Disease is one of the major factors limiting wheat yield and quality in Ohio and other Midwestern states. Yield losses as high as 30 to 50 percent are not uncommon in fields planted with susceptible varieties under disease-favorable conditions. Effective disease management re- quires knowledge and understanding of how―and under what conditions―each disease develops, at what growth stage the crop is most susceptible, and how the disease causing organism survives and spreads. In Ohio, the most frequently occurring and damaging diseases are caused by fungi that survive in crop residue left in the field from one growing season to another, and the greatest losses occur when flag leaves and spikes are damaged before grain-fill is complete. Producers should fine tune their disease management strategies for those diseases that are most prevalent in their area of the state and are most capable of causing substantial yield and quality losses. Correct diagnosis is critical for effective disease management, and producers with little experience identifying diseases should seek help from competent sources, such as plant pathology extension state specialists, Ohio State University Extension or an agricultural consulting service.
1. Planting disease-resistant varieties is the most effective and economical means for controlling diseases. Select resistant varieties based on research conducted by universities and seed companies. Varieties are available with moderate to high levels of resistance to leaf rust, powdery mildew, and wheat Spindle Streak mosaic virus, and moderate levels of resistance to Stagonospora leaf and glume blotch, and Fusarium head scab. When varieties have high resistance to a disease, they effectively limit losses in yield. However, resistance to leaf rust and powdery mildew may fail due to the development of new races of the pathogens. When selecting varieties, give priority to head scab resistance. Although this disease does not occur every year, it is by far the most important and damaging disease of wheat in Ohio. Most of the other important diseases can be effectively controlled (80 to 90 percent) with a single, well-timed fungicide application, but the best fungicides only provide about 50 percent control of head scab and vomitoxin when applied to a susceptible variety. Therefore, fungicides have to be used in combination with the most scab resistant variety in order to achieve the best results in terms of scab and vomitoxin reduction. While no variety is equally resistant to every disease, high-yielding varieties with moderate resistance to head scab and one or more of the other diseases are available (oardc.ohio-state.edu/wheat-trials/). So, in addition to scab, select varieties with resistance to the disease most common in your part of the state. Powdery mildew is most common throughout Ohio, except in the northwestern part of the state. Stagonospora glume blotch is most severe in central, west central, northwest and southern Ohio. Leaf rust has the greatest potential for damage in southern Ohio. Monitoring wheat diseases aids a producer in selecting varieties with resistance to the common diseases of his or her region.
2. Plant well-cleaned, disease-free seed, treated with a fungicide that controls seedling blights, bunt, and loose smut. Seed treatments will also provide protection against foliar diseases such as Stagonospora leaf blotch and reduce stand establishment problems due head scab, when scabby seeds are planted.
3. Plant in a well-prepared seedbed, after the fly-safe date.
4. Rotate crops; never plant wheat where the previous crop was corn, wheat or spelt. A two-to three-year rotation from wheat prevents most pathogens from surviving in fields. Planting wheat after other small grain crops, such as barley, may also increase the risk of some diseases.
5. Plow under residues from heavily diseased fields, especially those affected by head scab, Stagonospora, Cephalosporium stripe or take-all. Plowing enhances decomposition of residue and death of the disease-causing fungi.
6. Use a well-balanced fertility program based on a soil test. Apply sufficient amounts of phosphorus, nitrogen, and potassium in the fall for vigorous root and seeding growth. Spring topdress with nitrogen at the rate recommended to achieve the yield goal. Excessive nitrogen increases the severity of foliar diseases such as leaf rust, powdery mildew and lodging.
7. Control grass weeds. Destroying volunteer wheat, quack grass and other grass weeds in and around potential wheat fields reduces the amount of inoculum available to infect the crop. Weeds and volunteer wheat may also serve as hosts (or overwintering reservoir) for several viruses that affect wheat and the insects that transmit them.
8. Apply fungicide. The upper two leaves and the glumes of the heads contribute most of the sugars to grain fill. Thus, it is important to keep these upper plant parts free of disease to minimize yield loss. A well-timed foliar fungicide application is able to effectively control most foliar fungal diseases such as powdery mildew, leaf rust, Septoria, and Stagonospora nodorum leaf and glume blotch, and suppress head scab and vomitoxin, but such an application is not always warranted. For instance, applications made in the absence of diseases (for plant health), at green-up or at half-rates do not provide consistent yield gains and are not always cost effective in Ohio. The economic benefit of using a fungicide depends on grain price, application cost, and variety susceptibility. In Ohio, at any given grain price, the chance of obtaining a yield response high enough to offset fungicide application cost is highest for applications made at flag-leaf emergence (Feekes GS 8) or boot stage (Feekes GS 10), and lowest when applications are made at green-up (Feekes GS 4-5).
For foliar diseases, fungicides are often most warranted and beneficial when susceptible and moderately susceptible varieties are planted. Scout fields from flag-leaf emergence through flowering, and make your fungicide application decision based on disease threshold and risk. Disease thresholds are 1 percent of leaf area affected on the leaf below the flag leaf between Feekes 8 and 10, and 1 percent of leaf area affected on the flag leaf between head emergence and flowering (Feekes 10.1-10.5.1). When these disease thresholds are reached, a fungicide should be applied as soon as possible to protect leaf tissue before more becomes infected. One percent leaf area affected roughly translates to five to 10 leaf rust pustules, two to three powdery mildew pustules, or one to two Stagonospora nodorum blotches.
For head scab, disease thresholds cannot be used as a guide for making a fungicide application. Applications have to be made at flowering, or at the very latest, four to six days after flowering. This is 14 to 21 days before actual head scab symptoms are observed. Therefore, the head scab forecasting system (wheatscab.psu.edu) should be used as a guide for making a fungicide application for scab and vomitoxin control. Information on how to use and interpret the forecasting system can be found in fact sheet PLPTH-CER-03, Fusarium Head Blight Forecasting System at: ohioline.osu.edu/fact-sheet/plpath-cer-03/.
Growers should become familiar with symptoms of the common diseases affecting wheat in Ohio. Correct diagnosis and scouting are important steps in identifying the yield-limiting diseases on your farm. Scouting fields for disease is particularly important when growing moderately susceptible and susceptible varieties to determine the need for fungicide applications. This involves checking the level of disease on 30 to 50 individual tillers randomly selected throughout the field. Fields should be scouted for powdery mildew at flag-leaf emergence and boot stage (Feekes GS 8 and 10, respectively, see Figure 6-3) and scout for Stagonospora leaf blotch and leaf rust at boot stage and full-head emergence.
Help in diagnosis can be obtained from plant pathology extension state specialists, the C. Wayne Ellett Plant and Pest Diagnostic Clinic (ppdc.osu.edu/), OSU Extension, or other crop consultants. OSU Extension fact sheets with descriptions and pictures of the common diseases in the state are available on Ohioline at: ohioline.osu.edu. Symptoms and appropriate control measures for several important wheat diseases are provided in Table 6-9.
Table 6-9: Wheat Diseases and Disorders Common in Ohio.
|Head scab||Spikelets of head turn straw colored; glume edges with orange-pink spore masses; kernels shriveled white to pink in color.||Warm, wet, humid weather during flowering and early grain-fill.||1. Seed treatment for infected seed. 2. Crop rotation with non-host. 3. Plow down corn and wheat residues. 4. A triazole fungicide at flowering. 5. Do not use Strobilurin fungicide for scab. 6. Use scab forecasting system as a guide for fungicide application.|
|Powdery mildew||Powdery white mold growth on leaf surfaces.||High humidity; 60-75 F; high nitrogen fertility and dense stands.||1. Resistant varieties. 2. Crop rotation. 3. Delayed planting. 4. Fungicides. 5. Balanced fertility.|
|Leaf rust||Rusty red pustules scattered over leaf surface.||Light rain, heavy dew; 60-77 F; high N fertilizer; 6-8 hour leaf wetness for germination and infection.||1. Resistant varieties. 2. Balanced fertility. 3. Fungicides.|
|Septoria tritici leaf blotch||Leaf blotches with dark brown borders; gray centers speckled with black fungal bodies.||Wet weather from mid-April to mid-May; 60-68 F; rain 3-4 days each week.||1. Seed treatment. 2. Plant less susceptible varieties. 3. Crop rotation. 4. Balanced fertility. 5. Fungicides.|
|Stagonospora nodorum leaf and glume blotch||Lens shaped chocolate brown leaf lesions with yellow borders; brown to tan blotches on upper half of glumes on heads.||Wet weather from mid-May through June, 68-80 F; rain 3-4 days each week.||1. Seed treatment. 2. Plant less susceptible varieties. 3. Crop rotation. 4. Balanced fertility. 5. Fungicides.|
|Tan spot||Lens shaped, light brown leaf lesions; yellow borders.||Moist, cool weather during late May and early June.||1. Plow down infested residues. 2. Crop rotation. 3. Balanced fertility. 4. Fungicides.|
|Cephalosporium stripe||Chlorotic and necrotic interveinal strips extending length of leaf.||Cold, wet fall and winter with freezing and thawing causing root damage.||1. Crop rotation. 2. Bury infested residues. 3. Control grassy weeds. 4. Lime soil to pH 6.0-6.5.|
|Take-all||Black scurfy mold on lower stems and roots; early death of plants.||Cool, moist soil through October-November and again in April-May.||1. Crop rotation. 2. Control weed grasses. 3. Balanced fertility. 4. Use ammonium forms of N for spring topdress. 5. Avoid early planting.|
|Fusarium root rot||Seedling blight (pre- and post- emergence); wilted, yellow plants; roots and lower stems with whitish to pinkish mold. Root rot plants have brown crowns and lower stems.||Dry, cool soils; drought stress during seed filling.||1. Seed treatments for seedling blight. 2. Delayed planting. 3. Balanced fertility. 4. Avoid planting after corn.|
|Barley yellow dwarf||Stunted, yellowed plants, leaves with yellowed or reddened leaf tips.||Cool, moist seasons.||1. Delay planting until after the Hessian fly-safe date. 2. Balanced fertility.|
|Wheat spindle streak mosaic||Discontinuous yellow streaks oriented parallel with veins of leaves. Streaks with tapered ends forming chlorotic spindle shapes.||Cool, wet fall followed by cool spring weather extending through May.||1. Resistant varieties.|
Figure 6-3. Feekes Wheat Growth Stage scale.