnot yet in Florida. It is spread by the same fun-gus vector as that for SBWMV but the two vi-

ruses are distinct. WSSV infects both wheat andrye. WSSV has been reported to occur in patchyareas within a field, often in low wet areas, orthroughout the entire field. Symptoms of WSSVare similar to that of SBWMV except that thelinear chlorotic streaks between leaf virus tendto be tapered at their ends. Control for this vi-rus would be similar to that for SBWMV.

DISEASES CAUSED BY FUNGI

Glume Blotch (GB) is caused byStagnospora nodorum (Septoria nodorum). This fun-gus can live in old wheat debris for at least twoyears. Also, the fungus can be transmitted byinfected seed. Survival of the fungus for sevenyears in seed has been reported. Weed hostsare not known to be sources of inocula (spores).Wheat, barley, rye, and blue grass are suscep-tible, but wheat is the principle host and thedisease is most severe during spring monthsin Florida. Small amounts of GB occur duringthe fall and winter. GB reduces yield by reduc-ing seed number, seed size (grade), and tillers.

Stagnospora nodorum is capable of pro-ducing ascospores that result from sexual mat-ing, and pycnidospores that do not requiremating for formation. Pycnidospores are the thespores present in Florida. These spores areformed in flask-shaped structures (pycnidia)embedded within infected tissue (Fig. 4), andare formed in leaves 10-16 days after infection.Slightly longer intervals occur in stem tissues.Spores are splashed by rain or irrigation waterfrom nearby pycnidia located on old crop refuseor growing plants. Penetration occurs throughthe cuticle of leaves by germ tubes growingfrom the spores. At least six hours of moistureon the leaf is essential for the penetration pro-cess but is not required for growth of the fun-gus within the leaf after penetration. Survivalof spores outside of the pycnidia can occur butreports are varied as to how long.

Three to seven days after penetration,light-colored, chlorotic (yellow) to necrotic

(brown) blotches appear on leaves. From 10-16days after penetration, the tiny, black, dot-likepycnidia mature and form spores internally.Adding a drop of water to dried lesions orblotches will swell pycnidia and allow thespores to ooze out in a tendril. This life cycle isshortest when temperatures are between 68° to81°F. This fungus is capable of growth between40 and 90° F. Mature lesions on leaves are foot-ball shaped, tan-colored in the center (Fig. 5),and vary in size. The leaf spot may be sur-rounded by a yellow halo. On glumes, awns,and stem parts, blotches rather than distinctspots are formed. On glumes, these blotchestend to be limited to the upper half of the glume(Fig. 4). Leaf spots of GB may appear similar tothose caused by Helminthosporium sativum(Bipolaris sorokiniana) and sometimes one spotmay have both organisms.

Control of GB is best achieved by twoyears of crop rotation plus a seed treatment or,better yet, use of seed not infected with this fun-gus. In many growers’ situations, however, thisideal combination can not be achieved. There-fore, the use of a fungicide spray program asoutlined in Plant Protection Pointer 27 offers amethod to control GB as well asHelminthosporium leaf spot and leaf rust. Numer-ous cultural techniques (fertility, plantingdepth, row and plant spacing, and irrigationfrequency) have been tested for their affects onseverity of GB, but no clear results were found.Later planting dates in the fall would be ex-pected to reduce the amount of spores avail-able for spring months when GB epidemics arenormally seen in Florida.

Helminthosporium Leaf Spot or SpotBlotch (HLS) is caused by Helminthosporiumsativum (Bipolaris sorokiniana). This disease hasbeen more of a problem on wheat in the rye-growing areas of Florida, as HLS is commonon rye and triticale. This fungus has a wide hostrange on many grass species, but does not in-fect corn, sorghum, or oats. This fungus sur-vives on crop debris and is carried in or on seed.

All plant parts are susceptible; that is, this fun-gus causes spots or blotches on abovegroundparts as well as stem and root rots and seed-ling blight.

Two kinds of spores are formed by H.sativum: ascospores from a mating process andconidia without a mating process. Conidia arethe primary inocula of HLS in Florida. Conidiaform on the surfaces of lesions and thus aresuitable for spread by wind. With the aid of ahand lens the mass of spores appears as a min-iature forest. If they land upon susceptible tis-sue that is wet with dew or rain water, they ger-minate, form germ tubes, and with special struc-tures, the host tissue is penetrated. The life cyclevaries from three to 20 days, depending upontemperature. Growth of the fungus occurs be-tween 33 and 99° F, but temperatures from 71to 85° F are optimal for infection and symptomdevelopment.

HLS may appear as dark, pin-point spotsor may be up to one inch in length (Fig. 6). Spotsare somewhat linear, paralleling the leaf veins,and are dark green, black, or brown. A yellowhalo may or may not surround the dark lesion.Glume blotch lesions are lighter in color thanHLS. Where flower parts are infected, the headsappear dirty green to black (Fig. 7) in contrastto dirty brown heads affected by glume blotch.

Control of HLS is similar to that of glumeblotch. In addition, wheat plantings should beseparated from rye plantings to the extent pos-sible. Differences in susceptibility between cropvarieties exist, but resistant varieties to HLS arenot available.

Leaf Blotch (LB) of oats is caused byHelminthosporium avenae (Drechslera avenacea)andother plant species are not known to be sus-ceptible. The life cycle of this fungus and con-trol of this disease are similar to those forHelminthosporium leaf spot of wheat.

LB of oats is primarily a disease of the

leaves and panicles in Florida, although a blackstem phase has occurred. Leaf symptoms in-clude linear, reddish brown to black spots, usu-ally 1/2 to one inch long, which are water-soaked (greasy) when small (Fig. 8). Spots arenarrow, being somewhat limited by leaf veins,but they can be blotchy. With a hand lens, darkfuzzy spore masses may be seen near the cen-ter of the spots. Black stem symptoms of thisdisease display pinkish-brown blotches on theleaf sheaths or large black areas on the stem,the latter usually being associated with nodaltissue. A white mold growth occurs in the stemcavity near the blackened tissues. Stem break-age may occur. Control of LB is done with re-sistant varieties (if available), seed treatment,and crop rotation.

Powdery Mildew (PM) is caused byErysiphe graminis (Blumeria graminis). In Florida,the only strain of PM of consequence on smallgrains is the one in wheat, usually during thespring months. On some varieties, PM can besevere, covering the entire leaves with a white,powdery mold growth (Fig. 9). PM can causeyield loss on susceptible varieties that main-tain high disease severities from prior to joint-ing until flag leaves (uppermost leaf on a ma-ture plant) form. Excess nitrogen predisposesplants to PM.

The fungus causing PM produces twospore types: one is produced after a matingprocess (ascospore) and the other is producedin large amounts without a mating process(conidia). Conidia are formed among the whitepowdery growths on leaves and appear to bethe main source of inocula for PM in Florida.Upon germination, a conidium produces agerm tube from which an infection peg formsand penetrates directly through the leaf cuticle.Conidia of PM are unique as they do not re-quire free moisture for germination, but ger-mination is most rapid when free moisture ispresent. Conidia germinate from 34 to 86° F,but disease development is most rapid from59 to 72° F. Above 77° F, disease progression is

slowed; this is why PM is not as severe on up-per leaves of fully jointed wheat in Florida. PMconsists of numerous races which differ in abil-ity to cause disease in different varieties. ThePM fungus survives from one season to anotheron volunteer wheat plants, in susceptible wildgrasses, as a mycelial mat (dried up white fun-gus growth) and possibly by the ascosporewithin a cleistothecium ( a fungus encapsula-tion).

Control is best achieved with varietiesresistant to the current prevalent races. Somenew fungicides may be available for control ofPM, but discuss their use with your countyagent first. Use a balanced soil fertility; do notuse excess amounts of nitrogen.

Leaf Rust (LR) of wheat is caused byPuccinia recondita f. sp. tritici, and is a problemon susceptible varieties or previously resistantvarieties infected by new or different races ofthis fungus. In Florida, LR occurs from late win-ter through spring. Occasionally, rye or barleymay be infected by this fungus. This fungusproduces three kinds of spores in wheat,urediospores, teliospores, and sporidia. Theurediospore, contained in orange to brownuredia (Fig. 10), is thought to be the only sporeof consequence for spread of LR in Florida.Uredial pustules can form on all parts of theplant above ground level. They first appear onlower leaves and with time the disease spreadsto upper plant parts. When a urediospore ger-minates, it produces a germ tube from whichstructures are produced that penetrate thebreathing pores (stomata) on plant tissue. Aminimum of four hours of leaf wetness isneeded for penetration to occur but longer wet-ting periods will result in more penetrations.Within 7 to 14 days, or more if temperatures orvarieties are not ideal for the fungus, new pus-tules are formed in areas that began as yellowto tan flecks on susceptible tissue. If the tissueis less susceptible, no pustule is formed (highlyresistant reaction) or the original fleck will bepartially covered by a uredium (intermediate

responses). The life cycle is the shortest whentemperatures are from 59 to 81° F.

Telia are black and contain teliospores.Typically, telia form on older infected leaves.When teliospores germinate, they produce spo-ridia that infect the alternate hosts (meadow rueand others). On the alternate host, pycnia andaecia are formed, and each containing their re-spective spore types. However, in Florida, di-rect pertinence of spore stages other than theurediospores is probably negligible. The sig-nificance of the alternative host relates to thepotential for new races to form and aeciosporesare infectious to wheat.

Control of LR is by use of resistant vari-eties or with foliar sprays during springmonths; see Plant Protection Pointer No. 27.

Stem Rust (SR) is caused by Pucciniagraminis. Nearly all small grains are susceptibleto different “form-species” of this fungus. Theform-species tritici infects wheat; the form-spe-cies avenae infects oats; etc. Although SR can bedevastating, the SR epidemic in 1974 in wheatis the only known case in Florida and this wasrelated to the widespread planting of a highlysusceptible variety. Normally, SR is seen intrace amounts each year in Florida.

The SR fungus has a life cycle and tem-perature responses similar to leaf rust of wheat,but the alternate hosts (barberry, and others) aredifferent. The length of the life cycle on wheathas been as short as five days and as long as 85days, depending on temperature and variety-race combination. A seven to 15 day life cyclewould be typical.

SR can appear similar to leaf rust onleaves of wheat, but often the uredial pustulesare red to brown, rather than orange as with leafrust. Should SR be seen on stems, including leafsheaths, long red to brown uredia will be evi-dent (Fig. 11). With a microscope, the two rustscan be distinguished by urediospore shape;

spores of SR are oblong whereas those of leafrust appear round or block-shaped.

Control SR with resistant varieties. Fo-liar sprays may be useful, but this dependsupon the type of fungicide used.

Leaf Rust and Stem Rust of Rye can oc-cur but only the former has been seen on occa-sion in Florida. These rusts have similar symp-toms to their respective rusts on wheat. The al-ternate hosts for SR on rye is the same as forwheat, but the alternate host for LR on rye isdifferent (Anchusa spp.).

Crown Rust (CR) of oats is caused byPuccinia coronata. It has a life cycle similar tothat of leaf rust of wheat, except its alternatehost is Rhamnus spp. When free moisture ispresent, urediospores will germinate from nearfreezing to 86° F. Temperatures near 77° Fshorten the life cycle to five days, dependingupon the variety. A nine to 15 day life cycle istypical during the spring months in Florida.Penetration of leaf tissue by infection structuresmay occur in three hours near 70° F or take 12hours at 86° F or 24 hours at 41° F .

Symptoms and control for CR are simi-lar to those for leaf rust of wheat, except a foliarspray program is usually not advised.

Loose Smuts (LS) of wheat, oats, andbarley have occurred in Florida but none ofthese fungal diseases has been severe excepton occasions in some fields. LS is caused byUstilago tritici, U. avenae, U. nuda on wheat, oats,and barley, respectively. LS on wheat and bar-ley are seedborne within normal-appearingseed. Upon seed germination, the fungus growssystemically within the plant and after flower-ing, black spore masses are present within flo-rets instead of seed (Fig. 12). These black spores,called teliospores, are spread by wind to otherflowers within the crop where they germinateand infect the female portion of the flower. Theseed then forms normally.

With LS on oats, the life cycle is the sameon occasion, but most infections occur in thefield during the early seedling stage. The blackteliospores, which are carried on seed, germi-nate to form sporidia that in turn germinate. Ifa fusion occurs between the germ tubes of cer-tain sporidia, the fused-germ tube is capableof penetrating the seedling. Infection is favoredby somewhat dry conditions and is best at 64to 72° F.

Control of LS is done with resistant vari-eties and seed treatments that are highly selec-tive for control of smuts. Also, seed sourcesshould never be from fields that had smut.

Anthracnose of rye, caused byColletotrichum graminicola, appears during warm(77° F) and wet periods in the spring. Prema-ture ripening and bleaching of the lower stemoccur. Later, the lower stem turns purple tobrown where dark fungal structures with spinesmay be seen with a hand lens. Root rot, culmrot, lodging, and flower sterility are furthersymptoms. This disease is most apt to occur infields with acidic soil or low in phosphorus.Broom sedge fields are indicators of such con-ditions. Crop rotation with non grass crops issuggested.

Pythium Root Rot (PRR) of small grainsis caused by several species of Pythium whichare common in soils in Florida. These fungi pro-duce swimming spores which germinate inwater. From the germ tube, structures are pro-duced that are capable of infecting root tissue.Also, several types of resting spore types mayform that allow these fungi to survive harshconditions in the soil or in old plant debris.

Symptoms of PRR include lower leafyellowing (similar to nitrogen deficiency).Eventually, all leaves on a plant may be yel-low followed by stunting, leaf browning, anddeath of the plant (Fig. 13). Although entireplantings may display these symptoms, thisdisease occurs commonly in patches or linearstreaks in the field (Fig. 14). These fungi mayalso cause a pre- or post-emergent seedlingblight. After jointing occurs, PRR occurs but isless damaging than early season infections.

Control of PRR on small grains is withseed treatment, avoidance of planting prior tothe middle of November, and avoidance ofdeep planting. After stand establishment, soilmoisture, nitrogen levels, and overall fertilitylevels should be adequate to promote a consis-tently growing plant. Plants are more apt to beinfected when the root system is not expand-ing. Where the crop is to be used for grazingand planted early or in warm soils, the use ofan effective seed treatment is highly important.

Figure 1. Stunting of wheat caused by soil-borne wheat mosaic virus.

Figure 2. Mosaic symptoms in wheat causedby soilborne wheat mosaic virus.

Figure 3. Leaf symptoms in oats caused bybarley yellow dwarf virus.

Figure 4. Lesions of glume blotch in a wheatspike.

Figure 5. Leaf lesions of glume blotch inwheat.

Figure 6. Leaf lesions in wheat caused byHelminthosporium sativum.

Figure 7. Blotches on wheat spikes caused byHelminthosporium sativum.

Figure 8. Leaf lesions and effects of leafblotch in oats.

Figure 9. Powdery mildew of wheat. Figure 10. Pustules of wheat leaf rust.

Figure 11. Pustules of stem rust in stems andleaves of wheat.

Figure 12. Loose smut of wheat.

Figure 13. Pythium root rot in young rye plants.

Figure 14. Pythium root rot in wheat.