Soybean Disease Outlook for 2006

Craig R. Grau

Department of Plant Pathology

Weather conditions are a significant factor that influences the incidence and severity of soybean diseases each year.  Although soybean rust has garnered much attention, we must not lose sight of diseases that have plagued the Wisconsin soybean crop for years.  As predicted by many, May 2006 has been wet and cool and the summer outlook is for dry and hot conditions due to the return of La Nina (The Corn and Soybean Digest; Mid-March 2006 issue).  This sequence of weather conditions can provide insight on potential disease problems for 2006.  Soybeans planted prior to May rains will have been stressed due to seedrot and seedling pathogens.  Growers that planted early and used fungicide treated seed should discern a benefit from this practice.  Delayed planting due to wet and cool May reduces the risk of white mold (see previous issue of Wisconsin Soy Sentinel), but increases the risk of foliar disease including soybean rust.  However, hot and dry conditions are predicted for the summer of 2006 and should disfavor development of soybean rust.  If dry and hot conditions prevail this summer, watch for stem canker.  Stem canker was severe in 2003 and again in 2005, signifying hot and dry conditions as favorable for this disease. 

Delayed planting into warmer soils raises the risk of Phytophthora root rot, especially if rain occurs during seedling emergence.  For growers planting into warm and wet soils, the upcoming weeks are an excellent time to assess the effectiveness of management practices and soybean varieties designed for the management of Phytophthora root rot.  A review of “2005 Wisconsin Soybean Variety Test Results” (UWEX A3654) indicates that three resistance genes (Rps) are commonly present among soybean varieties available for Wisconsin growers. The Rps1a gene has been used for many years by soybean breeders, but is not effective in 95% of the fields tested in past surveys and is considered an outdated resistance gene.  The Rps1c gene is effective 75% of the time, but the most effective gene for protection against Phytophthora root rot is the Rps1k gene.  Although the Rps1k gene has lost efficacy in many neighboring states, failure of the Rps1k gene has been observed only once in Wisconsin.  There is currently no indication of widespread failure of the Rps 1k gene, but soybean fields should be monitored for the performance of varieties containing this gene.  The Rps1k gene is our most effective source of resistance until newly discovered genes can be incorporated into commercially acceptable varieties. Fungicide seed dressings containing formulations of metalaxyl or mefenoxam can supplement partial resistance. The North Central Soybean Research Program (NCSRP) and the Wisconsin Soybean Marketing Board (WSMB) are funding Phytophthora research at the University of Wisconsin-Madison.  Additional information on Phytophthora root rot and other soybean diseases is available on the WSMB funded Soybean Plant Health Website (http://www.plantpath.wisc.edu/soyhealth/index.htm)

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Photo:  Symptoms of wilt and brown stem lesion advancing from soil line associated with Phytophthora root rot of soybean.  Photo: Craig Grau