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. Help in diagnosis can be obtained from OSU Extension or other crop consultants. OSU Extension Bulletin 785, Wheat Disease Control in Ohio, provides descriptions and pictures of the common diseases in the state. This bulletin can also be found on OSU's web site Ohioline at http://www.ag.ohio-state.edu/~ohioline/b785/index.html. Additionally, information on wheat-disease diagnosis and control can be obtained on the web site Ohio Field Crop Diseases at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.
Planting disease-resistant varieties (Table 7) 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 yellow mosaic. Varieties with moderate levels of resistance to Stagonospora leaf and glume blotch and Fusarium head scab are also available. However, varieties rarely have good resistance to all diseases. When varieties have high resistance to a disease, they effectively limit losses in yield. Resistance to leaf rust and powdery mildew may fail due to the development of new races of the pathogens. Combining the use of resistant varieties with good crop rotations, planting after the Hessian fly-free date, and the use of seed-treatment fungicides will improve disease control.
Scouting fields for disease is essential 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 Growth Stage 8 and 10, respectively) and scout for Stagonospora leaf blotch and leaf rust at boot stage, full-head emergence, and after flowering.
The upper two leaves on tillers and the glumes on heads contribute most to grain fill. Thus, it is important to keep these upper plant parts free of disease to avoid yield loss. Fungicides should be used only on susceptible and moderately susceptible varieties in fields that have a yield potential of 60 or more bushels per acre. Disease thresholds are one percent of leaf area affected on the leaf below the flag leaf up to boot stage (Feekes Growth Stage 8 through 10), and one percent of leaf area affected on the flag leaf between head emergence to the end of flowering (Growth Stage 10.1 to 11) (Table 8). When these disease levels are present, 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.