Diseases
Part 2, Section 5: Small Grains Pest Management
Small Grains Pest Management
DISEASES
Disease management is an important part of winter wheat production in Pennsylvania. The most common diseases in the Commonwealth are powdery mildew and Septoria leaf blotch. Other diseases, such as Fusarium head blight (scab), barley yellow dwarf, and leaf rust can be severe, but fortunately disease epidemics occur only sporadically in our region. Many of these diseases can cause considerable yield losses if left uncontrolled. This section describes some of the disease management strategies available for small grains in Pennsylvania.
Plant disease-resistant varieties
Planting disease-resistant varieties is the most effective and economical way to manage many diseases of wheat. Many wheat varieties are available that have a high level of resistance to powdery mildew, leaf rust, and wheat spindle streak virus. Varieties with a moderate level of resistance to Septoria leaf blotch and Fusarium head blight can also help reduce symptoms and manage these diseases. Use the information provided by Penn State Cooperative Extension and seed companies to select varieties that have resistance to as many diseases as possible. However, be aware that disease reactions may change rapidly if new races of the pathogen develop. This type of “race shift” is most commonly reported for powdery mildew and leaf rust. Selection of disease-resistant varieties, when used in combination with crop rotation, appropriate planting date, proper soil fertility, and fungicide-treated seed, is an effective way to maintain consistently high wheat yields.
Scouting for disease
Monitoring wheat fields for disease is essential to identify fields that may need a fungicide application when growing susceptible and moderately susceptible varieties. Assess the average disease level on at least 40 tillers randomly selected from throughout the field. Fields should be checked for disease at flag leaf emergence (Growth Stage 8).
Description of Wheat Diseases
Seed Rot and Seedling Diseases As with other crops, wheat seed can rot if placed in wet, cold soils. They typical pathogens that cause seed rots and seedling diseases are similar to those of corn and soybeans: Pythium, Phytophthora, Rhizoctonia, and Fusarium species. Damping off and other seedling diseases are common in high-traffic areas where compaction is often a problem. The best method for preventing seed rot and seedling diseases is to use fungicide-treated seed, plant into a proper seedbed, and maintain adequate soil fertility.
Foliar Diseases
Powdery mildew can infect anytime after emergence. Commonly found on the upper surface of lower leaves, powdery mildew is a superficial disease except for the haustoria, which penetrates the plant surface and absorbs nutrients from the plant. The disease is often in patches of white, cottony growth that eventually turns dull gray brown. In severe cases, fungicides are used to control the disease in the spring.
Septoria Leaf and Glume Blotch (a complex of Septoria and Stagonospora) has two phases affecting wheat: after emergence and, often more economically important, at flag leaf emergence. In the fall, Septoria can cause seedling blight, reducing initial stands. In the spring, as the wheat further develops, Septoria will often infect lower leaves, near the ground. Symptoms appear as chlorotic (yellowed) flecks that expand into irregular lesions. Other symptoms may appear as dark-green, water-soaked lesions that become dry and yellow and then turn red brown. Lesions that begin at the leaf base (near the stem) may kill the entire leaf. The disease is considered a threat once there are at one to two lesions on the second leaf (first leaf below the flag leaf) at flag leaf emergence through boot stage. Glume blotch can attack the heads, causing brown spots on the glumes (outer chaff), lemmas (inner chaff), and the awns. These colonized florets have grain fill impaired, resulting in low test weight and shriveled seeds.
Rusts in wheat are composed of leaf, stripe, and stem rusts. All rust diseases have a similar appearance in that they protrude from the leaf surface, with a blister-like appearance. Stem Rust appears as orange-red oval lesions that tear through the outer layer of plant tissue on the stems, leaf sheaths, blades, and occasionally on the head. Leaf Rust is a brown, round or slightly elongated lesion commonly found on the leaf blades, and may be on the leaf sheaths. Stripe rust occurs as yellow-orange, small blister-like lesions that merge to form stripes on the leaf blades and occasionally on the head. Further information can be found at http://extension.psu.edu/agronomy-guide/pm/documents/rust-diseases.pdf
Barley Yellow Dwarf Virus (BYDV) can be a problem in early planted wheat or barley. BYDV appears as leaf discolorations of yellow, red, or purple from tip to base and margin to midrib. In early infections, if the plants survive, they often exhibit discoloration and slow plant growth along with brilliant yellowing of older leaves. Discoloration of the flag leaf, either yellowed or reddened, is indicative of a later infection. BYDV is transmitted by over 20 species of aphids, with the oat bird-cherry aphid, the corn leaf aphid, the English grain aphid, and the greenbug being the most important vectors. Although insecticides are available to treat the aphids, the most common and practical practice is to plant wheat after the Hessian Fly-free date, when most of the aphid populations have crashed, and avoid the disease. With a warm, early spring, BYDV can be relatively severe on both spring- and fall-planted small grain crops, including barley, oats, and wheat. Additional information can be found at http://pubs.cas.psu.edu/FreePubs/pdfs/xl0085.pdf
Soil-borne Wheat Mosaic Virus (SBWMV) is another viral disease of wheat that has been found in Pennsylvania. This virus appears in the early spring as new leaves with mosaics (mixtures of dark green, light green, and yellow spots irregularly distributed throughout the leaf) and reduced tillering. Areas in the field appear as though no nitrogen has been applied due to the yellow discoloration and reduced tillering. This virus is vectored by a soil-borne fungus, with no practical control measures known. Symptoms will disappear after the average daily temperature reaches 68°F. The best control method for this virus is crop rotation for 2-3 years if a significant problem.
Seed and Head Disease
Fusarium Head Blight infects wheat during anthesis, or flowering stage, that lasts for about 7 days. The results are then seen during the late heading stage when spikes on the green heads look bleached or prematurely ripened. After ripening, the blighted seeds appear shriveled, called tombstones, and may exhibit signs of pink mold. These tombstones have low test weight, often sterile, and may contain mycotoxins. The Fusarium species that cause head blight are also linked to stalk and ear rots in corn. The Fusarium species are excellent saprophytes that can colonize dead tissue and persist. Wheat no-till drilled into corn infected with stalk rot and taken off for silage is still likely to exhibit higher incidence of head blight than if the field was tilled.
Fusarium head blight is known to produce the mycotoxins deoxynivalenol (DON) and zearalenone in both infected grain and straw. DON is known to cause vomiting, feed refusal, and lower weight gain at low levels when fed to animals. Zearalenone has estrogen-like properties that can cause reproductive problems in animals. Although grain is commonly tested, caution should be used when using straw as an alternative roughage source in years of high levels of head blight.
Due to this narrow infection period, it is difficult to apply contact fungicides directly on the heads. Research has shown a correlation with higher levels of mycotoxins in wheat that was sprayed with strobilurin fungicides. Therefore, strobilurin fungicides (Quadris, Headline) or mixtures containing strobilurin fungicides (Quilt, Stratego, TwinLine) should not be applied after the boot stage. Triazole fungicides have been shown when applied properly to reduce the amount of mycotoxins in the wheat grain. Additional information can be found at http://pubs.cas.psu.edu/FreePubs/pdfs/xl0086.pdf and weekly web-based disease forecast can be found at http://www.wheatscab.psu.edu/ . Checked this site regularly to determine if a fungicidal spray is necessary to control Fusarium head blight.
Fungicide Applications
The upper leaves and glumes contribute the most to the wheat yield, and it is important to protect these leaves from disease damage. Fungicides should only be considered for susceptible varieties with a yield potential of 60 bushels per acre or more, and if weather forecasts indicate that conditions are likely to favor continued disease development. Disease thresholds and suitable fungicides for powdery mildew, Septoria, and leaf rust are provided in Table 2.5-9 and 2.5-10. Table 2.5-11 provides the estimated yield losses attributed to powdery mildew based on variety susceptibility and disease level.
Many varieties of wheat that are resistant to powdery mildew have been developed to have resistance to all races of the pathogen. This type resistance is not subject to dramatic failures when new races of the pathogen are introduced into a region. Varieties with this type of resistance will often develop low levels of disease, but that rate of development and severity of the disease is reduced.
Fungicide Seed Treatment Update
Seed treatments on wheat are usually targeted at preventing seedborne diseases such as common bunt or loose smut. These pathogens survive in association with wheat seed and can build to high levels in several years. Raxil-Thiram (tebuconazole and thiram) provides control for most of the common seed rots, as well as suppression of many early season diseases. Charter PB (triticonazole and thiram) is very similar to Raxil-Thiram but uses a different triazole component. Dividend Extreme (difenoconazole and mefenoxam) is a seed treatment focused on wheat. At high rates, Dividend Extreme is labeled to control seed rots as well as fall-season powdery mildew, leaf rust, and Septoria leaf blotch for up to 6 weeks after planting. In commercial applications, Dividend Extreme is paired with Maxim 4FS (fludioxonil), which increases the protection against Fusarium, Rhizoctonia, Helminthosporium, and some weakly pathogenic fungi.
The addition of a seed treatment insecticide may help reduce the incidence of barley yellow dwarf virus (BYDV) in fall-planted small grains. Cruiser Maxx Cereals offers two fungicides along with Cruiser (thiamethoxam) to provide early season protection against aphids. Gaucho XT and Sativa IM Max are seed treatments with two fungicides and imidacloprid to control insect pests during the early season.
Foliar Fungicide Update
A new strobilurin fungicide, Evito 480 SC (fluoxastrobin), has been labeled for use on wheat for rusts, Septoria leaf and glume blotch, tan spot, and powdery mildew. Stratego YLD (prothioconazole and trifloxystrobin) is a mixture that is labeled on barley, corn, soybeans, and wheat for controlling multiple diseases. Absolute 500 SC (tebuconazole and trifloxystrobin) is another mixture labeled for application on barley and wheat to control foliar diseases. As with other strobilurins, no application of these products should be made after boot stage. See Table 2.5-10 for fungicide information and university results on the control of wheat diseases.
