I have received numerous calls and digital and live samples of wheat symptomatic of infection with wheat streak mosaic virus (WSMV) this past week. Counties in the southwestern part of the state such as Randolph, Monroe, and others have been very severely affected. The curl mite has been detected in some but not all of the fields. In those fields that are symptomatic of WSMV where the wheat curl mite is actually present, it is a pretty good assumption that the virus present is actually WSMV; however, keep in mind that viruses can be diagnosed only from an ELISA test. Should you want definitive proof that WSMV is present in the field, you must send "live" affected tissue to a laboratory such as AgDia for testing. (For more in-depth information about the virus and wheat curl mite, refer to issue no. 4 of the Bulletin.) Some of the most common questions that have come up this week follow. |
Should I destroy my wheat crop? The answer to this has to do with when the symptoms of WSMV actually showed up in the field. The curl mite can transmit the virus in the fall and in the spring (or throughout the winter if it is mild). Fields that were infected last fall by the mite should have been exhibiting mosaic symptoms for several weeks now, and the plants may be severely stunted and leaves may already be dying from the bottom of the plant up. The plants will not recover from this and will likely produce only sterile heads, if they produce any heads at all. Fields such as these are good candidates for replant. If the mosaic symptoms are just now showing up, then the virus was probably transmitted this spring, and these fields will probably experience minimal loss from WSMV and are not good candidates for destruction.
When can I replant to corn? The concern about planting the field to corn has to do with the fact that corn can host both the wheat curl mite and WSMV. The corn is not affected by the virus, but it does serve as a reservoir for the disease. This will be a concern for wheat planting that takes place next fall, and "green corn" that has the virus and mites may still be around. Management strategies are limited. One of the best is to try to break what is called the "green bridge" so that the mite starves and can't transmit the virus. Fall recommendations call for 10 days between when secondary hosts die and wheat planting. The mite can actually only survive about 8 hours without live tissue, so the extra 9 days are to maximize risk reduction to the new wheat crop.
Well, as interesting as that is, how do you make a decision about what to do now? There isn't much information available to answer the question with complete surety. My recommendation is this: if you are going to destroy your wheat and replant to corn, do as good a job as possible of killing the wheat. If you are able to wait to plant corn for several days, it would probably be useful in trying to reduce the wheat curl mite population. However, realistically speaking, there are many other grassy hosts for the mite and the virus that are growing now as well. So the bottom line is to remember when you come back to the area to plant wheat next fall that planting beyond the Hessian fly-free date will be imperative so that the "green bridge" can be broken.
Kentucky is experiencing an epidemic of WSMV as well, and for some additional reading I recommend Doug Johnson and Don Hershman's article about the WSMV epidemic there. You can read it on the Web at http://www.uky.edu/Agriculture/kpn/kpn_00/pn000424.htm#whewhe .-- Suzanne Bissonnette