Monday, August 17, 2009

Soybean Disease Update

Sclerotinia Stem Rot: Over the past week, we have continued to receive numerous questions regarding Sclerotinia stem rot (aka, white mold) and the control of this disease using foliar fungicides. In most soybean fields, we are at the R4 growth stage, or full pod. Fungicides are not recommended at this time for managing white mold. The key point to always remember is that infection by the pathogen that causes Sclerotina stem rot occurred during the flowering period. Sclerotinia stem rot is a monocyclic disease. This means there is only a primary cycle for infection. There is no secondary spread of the disease; although plants that are adjacent or touching an infected plant may become diseased.

Sudden Death Syndrome: Also in the past week, we have also received reports and have seen symptoms associated with Sudden death syndrome (SDS), caused by Fusarium virguliforme. As a reminder, this is a relatively new disease in Wisconsin and the foliar symptoms of SDS can easily be confused with Brown stem rot (BSR) (Figure 1). Symptoms of SDS include a yellow to brown discoloration of the leaves around veins. These often begin as small, circular spots. A differentiating characteristic to BSR us that roots may be black and rotted with a slightly blue hue (growth of the fungus). Conditions in Wisconsin during the 2009 growing season have been favorable since there was ample soil moisture during early vegetative growth in many parts of the state as well as the cool temperatures around flowering.

Figure 1. Images showing symptoms of Brown stem rot (top) and Sudden death syndrome (bottom).