Leaf rust has been reported at multiple areas of the state, suggesting that fall infection is widespread this year. Leaf rust infection is common in Kansas but the disease rarely reaches damaging levels.
There is no need for fungicide applications this fall, because the disease only survives the winter in 30 to 40% of the years in Kansas. Winter survival of leaf rust is favored by above-normal temperatures and snow cover. Snow cover is important because it insulates the wheat from cold temperatures and drying winds that cause tip dieback and desiccation of wheat leaves.
This loss of leaf area is not critical to the plant because it will produce a lot of new growth in the spring. However, the loss of leaf area is important for the survival of the leaf rust, because the rust fungus needs a living host to survive. The loss of leaf area during the winter months often results in mortality of the leaf rust fungus.
It is helpful to know that leaf rust is present in the fall because this information can help us establish scouting priorities next spring. The risk of severe disease increases significantly when leaf rust survives the winter locally and becomes established on the new growth next March.
Figure 1. Leaf rust on wheat this fall. Photo by Erick DeWolf, K-State Research and Extension.
Wheat Streak Mosaic
I have also received multiple reports of wheat streak mosaic this past week. The images submitted by area Extension specialists, county agents, and consultants are consistent with this viral disease, but I am waiting for laboratory confirmation of the virus. In nearly all the reports, the infection of wheat streak mosaic can be traced back to volunteer wheat and/or early planting dates. The volunteer wheat serves as a reservoir or "green bridge" for the virus and wheat curl mites that spread the disease.
Unfortunately, there is nothing that can stop the development of the disease or activity of the curl mites once the problem has begun. Some producers have asked about the possibility of destroying a wheat stand and replanting this fall. The replant option seems risky. We need time for the current wheat stand and the mite population to die before we could replant. This delay could result in late-planted wheat with a poor yield potential. It is hard to do, but it is probably best to wait and evaluate the yield potential next spring. If yield potential is poor because of disease, it might be possible to use the wheat as a forage crop or consider switching to another crop in the spring.
Figure 2. Wheat streak mosaic this fall. Photo by Jeanne Falk Jones, K-State Research and Extension.
Erick DeWolf, Extension Plant Pathology