No. 8/May 16, 1997
Wheat Disease Update
Writing strictly as a plant pathologist, I find that this season is not off to a very exciting start. The wheat, corn, and soybean crops appear to be relatively disease-free at this time, and conditions do not seem to favor the development of disease problems. Hopefully, this will continue and the season will be one of only light disease pressures.
The wheat crop is now maturing in much of the southern part of Illinois. Reports from Noel Troxclair, IPM Extension educator, and plant pathologists at the University of Kentucky indicate very few disease problems. The only significant one is the presence of wheat yellow mosaic virus, formerly known as wheat spindle streak virus. This virus is favored by cool temperatures and causes yellow parallel streaks in the leaves of wheat plants. The infected plants tend to remain small, even when growing conditions improve. Plants infected with another virus, soilborne mosaic, may also appear yellowish initially but recover when temperatures warm. Thus, patches of yellowish plants in fields that do not improve in growth and in color as the season progresses probably have yellow mosaic virus, and those that do improve were infected with soilborne mosaic virus.
Testing for identification of viruses involves lab procedures and cannot be made accurately in the field. Symptoms of a virus include off-color leaves, rosetting of plants, reduced tillering, and poor growth. These symptoms can help identify a virus problem, but determining which virus is infecting plants must be done in a lab. A quick test to help determine if soilborne mosaic or yellow mosaic is present involves digging whole plants and putting them (with soil) in a container in a warm, sunny location. If the new growth is green and healthy, then the plant probably has soilborne mosaic. If no recovery is noted, then it is likely that yellow mosaic is present.
What to do about viruses in wheat? Unfortunately, there is little to be done with infected plants. No sprays are effective against viral diseases. Producers must determine the potential yield and assess whether or not to keep the stand. Viral infections should not be the only criterion used because some stands may appear to have little potential now but will recover as growing conditions improve.
In southern Illinois, Noel Troxclair has reported low levels of foliar diseases on wheat. Septoria leaf blight is present, but only on the lower leaves. Fungicides should not be applied until this pathogen is seen at the leaf directly below the flag leaf. At that time, applying a protectant such as a mancozeb-containing fungicide or a tank mix of mancozeb plus Benlate will protect from additional infections.
H. Walker Kirby, Department of Crop Sciences, (217)333-8414