|
Planting Date Affects Wheat Disease Incidence and Severity September, 2002 |
|
Using wheat as forage for cattle and to produce grain is a common practice in Oklahoma that greatly enhances the economics of farming. In such a dual-purpose system, wheat is planted as early as late August to optimize forage production, whereas in a grain-only system wheat would be planted in October. Although this early planting optimizes forage production, it also greatly favors certain diseases such as wheat streak mosaic virus, the aphid/barley yellow dwarf virus complex, and the root and foot rots. Wheat streak mosaic virus (WSMV) is carried by wheat curl mites that obtain the virus from feeding on infected crops such as corn, grassy weeds, and volunteer wheat. In the fall, WSMV-infected mites spread the virus to emerging seedling wheat, which either kills or severely damages the wheat by heading time the next spring. Planting late in the fall (generally after October 1 in northern OK and after October 15 in southern OK) and controlling volunteer wheat are two practices that help control WSMV. The wheat curl mite has a life span of 10-14 days, so it is imperative to destroy volunteer wheat in all production fields at least two weeks prior to emergence of seedling wheat. In the spring of 2002, WSMV was observed across western OK from the southern to the northern border. Hence, producers should diligently control volunteer wheat before planting this fall. Root and foot rots refer to several diseases caused by fungi including dryland root rot, Rhizoctonia root rot (sharp eyespot), common root rot, take-all, and eyespot (strawbreaker). Controlling the root and foot rots is difficult because there are no resistant varieties or fungicide treatments that consistently control all these diseases. Late planting helps reduce severity of the root and foot rots, but will not eliminate their presence. If you have a field with a history of root rot, consider planting that field as late as possible or use it in a “graze-out” fashion. Additional methods can be used to help control take-all. For example, take-all is greatly favored by a high soil pH (>6.5), so do not raise the pH above this level when liming a field to correct for acid soil. Elimination of residue also helps control take-all, and can be somewhat helpful in reducing the other root rots. However, residue removal by tillage or burning does not seem to affect the severity of eyespot (strawbreaker). Aphids and barley yellow dwarf virus (BYDV) should be thought of as a complex because both can damage wheat. While aphids feed, they can also infect wheat with BYDV. Fall infections with BYDV are the most severe because the virus has a longer time to damage the plant as compared to infections that occur in the spring. Controlling the aphid/BYDV complex can be approached in several ways. First, planting after October 1 in northern OK, and after October 15 in southern OK, helps reduce the opportunity for fall infestations of aphids and BYDV. Second, some varieties such as Custer, 2174, and 2137 show some tolerance to the aphid/BYDV complex, but remember, this tolerance can be overcome at high levels of infection. Finally, systemic insecticide seed treatments such as Gaucho (Gustafson Corp.) or contact insecticides can be used to control aphids. Unfortunately, aphids may have already transmitted BYDV by the time contact insecticides are applied, and although seed-treatment insecticides can effectively control aphids for up to 70 days after planting, in some falls no aphids occur and hence the treatment was not necessary. Be sure to thoroughly read the label before applying any chemical. For more information on these (or other) diseases, contact your extension educator and ask for OSU Extension Facts 7636 (WSMV), OSU Extension Facts F-7622 (root rots), or examine the information available at the web page for the Plant Disease and Insect Diagnostic Laboratory at: http://www.ento.okstate.edu/ddd/hosts/wheat.htm.
Submitted by Bob Hunger, on behalf of OSU’s Wheat Improvement Team.
|