SDS Disease Cycle and Epidemiology
Monday, August 31, 2015
By: Andreas Westphal1, T. Scott Abney2, Lijuan Xing1 and Gregory Shaner1,
1 Purdue University, and 2 USDA-ARS, West Lafayette, IN.
The fungus may infect roots of soybean seedlings soon after planting, but above ground symptoms of SDS in the midwestern USA rarely appear until mid-July, when soybean plants have reached reproductive stages. At this later time, the fungus penetrates into the vascular tissue of the plant (Roy et al., 1997). The fungus produces toxins in the roots that are translocated to the leaves. Often, symptoms first appear after heavy rains during reproductive stages; high soil moisture increases the disease severity (Xing and Westphal, 2006). The toxins cause foliar symptoms; the fungus itself does not invade the stems more than a few centimeters above the soil line (Roy et al., 1997).
The first noticeable symptoms of SDS are yellowing and defoliation of upper leaves. When symptoms first appear in a field, they may be confined to a few small areas or strips in the field, often in wetter or compacted areas, such as turn rows. Over the following two or three weeks, affected areas may enlarge and plants in other areas in the field may show symptoms. The extent of yield losses due to SDS depends on the severity and timing of disease expression relative to plant development in regards to yield components. If the disease develops early in the season, flowers and young pods will abort. When the disease develops later, the plant will produce fewer seeds per pod or smaller seeds. The earlier severe disease develops, the more the yield is reduced. Because the SDS fungus can persist in soil for long periods, larger areas of a field will show symptoms of the disease each growing season until most of the field is affected.