Late Season Soybean Diseases

The 2011 growing season has been one of great variability in terms of the weather and its impact on soybean diseases. Early season conditions were cool and wet planting which was then followed by high heat and humidity. Now, as reported in the August 19th blog posting, abnormally dry conditions are being reported in several areas of the state. Thus, a common question as we get closer to harvest is, “Given these variable weather conditions, what is the potential effect of soybean diseases on yield?” Based on recent reports and our observations of different field trials, we highlight below several diseases that are popping up in soybean fields and provide specific links for further information about each disease.

In spite of the heat during flowering in many areas, we have seen white mold occurring. This is not necessarily a surprise as we do find the disease each growing season. Based on our current observations and reports though, it appears that the disease intensity is low in many locations, although variation to the soybean variety is being noted.

Also, over the past week to two weeks, we have seen an increase in Frogeye leaf spot. This disease, caused by the fungus Cercospora sojina, was observed in 2010 across the state and conditions in 2011, especially the hot and humid periods that occurred a few weeks ago, were favorable for disease development. In some of the plots that were recently visited, the severity of the disease appears higher than what we observed in 2010, however, levels still appear to be low.

Similar to some of the conditions we saw in 2008, we have seen fields and soybean plants with symptoms of either Phytophthora root and stem rot or stem canker. These two diseases can easily be confused for one another so it is important to make sure a proper identification.

Last, but not necessarily least, we have had several reports and also seen symptoms of sudden death syndrome (SDS). Reports are still being compiled, but based on our observations, the severity of SDS is probably going to be lower than in 2010. With soybean in the R6 growth stage in many areas, it is also important to make sure that the disease identification is correct for SDS, since foliar symptoms are similar to brown stem rot (BSR). In particular, make sure to examine the whole plant, including stems (looking for internal browning due to BSR) and roots (looking for a root rot and also a bluish hue that is the fungus of SDS). Do not just rely on the foliar symptoms to verify your diagnosis of either disease.