9. Disease Management in Wheat
By Roberta Armenta
Great Plains wheat production can be affected by a number of diseases caused by viruses, fungi, and bacteria. While many of these were once of great concern to Great Plains producers, varietal resistance, cultural practices and effective pesticides have decreased the severity and incidence of disease. Those still of econom- ic concern across the Great Plains include take-all, barley yellow dwarf virus, wheat streak mosaic virus, High Plains virus, Triticum mosaic virus, wheat soilborne mosaic virus, root rots, bunts and smuts, and rust diseases.
Bunts and Smuts
Bunts and smuts are a group of similar fungal diseases that attack developing ker- nels, replacing them with spore masses. In addition to reducing grain yield and qual- ity, spore clouds are flammable and can be a hazard during threshing. In the Great Plains region, those of economic importance include common bunt or stinking smut, dwarf bunt, and loose smut.
Common Bunt / Stinking Smut (Tilletia foetida & T. caries)
Disease Cycle. Common bunt and stinking smut are most commonly seed-borne diseases, but they can be soilborne and wind-borne as well. Seeds become contaminated during harvest, when smut spores from diseased plants stick to healthy kernels. Smut spores can survive in the soil for at least ten years, and seeds and young seedlings also may become infected when they are sown near these spores (soilborne). Smut spores and wheat seeds germinate at the same time, allowing the fungus to penetrate the seed before seedling emergence. The fungus continues to grow in its host until it has invaded the head and developing ovaries. The healthy plant tissue is replaced by the fungus, and the kernels are converted into spores by the time the plant reaches maturity. Hosts include wheat, rye, triticale, barley, and grassy weeds. These diseases are favored by cool, moist soil conditions (40-60°F or 4-16°C), so they are more prevalent and severe in fall sown wheat than in spring-sown wheat.
Figure 9.1 (a) Winter wheat head infected with common bunt.
Figure 9.1 (b) Wheat plants infected with common bunt exhibiting stunting and extended greening.
Symptoms. The symptoms of common bunt and stinking smut generally are not apparent until heading. Diseased plants may be stunted, and bunted heads may be a darker color of green and may stay green longer than normal plants (Figure 9.1 a & b). Heads of infected plants may also be smaller and their numbers reduced. After heading, diseased heads can appear slightly open due to the expansion of infected kernels causing the glumes to spread apart. In infected heads, diseased kernels, or bunt balls (Figure 9.2), are dull, grayish-brown in color and are filled with dark brown spore masses. The heads rupture at harvest and release spores that have a fishy odor due to the chemical trimethylamine.
Management. Stinking smut and common bunt are controlled through seed certification and seed treatment.
Figure 9.2 Winter wheat spike, with glumes removed to reveal smut balls.
Loose Smut (Ustilago tritici)
Disease Cycle. Loose smut is a seed and wind-borne fungal disease. The pathogen survives in the wheat seed until germination and then grows up the shoot and infects the head. Healthy wheat plants can be infected during the first two days of flowering by wind-borne spores from infected plants. Rain and insects can also help spread the fungus. Humid weather, including light rain and heavy dew, and cool to moderate temperatures, between 60 and 71°F (16-22°C), promote infection. When spores land on healthy flowers, they germinate and become dormant within the ovary until seeds germinate. Yield loss is in direct proportion to the number of smutted heads present.
Symptoms. Disease symptoms usually are not apparent until heading. However, diseased heads tend to emerge earlier than normal plants from the boot stage (approximately 1-3 days). Brown to black fungal spore masses develop in the diseased heads of the plants. The membrane ruptures during flowering and smut spores are dispersed leaving only the dark, bare rachis (Figure 9.3).
Management. It is not possible to visibly tell the difference between infected seed and healthy seed. Plant high quality, certified seed treated with an effective fungicide.
Common Root and Foot Rot
Dryland Foot Rot (Fusarium culmorum and F. graminearum)
Also known as Fusarium foot rot, dryland foot rot is a soilborne disease that can survive and multiply on crop residues. The occurrence of dryland foot rot has increased with reduced tillage practices. Hosts of dryland foot rot include many cereals, especially barley, and grasses. The primary causes of this disease are the fungi Fusarium culmorum and F. graminearum. F. culmorum is important for the northern Great Plains and interior northwest,while F. graminearum is more important in the southern Great Plains.
Disease Cycle. Dryland foot rot infects the roots and crowns of wheat plants and is more prevalent in loose, dry soil. Areas with low annual precipitation (below 16-18 inches) are susceptible to the disease. Stress, including drought, can increase the damage. Associated with areas of high fall soil temperatures and low fall soil moisture, dryland foot root is most common in dryland winter wheat and no-till spring cereals. Spring wheat usually is not affected. Stressed or droughty areas, such as hilltops, sandy areas, slopes, and ridges, tend to experience the most severe damage.
Symptoms. In the late fall and early spring, discolored root and crown tissue, appearing brown to reddish-brown and rotted, is the most apparent sign. The stem may also be brown to reddish-brown several nodes up the plant (about 4 to 5 inches) (Figure 9.4). During the final stages of development water stress due to root damage causes the plant to ripen prematurely resulting in white heads (Figure 9.5). Heads may either be void of kernels or contain shriveled kernels.
Figure 9.3 Wheat infected with loose smut exhibits typical symptoms- after flowering the spores are dispersed and only a bare rachis remains.
Figure 9.4 The browning of wheat stems is a common symptom of dryland foot rot.
Figure 9.5 Infected plants produce white heads.
Management. Control methods include crop rotations, seed treatments, and water conservation practices. Crop rotation with broadleaf crops, corn, millet, or oats can decrease the incidence of dryland root rot. At least two years between cereal crops is recommended to reduce the risk of infection. Avoid plant stress by implementing water conservation practices and applying fertilizers effectively. Chisel plowing can improve infiltration and decrease runoff. In addition, early seeding can result in bigger, more water-stressed plants, so plant seeds when soil is below 60°F (16°C) at seed depth. Plant seeds at shallower depths in warmer soil.
Common Root Rot (Cochliobolus sativus)
The common root rot fungus survives as spores in crop residue, but unlike many other root rots, it can also survive several years in the soil. Because of this and because the fungus infects many grasses, it is not practical to rid a field entirely of common root rot.
Disease Cycle. Common root rot is most common between September and June, during moist, warm weather. Root and crown tissues are infected, and flower parts also may be infected if spores are splashed onto spikes that will remain wet for several days. Water stress after infection can worsen damage.
Symptoms. Common root rot mainly affects the roots and crown. Roots may be poorly developed and spotted with brown to black lesions. The crown is affected later in the season with similar symptoms-poor development and areas ofbrown discoloration. The infection is especially noticeable on the subcrown internode and coleoptiles where symptoms mirror those of the roots and crown. Diseased spikes turn white before healthy plants mature. Plants may be stunted and produce fewer tillers, and infected heads turn white and contain shriveled kernels.
Management. Control common root rot with good cultural practices, good weed control, crop rotation, and seed treatments. Plant high quality wheat seed late in the fall into firm seedbeds, since loose seedbeds and warm soil conditions promote disease. Do not over fertilize, especially with nitrogen.
Take-all (Gaeumannomyces graminis var. tritici)
Take-all got its name over 100 years ago in Australia when a severe seedling blight emerged killing entire fields, destroying entire stands of wheat, and "taking-all" seedlings it infected. The disease affects the root, crown, and stem base of wheat and interrupts plant development. Cool, damp conditions and alkaline soils promote infection, and irrigation increases damage.
Disease Cycle. Take-all is caused by a soilborne fungus that survives year-to-year in wheat residues and on volunteer wheat and grassy weeds such as bromegrass, quack grass, and bent grass. Wheat becomes infected when plant roots come in contact with infested residues or infected plants. The fungus moves to its new host via the growth of runner hyphae through the soil. Spores are produced, but are not important in spreading the disease.
Soil conditions affect the severity of the disease. Sandy, light, poorly drained soils promote take-all severity as do soils with low fertility and a high pH and heavy, poorly drained soils. Wet weather, particularly in the second half of the growing season, promotes take-all fungal growth. Increased damage occurs when soil temperatures are between 54 and 68°F (12-20°C). Usually, damage is worse the earlier plants are infected.
Nutritional stress also plays a part in determining the severity of take-all in wheat plants. Take-all incidence is decreased with adequate soil fertility, particularly with nitrogen. Spring nitrogen application in a deficient wheat crops can reduce take-all development.
Wheat plants can endure mild to moderate infection with no apparent symptoms and minimal yield loss. However, when weather and soil conditions favor the disease, symptoms may be severe and yield losses as high as 50 percent may occur.
Symptoms. Symptoms of take-all are most noticeable near heading and include plant stunting and early maturation. Circular patches of stunted, yellow plants may appear during the early growth stages, commonly occurring in wetter areas of the field. Infected plants tend to be yellow in color and produce fewer tillers (Figure 9.6). Because plants are killed prematurely, bleached and sterile heads are produced (“white heads”). The white heads may be void of grain or produce only a few shriveled kernels. Wet weather promotes fungal growth that blackens the dead, white heads.
Figure 9.6 Wheat plants infected with take-all root disease tend to be yellow in color and infection usually occurs in circular patches.
Figure 9.7 Root rot is common in plants affected by take-all; Roots may appear blackened and brittle compare to healthy roots.
Root rot is another take-all symptom, resulting in blackened, and brittle roots (Figure 9.7). Diseased plants can be easily pulled out of the ground or may break
off near the soil line. Under prolonged wet soil conditions, take-all extends into the crown and stem base. An infected stem will be covered with black, shiny fungal growth.
Management. Rotation with crops not affected by take-all, e.g., corn or sunflower, is an effective management strategy. Eliminate volunteer wheat and grassy weeds, such as downy brome, for these may serve as take-all hosts and allow the fungus to persist from year to year. If tillage is used, till as late in the year as possible. Early planting promotes take-all, so plant wheat after the Hessian fly-safe date for your area.
Barley Yellow Dwarf Virus (BYDV)
Disease Cycle. Hosts of barley yellow dwarf virus, a Luteovirus, include wheat, barley; oats, triticale, and over 150 grassspecies. Aphids feed on infected plants and transmit barley yellow dwarf to healthy plants in subsequent feedings. It is vectored by all the cereal aphids except Russian wheat aphid (Figures 9.8 & 9.11) (Reference Chapter 7-"Arthropod Pests of Wheat"). Each aphid species transmitsspecific virusstrains, but some strains can be vectored by multiple aphid species. There are five strains of the disease common to the United States including MAV, PAV, SGV, RMV, and cereal yellow dwarf. BYDV cannot be transmitted through seed or soil.
The barley yellow dwarf virus survives year to year in wild grass hosts and volunteer small grains or is introduced to the field by virus-carrying aphids. Winged aphid migrations can be either localized or occur over several miles with favorable winds. BYDV is often associated with environmental conditions that favor the buildup of aphid populations including wet, cool summers, warm falls, and mild winter conditions. Irrigated areas also support aphid populations.
Severity of disease depends on many factors including efficiency with which the aphids transmit the virus, the source and strain of the virus, aphid mobility and feeding habits, environmental conditions, and the age and susceptibility of wheat plants when infected. There is increased injury in early-seeded winter wheat, and fall infections tend to be more damaging than those in the spring and leave plants more vulnerable to winterkilling. However, even though plants may become infected in the fall, symptoms may not be apparent until the spring. Losses attributed to barley yellow dwarf are usually between 5 and 25 percent.
Figure 9.8 Adult bird cherry-oat aphids.
Figure 9.9 The corn leaf aphid in both the adult and nymph stages.
Figure 9.10 Adult greenbug.
Figure 9.11 Adult English grain aphid and nymphs.
Figure 9.12 Winter wheat exhibiting barley yellow dwarf virus symptoms, including stunting and leaf discoloration.
Figure 9.13 Infection is apparent due to the yellow and purple discoloration of wheat leaves.
Symptoms. Symptoms of BYDV become obvious by jointing and include plant stunting and slight to severe leaf discoloration (Figure 9.12). However, BYDV symptoms can closely resemble those of environmental stress, nutritional stress, wheat streak mosaic virus, and crown and root diseases, so it is important to have samples tested in a lab to confirm disease. Plant leaves begin to yellow and sometimes turn red or purple beginning at the leaf tips or margins, and discoloration progresses towards the base (Figure 9.13). Serration may also occur at the leaf margin, and diseased leaves can havea more erect appearance compared to healthy leaves (Figure 9.13). Infection occurring at early growth stages results in increased injury. Severely infected plants exhibit stunting, poorly developed tillers, reddening of flag leafs, delayed maturity resulting in poorly developed heads, shriveled grains, and reduced yields. Infection often occurs in small patches due to localized aphid feeding, and these patches tend to occur in a row due to aphid feeding along rows of plants or may also be associated with field margins. Symptoms become apparent two weeks after a plant becomes infected at 68°F (20°C) and four weeks at 77°F (25°C); bright, sunny weather favors symptom expression. If temperatures exceed 86°F (30°C) the virus will be suppressed, and symptom development will cease.
Management. See Chapter 7-“Arthropod Pests of Wheat” for details on managing aphids in wheat.
Wheat Streak Mosaic Virus (WSMV), High Plains Virus (HPV), and Triticum Mosaic Virus
Wheat streak mosaic virus, High Plains virus, and Triticum mosaic virus are very similar wheat diseases, with the same vector, wheat curl mite, Aoeria tosidlella Keifer. They are common in the Great Plains and have similar disease cycles, symptoms, and management approaches. In addition. the three diseases often occur in conjunction with one another, making it hard to distinguish which disease is the cause of infection without sending samples to a virus lab.
The virus complex can cause serious losses in wheat, especially in fields planted next to or near volunteer wheat. HPV was identified in the Great Plains in 1993. It is now reported to be widespread in this area from Nebraska and the Texas Panhandle, to Colorado and Kansas. Triticum mosaic virus was first identified in 2006, and it has since been found across most of the Great Plains. The impact of these viruses in combination are not well known.
Hosts of WSMV include wheat, oats, barley, corn, triticale, rye, and several annual and perennial grasses (green foxtail, giant foxtail, sandbur, crabgrass, barnyard grass, stinkgrass witchgrass, hairy grama, Canada wild rye, Virginia wild rye, and Bermuda grass), while H PV hosts include wheat, corn, barley, yellow and green foxtail, and witchgrass. While virus hosts are not always the same as vector hosts, many of them tend to overlap.
For details on the cycle of these diseases and their vector, the wheat curl mite, reference Chapter 7-"Arthropod Pests of Wheat '
Symptoms of High Plains Virus: Infection caused only by the High Plains virus occurs in mid-May through july and also in September and October. Disease symptoms are apparent when leaves ehibit a mosaic pattern of yellow chlorotic spots and streaks (Figure 9.14). Leaf symptoms also may be similar to those of WSMV, with green or yellow stripes near the leaf tip or wheat soilborne mosaic virus, with green spots on a light green background. Laboratory testing is needed to reliably diagnose High Plains virus.
Figure 9.4 High Plains Virus disease symptoms on three leaves of winter wheat.
Figure 9.15 (a) Yellowing of winter wheat.
Figure 9.15 (b) Patches of diseased winter wheat appear yellow and stunted when compared to healthy plants.
Wheat streak mosaic virus (WSMV)
Symptoms. Symptoms of WSMV first appear in the spring when temperatures begin to warm. They are often most noticeable on the edge of fields or in areas near volunteer wheat. Severe infections occur in the fall but may not show symptoms until the spring when warmer temperatures favor disease development.
Diseased plants have a general appearance of being yellow and stunted (Figure 9.15 a & b). Infected leaves are mottled and exhibit light green-yellow, parallel, and discontinuous streaking (Figure 9.16 a & b). As infection worsens, infected leaves turn brown and die. In addition, infected plants develop fewer tillers than normal, some of which may lie on the ground (Figure 9.17a & b). Time of infection is a very important factor in determining how great losses will be. Plants infected early in the season, before early tillering, exhibit severe stunting, and produce few, if any, heads. If plants are not infected until the spring, there is little impact. Losses also tend to be more severe in dry years.
Figure 9.16 (a) Diseased wheat leaves exhibit discontinuous streaking.
Figure 9.16 (b) Light yellow-green, parallel streaking due to wheat streak mosaic virus.
Figure 9.17 (a) Infected wheat plants develop fewer tillers, some of which may lie prostrate on the ground.
Figure 9.17 (b) Diseased leaves fail to unfurl due to the feeding habits of the wheat curl mite.
HPV & WSMV
Plants infected with both High Plains virus and wheat streak mosaic virus will exhibit severe chlorosis and stunting, strong mosaic patterns, and premature death.
Management. See the section on wheat curl mite in Chapter 7-"Arthropod Pests of Wheat" for details.
Wheat Soilborne Mosaic Virus (WSBMV)
Wheat soilborne mosaic virus is carried by the soilborne fungal vector Polyrnyxa graminis which is found in cool, wet soils. The symptoms, life cycle, and disease patttern of WSBMV are very similar to those of wheat spindle streak mosaic virus, but with two distinct differences. WSBMV causes greater yield losses and disease symptoms persist longer into the spring.
Disease Cycle. WSBMV is carried into roots by fungal zoospores (Figure 9.18). In cool, wet soil conditions resting spores produce these zoospores which swim to young wheat roots and enter through root hairs or epidermal cells. When infected roots decay, resting spores are released into the soil where they, and the virus, can survive for many yean. The virus also persists for as many as 10 years in dry plant tissue. WSBMV is spread only through infested soil. Infection is spread when infested soil is dispersed by wind, water, or contaminated equipment.
Figure 9.18 Fungal zoospores carrying the vector Polymyxa graminis.
Favorable conditions for WSBMV include prolonged cool temperatures, below 65°F (18°C), soil temperatures between 50 and 60°F (10-16°C), and short day lengths. Therefore, root infections occurring in the fall are most important. Wet soil conditions and low-lying areas of fields also promote disease.
Losses caused by wheat soilborne mosaic virus are variable and depend on the area, weather, and wheat variety. The earlier a plant is infected, the greater the injury is. Temperatures below 60°F (16°C) promote fungal development, but as the season warms and temperatures exceed 68°F (20°C), fungal development ceases. Therefore, a long cool spring promotes the disease and may result in yield losses of 30 to 50 percent, while a warm spring hinders disease development and results losses of only 10 to 20 percent. Reduced yields are associated with fewer kernels per spike and reduced test weights.
Figure 9.19 Wheat infected with wheat soil-borne mosaic virus exhibits stunting and yellowing.
Figure 9.20 Irregular patches of yellow, stunted wheat may be due to wheat soil-borne mosaic virus.
Figure 9.21 Leaves infected with wheat soilborne mosaic virus exhibit mottling and discoloration.
Symptoms. Symptoms are most apparent in the early spring, right after green up and include plant stunting and leaf mottling and streaking (Figure 9.19). Leaf mosaic symptoms diminish by the time of jointing, but stunting tends to persist through maturity. Wheat soilborne mosaic virus is first evident by the presence of large, irregular patches of yellow, stunted wheat (Figure 9.20). Unlike many other diseases, the infected patches do not grow in size throughout the season; instead field patterns follow drainage or irrigation patterns because the zoospore vector needs water in order to spread the disease. Leaf mosaic symptoms include mottling and light green spots or dashes against a yellow background (Figure 9.21), often referred to as a mosaic of "green islands.' Reddish streaking and necrosis may sometimes occur on the tips of leafs, and infected plants may produce a reduced number of tillers and heads (Figure 9.22).
Management. Because the virus can survive in the soil and in crop residues for up to 10 years, crop rotation is not an effective control method. Planting resistant varieties is the most effective control strategy for wheat soilborne mosaic virus. Late planting may reduce the risk of infection but is not always successful. Plant winter wheat after the Hessian fly safe date for your area to reduce the incidence of WSBMV and other viral diseases.
Figure 9.22 Reddish streaking may occur at leaf tips.
Rust Diseases of Wheat
Rust diseases occur worldwide and are of economic importance due to their capacity to rapidly develop new races, making previously resistant varieties susceptible. In addition, rust diseases are capable of disseminating over long distances and develop rapidly under favorable conditions. Leaf rust, stem rust, and stripe rust all affect wheat, but in the Great Plains leaf and stripe rust are of the greatest importance.
Leaf rust is a worldwide disease of wheat caused by the fungus Puccinia recondita f. sp. tritici. There are many races of the leaf rust fungus, and no variety is resistant to all of them. New races emerge frequently, making the lifespan of a resistant variety only a few years.
Disease Cycle. The severity of leaf rust is affected by the growth stage at the time of infection, weather conditions, and the amount of rust inoculum present. Damage is greater when plants are infected before flowering, especially when the flag leaf becomes infected. Late-maturing varieties of wheat and cool (60-75°F or 16-24°C), wet weather, including rain and dew, also promote the disease. However, heavy rain washes spores off of the plant, and dry, windy conditions favor spore dispersal. Losses due to leaf rust are caused by a reduced number of kernels per head, reduced size of kernels, lowered test weights, and reduced protein content of the grain.
Symptoms. Leaf rust pustules form on infected wheat and are small (.04-.08 mm in length), reddish-orange oval fruiting bodies (uredinia) on the leaf surface (Figure 9.23). Pustules can be either scattered or clustered. Each pustule contains thousands of orange, powdery rust spores that rupture the epidermis of the leaf surface as the fungus matures and then are disseminated by the wind and rain. Pustules are usually surrounded by orange dust, and sometimes also a narrow, yellow or white border or “halo” (Figure 9.24). Unlike other rusts, the orange spores will rub off of your finger and, if infection is severe, field scouts may find the orange dust on their hands and clothing.
As plants mature, pustules begin producing black spores. These pustules resemble tar spots and are most noticeable on lower leaves and leaf sheaths. Orange spots (not pustules) may also form on the heads and culms of diseased plants. Leaf rust, unlike stem rust, does not form pustules on these organs.
Leaf rust infection occurs uniformly across fields, usually from mid May through early July and again in September and October. When leaf rust overwinters in a field, disease is more severe on the lower leaves because the fungus develops here first before advancing up the plant to the flag leaf. Disease is more severe in the upper part of the plant when spores are blown in from adjacent areas. Severity of disease increases exponentially, and during favorable weather pustules development can result in 30 to 50 percent coverage of the leaf surface.
Figure 9.23 Leaf rust pustules occur randomly across the leaf surface and are ellipsoidal in shape.
Figure 9.24 Leaf rust pustules surrounded by a yellow or white halo.
Management. Plant varieties with at least moderate resistance to leaf rust. Planting varieties that vary in parentage, maturity, and disease reaction can reduce the chances of leaf rust taking out entire fields. Some varieties are susceptible to disease, but tolerate infection better than other varieties. Early maturing varieties may escape late season rust problems.
Susceptible varieties can be protected with foliar fungicides. However, fungicide treatment is recommended only after consideration of the following risk factors:
I. Yield potential in a field (at least 45 bubu/ac dryland and 75 bu/ac irrigated)
2. Wheat variety susceptibility to leaf rust
3. Time of infection (early rust increases damage)
4. Dryland or irrigated wheat (irrigated wheat more vulnerable to injury)
5. Planting dates of winter wheat crops (late seeded crops more at risk due to delayed development)
6. Current and 30-day weather forecasts from mid-May to mid-june (wet weather conditions favor disease development)
Seed treatments can control fall infections but may not persist through to spring.
Plant winter wheat after the Hessian fly-safe date for your area to reduce fall infections, but keep in mind that delayed maturity can lead to increased injury in the spring. In addition, control volunteer wheat in the summer because it is an important source of infield inoculum in the fall, from over-seasoning uredinia, but this does not prevent infection from windbome spores.
Stripe Rust (yellow rust)
Stripe rust is caused by the fungus Puccinia striiformis. Stripe rust incidence was once rare in the High Plains due to the hot, dry climate. The development of new strains tolerant of a wider range of temperatures has led to increasing stripe rust problems in the central Great Plains.
Disease Cycle. Stripe rust develops in cooler temperatures (55-75°F or 13-24°C) than other rust diseases, allowing it to develop earlier in the season. Stripe rust develops most rapidly between 50 and 60°F (10-16°C), and development slows when temperatures exceed 75°F (24°C). Cool, wet falls, mild, open winters, and long, cool, and wet springs all promote disease development.
Stripe rust over-summers on volunteer wheat and perennial grasses. It also develops in the fall and winter in the southern United States, and then spores are carried north into the central Great Plains in the spring. The fungus can persist through cold climates (as low as 23°F or -5°C), overwintering on wheat and grassy weeds or as dormant mycelium under snow cover.
Symptoms. Infected wheat leaves develop long, narrow stripes, usually about 1/16 of an inch wide and irregular in length, of yellowish-orange pustules (Figure 9.25). Pustules are small (1/100 of an inch) and round, contain masses of rust spores, and develop on the head as well as leaf sheaths. Stem rust spores are lighter in color than those of leaf and stem rust. In moderately resistant varieties pustules may be absent or hard to see, resulting in symptoms similar to those of black chaff. As diseased plants mature or become stressed, tissues appear dry and brown, giving plants an overall scorched appearance (Figure 9.26).
Management. The most effective control method for stripe rust in wheat is to plant resistant varieties (see Chapter 3-“Variety Selection” for resources on current varieties). However, a new race of stripe rust is currently emerging, and previous management practices may be ineffective.
Cultural practices also can help decrease disease incidences. Control grassy weeds and volunteer wheat at least three weeks prior to fall seeding to reduce the risk of disease transmission by “green bridge.” Avoid early planting of winter wheat to reduce this risk as well. Scout fields for infected perennial grasses, because these are an important reservoir of disease.
Figure 9.25 Long, narrow stripes of yellowish-orange pustules are indicative to the stripe rust fungus.
Figure 9.26 As diseased wheat plants mature, they turn brown and dry out giving plants a scorched appearance.
Fungicides may control stripe rust effectively and economically after taking into account the following risk factors:
1. Susceptibility of wheat varieties to stripe rust.
2. Cwrent and 30-day Mather forecasts from mid-May to mid- june (cool, wet weather conditions favor disease development).
3. Dryland or irrigated wheat (irrigated wheat more vulnerable to injury).
4. Yield potential in a field (at least 4 5-bu/A dryland and 75 bu/ac irrigated).
5. Development of rust on lower leaves (early infection inaeases losses).
6. Incidence of rust on local wheat.
Stem Rust (black rust)
Stem rust, caused by the fungus Puccinia graminis f. sp. tritici, is the most damaging of all rusts, capable of causing complete crop loss. Stem rust outbreaks have been rare for several decades, but a new race, Ug99, is spreading from East Africa. Little resistance is available to Ug99, and the potential for severe epidemics is greater than it has been in many years.
Figure 9.27 (a) European barberry serves as an alternate host for the stem rust fungus.
Figure 9.27 (b) Aecia of stem rust on barberry.
Figure 9.28 Telia of wheat stem rust overwintering on stubble and plant residue.
Figure 9.29 Stem rust pustules develop on infected plant parts including leaf sheaths, stems, spikes, and occasionally leaves as well.
Figure 9.30 (a) Stem rust pustules.
Disease Cycle. The stem rust fungus requires two hosts to complete its life cycle. Telial stage hosts include wheat, barley, and several grasses, and aecial hosts, or alternate hosts, include European barberry such as Berberis vulgaris (Figure 9.27a & b), B. fendleri, and B. canadensis.
The fungus overwinters as teliospores on plant residue (Figure 9.28) or in the soil in colder climates and as urediospores on winter wheat grown in warmer climates. Wind-borne urediospores from southern states are the primary inoculum for disease in the Great Plains. Diseased plants produce more urediospores, creating a second inoculums.
Stem rust occurs worldwide, and is especially important in areas exhibiting warm, humid conditions (65-85°F or 18-29°C). Losses tend to be greatest when severe infection occurs before grain fill. Diseased plants produce shriveled grain and lodging results in loss of spikes.
Symptoms. Fruiting bodies (uredinia) develop on diseased leaf sheaths, stems, spikes and occasionally on leaves as well (Figure 9.29). The brick red, oval-shaped pustules (Figure 10.9 a & b) eventually turn dark brown or black (Figure 9.31) and rupture the epidermis of its host.
Figure 9.30 (b) A close-up view of stem rust pustules.
Figure 9.31 Pustules turn black as plants mature.
Management. The most effective control method for stripe rust in wheat is to plant resistant varieties. See chapter 3-"Variety Selection" for sources of information on current varieties. Early maturing varieties also reduce the risk of injury because plants have time to ripen before becoming severely infected.
Eradicating barberry, the alternate host, may help control local buildup of disease causing spores. However, Great Plains wheat is most commonly infected when windborne spores are dispersed in the area from southern states and Mexico. Fungicides are usually not necessary when resistant cultivars are planted.
Tan Spot (Pyrenophora tritici-repentis)
Tan spot, caused by the fungus Pyrenophora tritici-repentis, is an important leaf spot disease in the Great Plains region. Tan spot often occurs in conjunction with leaf rust and Septaria leaf blotch and is associated with reduced tillage. Losses due to tan spot are reflected in reduced yields and grain weight.
Disease Cycle. The tan spot fungus overwinters as pseudothecia on wheat stubbles (either standing, buried, or lying down). Spores produced in the pseudothecia are disseminated by wind and rain and are the primary source of infection. Secondary infection is spread through a field or to adjacent fields by spores produced by the tan spot lesions, also disseminated by wind and rain. Fungal development is favored by wet weather conditions, especially in May and June.
Symptoms. Symptoms of tan spot appear in the spring as oval to diamond-shaped, elongated brown leaf spots that are often darker in the center and yellow around the outside (Figure 9.32). The earlier the plant is infected, the more distinct the yellow border usually becomes, creating an "eye-spot" appearance. As the disease progresses, more spots develop on the leaves and start to coalesce, producing large areas of dead tissue. In addition, tan spot may kill leaves after heading, resulting in the early death of plants. In the late summer (August), characteristic, small, black fruiting bodies called pseudo-thecia appear on the stubble.
Figure 9.32 Infected wheat plants develop brown spots with a yellow halo.
Management. Crop rotation at least one year out of wheat is the best option for controlling tan spot. In addition, there are many tan spot resistant varieties available. See chapter reference for information on tan spot resistance sources. Foliar fungicide applications are recommended to protect the flag leaf in high risk situations.
Powdery Mildew (Blumeria graminis f. sp. tritici)
Powdery mildew is caused by Blumeria graminis f. sp. tritici and is common in humid or semi-arid wheat growing regions. Factors that favor powdery mildew problems include mild temperatures (59-71°F or 15-22°C), high humidity (between 85 and 100 percent), dense stands, high nitrogen fertilization, and varietal susceptibility. Varieties are most susceptible to injury from jointing to flag-leaf emergence, and plants sustain the most damage when infected early in the spring. Damage increases as the mildew develops further up the plant before flowering, and severe losses occur when the flag leaf becomes diseased before heading. Severe infections may result in lodging, early death of leaves, reduced kernel size and test weight, failure to produce heads, and yield losses of up to 40 percent.
Figure 9.33 Powdery mildew on winter wheat.
Disease Cycle. Fall infections of newly planted wheat occur when spores develop on volunteer wheat or within cleistothecia. The powdery mildew fungus overwinters as cleistothecia on plant debris or as mycelium on infected plants. Conidia form on infected winter wheat plants and serve as the primary means of inoculum. Conidia are wind dispersed and germinate under cool, humid conditions. Under favorable conditions the disease can complete a life cycle in 7 to 10 days. Development ceases at around 77°F (25°C).
Symptoms. Symptoms of powdery mildew include patches of powdery white or grey fungal growth on leaves, stems, and heads (Figure 9.33). Infection usually occurs on the lowest leaves of plants first and eventually works its way up the plant. The opposite sides of infected leaves become chlorotic and turn yellow and brown in color. As plants mature, the fungus changes color, getting darker grey and brown. Small, round, black fruiting bodies form on leaves in June.
Management. Growing resistant varieties is the easiest way to control powdery mildew. However, new races are constantly developing, so it is important to stay informed about current varieties and resistance (reference Chapter 3-"Variety Selection'' or your local university extension office).
Utilize crop rotation, destroy volunteer wheat, and use a balanced nitrogen fertilization program to reduce the likelihood of inoculum.
Other Wheat Diseases
Similar to wheat streak mosaic virus, agropyron mosaic virus in wheat is transmitted by the cereal rust mite (Abacarus hystrix). Symptoms of this disease are also similar in appear ance to those of wheat streak mosaic, but they are not as severe. Agropyron mosaic is often associated with quack grass, and is usually found in patches or along grassy field borders. Management practices are similar to that of wheat streak mosaic virus.
Disease Cycle. Black chaff is a bacterial disease caused by the bacterium Xanthomonas translucens. The bacterium survives in and on seeds and may also persist on crop residue and in soil. Because it can also survive on plants during the growing season, it can be transmitted by splashing water, plant to plant contact, and insects. However, the most important source of inoculum is contaminated seed. Volunteer and grassy weeds also are sources of inoculums. Black chaff is promoted by irrigation and plentiful rainfall. Losses of up to 40 percent have been attributed to black chaff.
Symptoms. Black chaff gets its name from the dark discoloration of the glumes characteristic of diseased plants. Brown to black interveinal streaks develop on infected glumes and leaves, and stripes of alternating healthy and necrotic tissues on awns create a "barber's pole" appearance. Cream to yellow colored slime or droplets appear in wet weather.
Dried droplets are light in color and scale-like. Infected stems, below the head and above the flag leaf, may develop a brown to purple discoloration and leaves may produce irregularly shaped lesions that first appear as water spots. These spots turn brown as infection progresses, giving diseased plants an overall orange appearance.
Management. Plant high quality, disease-free seed. Control volunteer and grassy weeds and do not overirrigate.
Cephalosporium stripe (Cephalosporium gramineum)
The fungus causing Cephalosporium stripe is soil and residue-borne and often associated with minimum tillage. Low, wet areas of fields favor disease as do heavy, wet, low pH soils.
Symptoms. Symptoms of cephalosporium stripe are first noticeable in the spring, in jointing and heading, as yellow, chlorotic stripes on leaves, blades, and stems. Sometimes brown necrotic tissue can be seen inside the yellow stripes. Plants are stunted and nodes are darker in color than normal. Diseased plants occur randomly, and heads are white and sterile.
Management. Control cephalosporium stripe with crop rotations, tillage, and tolerant cultivars. Destroying straw reduces inoculum since the cephalosporium stripe fungus can survive for several years on straw.
Ergot (Claviceps purpurea)
Ergot, caused by the fungus Claviceps purpurea, can result in significant loss in yield and quality. However, mycotoxins produced by the fungus are of greatest concern. These cause ergotism in livestock and humans, resulting in constriction of blood vessels, muscle contractions, gangrene, convulsions, and hyperexcitability.
Disease Cycle. Initial infection occurs when wind-borne sexual spores that land on open flowers and germinate, causing infection. The infected flowers produce cloudy, sticky honeydew that contains fungus spores that are disseminated with the help of insects, splashing water, and plant-to-plant contact. The fungus is favored by wet, cool conditions during flowering, and susceptibility to disease increases with prolonged flowering periods.
Figure 9.34 The honeydew stage of the ergot fungus.
Symptoms. Diseased plants produce hard, purplish-black sclerotia about ¼ to ½ inch in length, called ergot bodies, in place of healthy kernels. The ergots are similar in size to healthy wheat kernels and are tannish-white internally. In addition, a yellowish, sugary honeydew (Figure 9.34) forms on infected heads during flowering and prior to the development of ergots. This honeydew may be present on other infected plant parts as well.
Management. Plant clean seed, free of sclerotia, to avoid introducing disease into a field. Sclerotia will not germinate at a depth greater than one inch, so deeper planting may help reduce disease incidence. Varieties with shorter flowering periods may decrease initial infection. Rotate with non-host crops such as legumes or corn. Control grass and weeds in and around the field to reduce disease reservoirs.
Compendium of Wheat Diseases and Pests. 3rd Edition. 2010, APS Press. UNL Plant Disease Control: [http://pdc.unl.edu/agriculturecrops/wheat]