Wheat Problems Continue to Expand

May 5, 2000
Reports of wheat streak mosaic virus (WSMV) continue to come in from the wheat-growing areas in the southern part of the state. I just received the first report of WSMV symptoms from Iroquois County this morning, so it would seem the epidemic may not be limited to the southern part of the state. The southwestern part of the state seems to have been hardest hit by the epidemic. Omar Koester, Extension assistant, Randolph/Monroe Unit, and Mark Hoard, Dennis Epplin, and Robert Bellm, Regional Extension IPM and crop educators, have put in considerable time with regard to the virus epidemic this season. Please refer to the previous two issues of the Bulletin for further discussion of symptomatology and epidemic development.

Unfortunately a few other diseases are also present in the wheat crop. Ria Barrido of Growmark reports both barley yellow dwarf virus and a fungal root disease, take-all, on wheat in Perry County. These are diseases we are certainly more familiar with in Illinois.

Barley yellow dwarf virus (BYDV): Aphids spread BYDV disease. Aphids carrying the virus transmit the virus to wheat plants through their saliva when they feed. Serious yield loss results from fall infection by viruliferous aphids feeding on wheat seedlings. Fall infections typically are expressed as stunted plants with fewer tillers when spring growth resumes. Leaf discoloration is usually the most notable early-season symptom. Leaves may be varying shades of red to purple, pinkish-yellow to brown. As the plant continues to grow, older leaves typically begin to die back from the tip and may feel somewhat leathery while the new leaves begin to discolor. Symptoms from fall infections should have shown up 1 to 1 1/2 months ago. Spring infections occur as well but commonly only discolor the flag leaf and do not cause significant yield reductions. This is a similar situation to wheat streak mosaic where spring infections are not nearly as significant as fall infections.

The most common method of virus management is to plant resistant wheat varieties. There are very limited choices with regard to BYDV in that arena, though. Other control measures are directed at reducing the time the plants are in the field when vectors are active, which explains the recommendation to plant after the fly-free date when insect activity is reduced. Systemic insecticide seed treatments such as Gaucho have also shown some success.

Yield loss from foliar blights in wheat: Recent timely rains may have been conducive for development of foliar fungal blights on wheat crops. Plants already infected with virus disease can also host fungal infections. Keep an eye out for fungal leaf blight development this week. How can you decide if the infection warrants fungicide treatment? With wheat about $2.30 as I write today, it isn't likely that a fungicide application is a good economic decision, although it may still be a good disease-management decision.

The flag and flag-1 leaves contribute the most to final yield of the plant. In general, you can expect a yield loss in the range of 1 bushel per acre for each 5% leaf tissue infected on either of these two leaves. To make your decision whether or not to spray, you will need to determine your yield potential. A rough way to determine yield potential is to count the number of tillers per square foot and multiply by 1.5. Then consider that fungicides can generally provide yield increases of 10% to 20% in fields where diseases are economically important compared to an untreated field. Next determine the cost of the fungicide plus application at approximately $16 to $26 per acre, depending on material and type of application. With wheat at $2.30, yield increases of 7 to 11 bushels would be needed to cover the cost of treatment. For this scenario you would need approximately a 70- to 100-bushel yield potential. Putting a pencil to it with your own cost estimates and yield potential will help you make the decision whether a fungicide application will benefit you this season.

That is enough news on wheat disease this week. Look for a discussion of take-all in next week's Bulletin.--Suzanne Bissonnette

Author: Suzanne Bissonnette