The relationship between the causal agent of SDS, SCN
Cross Relationships: The relationship between these two diseases has been studied for almost 30 years and has yielded inconsistent results. Studies have shown positive associations between SCN and SDS foliar symptom development, where more severe SDS symptoms occur when SCN is present (McLean and Lawrence, 1993; Melgar et al., 1994; Roy et al., 1989; Sherm et al., 1998; Xing and Westphal, 2006). However, other studies report weak or no association between SDS symptom development in the presence of SCN (Gao et al., 2006; Roy et al., 1993; Sherm et al., 1998). The relationship between the actual presence of the SDS fungus in the soil as it relates the presence of SCN has been under-studied.
More research is needed to not only understand the relationship between SDS and SCN but also specifically between the two causal agents of these diseases. In WI, we have the unique opportunity to address this relationship. Until recently, the distribution of the SDS fungus throughout WI was not well understood, while the range of SCN was well documented in the state. We conducted a study to determine the presence of both causal agents, SCN and SDS fungus, from commercial soybean fields in WI and to determine if establishment of these two causal agents in production soybean fields is correlated.
Sample Collection Program: This study was possible through the checkoff-funded Wisconsin Soybean Marketing Board (WSMB) SCN soil testing program which offers free testing to WI growers. Soil samples that were voluntarily submitted during the 2011 and 2012 growing seasons were tested for SCN by wet-seiving methods and for the SDS fungus using DNA based detection methods.
WI Results: Sample submission totals are presented in table below for 2011 and 2012. In 2011, 56 of 135 (41.5%) samples were positive for SCN while 10 of 135 (7.4%) samples were positive for the SDS fungus. In 2012, 64 of 318 (20.1%) samples tested positive for SCN while 13 of 318 (4.1%) tested positive for the SDS fungus.
Wisconsin counties testing positive for SCN in 2011 and 2012 were representative of the confirmed SCN-positive region of the state. Soil samples where both SCN and the SDS fungus were found in the same sample occurred infrequently (data not shown), and counties where both SCN and the SDS fungus were found were not common. Our results also show the SDS fungus was found in counties farther west and north of the area where Bernstein and colleagues first found the pathogen.
Conclusions and Recommendations
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