A number of diseases can develop in winter wheat and their effective management involves taking some actions before planting. Some of the diseases that can develop during the fall include Wheat streak mosaic, root rots, fungal leaf spots, and stripe rust.
Wheat Streak Mosaic Virus (WSMV)
Wheat streak mosaic can be severe, leading at times to total loss where a wheat field is plowed under because of stunted plants that cannot be harvested by a combine, or even when the yield is so poor that it is not worth combining (Figure 1).
Wheat streak mosaic disease is caused by a virus, called Wheat streak mosaic virus (WSMV). This virus is transmitted by a tiny mite, the wheat curl mite (WCM) that can only be seen at least with 20x magnification (Figure 2).
The WCM is unable to move long distances on its own (WCMs do not have wings), they are picked by wind and deposited in nearby areas as the wind dies down. The WCM’s feeding alone is not as detrimental although heavy feeding can cause leaf curling, hence the name wheat curl mite. The main damage done by WCM is transmission of WSMV.
Once WCMs are deposited on the leaf surface, they move to the inner whorl near the growing point where they are protected from harsh environment.
Management of WSMV starts with understanding the origin of the inoculum. This virus is not spread through soil or residues and rather through WCMs when they move from infected sources (volunteer wheat, grassy weeds) to healthy young wheat plants. Therefore, while practices like rotation can be effective in management of several residue- and soil-borne diseases, rotation won’t be as effective if the neighboring field had WSMV.
- Because the WCMs which spread the virus come from volunteer wheat and grassy weeds, managing requires eliminating the source. Destroy any volunteer wheat and grassy weeds (the “green bridge”) at least two weeks before planting wheat in the fall. This can be achieved by treating with a desiccant like a herbicide or through tillage. Ensure that all green areas of treated volunteers have browned because WCMs can move to remaining green patches on treated plants as they brown. If wheat is to be planted next to a field previously planted with wheat, the volunteer wheat and grassy weeds in this field should also be destroyed.
- Delay wheat planting in the fall for areas with a history of recent WSMV outbreaks. Planting early in fall increases the chances of WCM moving off drying wheat or grasses, to young emerging wheat in the fall. Infections with WSMV that take place in the fall cause the most significant yield losses.
- Plant WSMV resistant/tolerant wheat cultivars. Rating for WSMV for various cultivars can be accessed in the 2016 South Dakota Winter Wheat Variety Trial Results.
- For areas with history of WSMV, avoid rotating wheat with small grain crops like pearl millet, oats, barley, etc. Rotate with broadleaf crops like sunflower, field peas, lentils etc.
Herbicide Resistance Info
Root Rot Diseases
A number of root rot diseases caused by a number of fungal pathogens can develop in wheat. These may cause poor stand establishment, when the seeds or seedlings are rotted, however, most often the root rot pathogens infect the plant but symptoms are not seen until around heading time.
Symptoms of root and crown rot seen at the time of heading include bleached wheat heads which may be confused with other diseases such as Fusarium head blight. For fields with a history of poor stand establishment due to root rots, seed treatment may be recommended.
Leaf Spot and Rust Diseases
In some years when the fall weather is mild and a hard freeze comes in late fall, winter wheat may have substantial tillering and several leaves (as was last year). Foliage growth in the fall can get infected with leaf spots diseases mainly tan spot (Figure 3). The falling temperatures during this season can also be conducive for stripe rust to develop.
Producers wonder if a fungicide could be applied in the fall. Since the leaves would soon be lost due to freezing conditions over the winter period, it may not be worth the investment protecting leaves that will soon be lost anyway.
Other management practices that limit disease development
Crop rotation helps break the disease life cycle. Wheat should follow a broad leaf crop.
Plant resistant/tolerant cultivars. While resistance may not available for some of the diseases, tolerant cultivars to several diseases are available. Consult the 2016 South Dakota Winter Wheat Variety Trial Results for ratings of different cultivars.