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Implications of Excessive Soil Moisture for Disease Development in Winter Wheat

Written collaboratively by Emmanuel Byamukama, former SDSU Extension Plant Pathologist and Shaukat Ali.

Some of the areas in the state are experiencing excessive soil moisture or flooding from the melting snow and the recent rainfall. Although it is too early to start thinking about disease issues in winter wheat at this time, current flooding in some areas may have implications on diseases that may develop in winter wheat.

One disease that is favored by excessive soil moisture in winter wheat is Pythium root rot. This disease is caused by a water mold, a fungus-like pathogen that produces motile spores that swim in water in saturated soils. However, the highest risk for this disease to develop is during and within the first few weeks after seedlings emergence. There is still a risk for this disease to develop when winter wheat breaks dormancy and there is new root growth under saturated soil. Pythium root rot developing during the season cannot be managed; however,  for fields with a history of Pythium root rot disease, use of seed treatments and improved drainage to control this disease in future seasons.

Another disease that can potentially develop due to floodwater is downy mildew. This disease is also caused by a fungus-like organism that depends on free water for infecting the plant. However, for this disease to develop, wheat leaves need to be submerged in water. The fact that, at the moment, wheat has not broken dormancy makes the risk for downy mildew development minimal. Downy mildew also has no in-season disease management practices. General sanitation and drainage reduce the risk for this disease to develop.

Wheat soilborne mosaic can also develop in parts of wheat fields with excessive soil moisture. This disease is caused by a virus called wheat soil borne mosaic virus (WSBMV) that is transmitted by a waterborne pathogen, Polymyxa graminis. Symptoms of this disease are more distinct later in the season when temperatures warm up. Young wheat plants are more at risk in the fall, but some limited infection may occur in spring when developing roots are submerged in water. Wheat soilborne mosaic is not a widespread disease in South Dakota, but flood waters may spread the vector in the soil.

Floodwaters may also spread pathogen-infested wheat residues as well other soil-borne pathogens. Growers are encouraged to scout winter wheat fields early to assess the extent of root rot and other early diseases that may develop because of floodwaters. Symptomatic wheat samples can be sent to the SDSU Plant Diagnostic Clinic (1205 Jackrabbit Drive, Brookings SD 57007; Phone: 605-688-5545), if confirmation of diagnosis is needed.