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Plant Disease Report

September 6, 2002
Fall virus concerns for winter wheat: Management of viral disease of wheat can be tricky business. There isn't anything out there to spray on the plants once they are infected to minimize the yield loss from virus infection. Management balances around avoidance, disease resistance through variety selection, and sanitation of secondary hosts.

Cultural control ("avoidance"): Table 4 describes and compares the variety of virus diseases that are important to wheat production in Illinois. For example, two serious diseases, barley yellow dwarf virus (BYDV) and wheat steak mosaic virus (WSMV) infections, can take place both in the fall and again in the spring. Fall virus infections are generally the more damaging, both in terms of yield loss and in direct plant loss from winterkill. You can reduce the impact of virus diseases by reducing fall infections, which is called "avoidance."

To accomplish avoidance, winter wheat should be planted after the Hessian fly-free date. This takes advantage of the fact that a virus is not alive and needs a vector (e.g., typically an aphid or wheat curl mite) to get around. Vector populations should be much reduced after the fly-free date. Utilize the information that typically for wheat viruses neither a vector nor a virus can function without a living host in the fall (called a "green bridge"), nor can they function at low temperatures.

Delayed planting breaks up the green bridge, or the overlap in the growing seasons between two viral host crops such as corn and winter wheat. Essentially, the virus and vector population crash because virtually no live hosts are left, and then it gets too cold for the vector to function. Virus diseases do not disappear forever because they typically overwinter in weed or volunteer crop hosts and await the appearance of their insect vector in the spring, or they arrive with the vector as it moves north in the spring. Fly-free dates for each county are listed in the Illinois Agronomy Handbook, available through your local Extension office or online at http://web.aces.uiuc.edu/iah/.

Variety selection: Check the Wheat Performance in Illinois Trials 2002 published by the University of Illinois (contact your Extension office for a copy or go online to http://vt.cropsci.uiuc.edu/wheat.html) for variety selection information on yield, test weight, and plant height data for numerous locations across Illinois. The report contains information about many other important agronomic and pest-related characteristics. However, for a number of reasons, published data on varietal resistance to virus diseases are scarce in the Midwest. In most cases, you should be able to get some resistance information from your seed sales representative.

Field diagnosis of viral diseases is a not an absolute confirmation of the actual virus that is out in the field. Symptoms, while they can be "typical" for various viruses, get murky when dual infections or nutrient problems are factors as well. So if you really need to know for certain what virus is out there, confirm the observations via laboratory diagnosis. A good example of something that should be tested would be a field that was, for instance, planted with a soilborne mosaic virus resistant variety that seems to have SBMV symptomatology.

Virus pathogens cannot be cultured in the lab like fungi or bacteria, so plant disease clinics around the country often send suspected virus-infected samples to a company called Agdia, Inc., for ELISA testing. It's expensive, often $50 or more per sample, plus shipping. In the past, Agdia offered a reduced fee structure when samples were routed through plant diagnostic clinics, but this is no longer the case. As a result, the University of Illinois Plant Clinic recommends that growers and consultants work directly with Agdia, or similar companies, when seeking laboratory virus confirmation. Agdia's address is Agdia, Inc., 30380 County Road 6, Elkhart, IN 46514. Call them for the current "wheat screen" test price and for instructions on submitting the sample [(219)264-2014 or (800)622-4342]. You can also access Agdia on the Internet at http://www.agdia.com/testing.--Suzanne Bissonnette and Bruce Paulsrud

Author: Suzanne Bissonnette Bruce Paulsrud


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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