October 12, 2007

Early disease management for 2008 starts now with the careful selection of disease resistant and tolerant corn hybrids. The word "resistance" implies a reduction in disease severity, but rarely implies that plants have immunity to disease. With that said, there are varying degrees of disease resistance.

A hybrid's level of resistance to a particular disease can range from low to high. A low level of disease resistance may be adequate under low disease pressure, but a moderate or high level of resistance may be necessary, either alone or in combination with other disease management strategies, to effectively reduce disease severity under high risk situations where conditions are favorable for disease. For example, continuous corn favors some diseases, such as gray leaf spot in the eastern half of Nebraska, where the pathogen survives in crop residue.

Fields with a history of gray leaf spot that are planted with continuous corn are at especially high risk for the disease again. Under some circumstances, such as this, it may be necessary to employ multiple disease management strategies, such as both the selection of resistant hybrids and a timely fungicide application program to adequately manage disease to minimize or prevent yield loss.

Another important disease of Nebraska corn, Goss's bacterial wilt and leaf blight, has become increasingly important, particularly in western Nebraska and the tri-state region with northeast Colorado and southeast Wyoming. Selection of resistant hybrids is especially important for planting in fields with a history of this disease because fungicides are not effective for its management and the bacteria can survive for long periods of time in crop residue.

Evaluation of hybrids for their disease resistance can be difficult, costly, time consuming, and/or impractical due to the nature of the pathogen(s). Therefore, not all companies evaluate their hybrids for disease resistance. Table 1 lists the number of companies that evaluate their hybrids' reactions to the listed corn diseases out of at least 22 companies in Nebraska. Be sure to remember when you are comparing hybrids that seed companies often use opposing rating scales when describing their traits. For example, 1 - 9 might mean "best to worst" for one company, but may mean "worst to best" for another. Keep this in mind when comparing hybrids across companies.

If you have had problems with lodging due to stalk rots or any of the ear rot or grain molds, an additional management strategy is to select hybrids that have insect resistance traits. The wounds that these pests create are used as entryways by many pathogens to infect plants. Reducing general plant stress and using good agronomic practices will help reduce damage caused by opportunistic plant diseases.

Tamra Jackson
Extension Plant Pathologist