C.O.R.N. Newsletter 2010-10

SCOUT FOR FOLIAR DISEASE: The wheat crop is already at Feekes 8, flag leaf emergence, in some parts of southern and central Ohio and will reach this growth stage within the next 7 to 10 days in northern counties. Foliar diseases such as powdery mildew and Septoria leaf blotch are beginning to show up and Feekes 8 is an important growth stage for making fungicide use decisions to manage these diseases. Scout wheat fields to determine which disease is present and at what level before making a decision to apply a fungicide. Although several fungicides are available to control foliar diseases in Ohio, the decision to use these products should be based on the susceptibility of the variety planted, the level of disease in the field, weather conditions, the yield potential of the field, fungicide cost, and the market price of wheat. 

Powdery mildew is important during the month of May and early June in mild seasons with high relative humidity. There are many varieties grown in Ohio that are susceptible to powdery mildew, so under mild conditions with temperatures in the 60's and low 70's and high humidity the disease can develop rapidly. Plan to scout those fields planted to susceptible varieties because these are the fields most likely to sustain yield loss. Research has shown that if disease affects the upper two leaves by heading, yield losses can be as high as 25% on susceptible varieties. Scout fields by pulling about 50 individual tillers randomly from throughout the field and look for the small white pustules on the lower leaves and leaf sheaths. Evaluate individual tillers by looking at the top (flag) leaf, then the second leaf. One percent of the leaf area (about 2-3 mildew pustules) on the second leaf between growth stage 8 (flag leaf emergence) and 10 (boot) is the threshold level for applying a fungicide. By the time the second leaf becomes infected, the lower leaves generally have a lot of mildew on them. Fields that needs a fungicide application will have lots of powdery mildew in the lower canopy, but mildew becomes an economic problem only if the disease advances to the upper leaves before flowering. Yield responses to fungicide application are generally dependent on the amount of disease on each plant, how early disease attacks the upper leaves and the level of susceptibility of the variety to the disease. If powdery mildew is present in a field planted to a susceptible variety you should watch its development over the next week or so and decide whether fungicides should be applied. 

Stagonospora leaf and glume blotch is most severe when frequent rains occur during the months of May and June. Scout fields between growth stages 8 (flag leaf emergence) and 10.5 (full head emergence) and if 1 to 2 lesions are detected on the leaf below the flag leaf on a susceptible variety, fungicide should be applied. 

Remember, for foliar disease management (powdery mildew, Septoria, Stagonospora and Rust) fungicides are usually not needed when resistant varieties are grown. An updated list of fungicides registered for wheat disease management can be found on the field crops disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/OFCDwheatfungicides.pdf . 

COLD TEMPERATURE CONCERNS: Colder weather is forecasted for later this week and this has prompted some concerns about possible injury to the wheat crop. Once it warms up in the spring and the wheat starts to grow, it loses its cold-temperature hardiness, becoming more prone to injury from freezes. At the current growth stage (between Feekes 6 and 8), the growing point is no longer protected by the soil, however, it is somewhat protected by the vegetation and its nearness to the soil surface. Wheat is a cool season crop and can tolerate temperatures well below freezing. At this growth stage, wheat can tolerate cold temperatures down to about 24 to 26 degrees F. 

Whether or not the crop is damaged by cold spring temperatures and the extent of the damage depend on three main conditions: 1) how cold it gets, 2) the length of time the crop is exposed to the cold temperatures and 3) the growth stage of the crop at the time of exposure. Freezing temperatures are most damaging when the crop is at more advanced stages of development. Injuries are most severe when freezing temperatures occur during boot and heading growth stages. 

Based on information coming out of a Kansas State University publication http://wheat.colostate.edu/freeze.pdf , temperatures below 12 F are injurious during tillering, whereas, during jointing, 2 hours of exposure to 24 F may be injurious. At these growth stages, injuries tend to be greatest if the wheat is lush and actively growing (especially after spring nitrogen application) and are more common in low areas of the field.