Friday, July 30, 2010

Scouting for Sudden Death Syndrome in Soybean

With soybean in the R4 to R5 growth stage in many areas of the state, we are starting get reports of plants expressing symptoms that for many (given the longer history in the state) would be identified as Brown stem rot (BSR) (Figure 1). A sample received into the lab earlier this week, however, showed symptoms of Sudden death syndrome (SDS) (Figure 2) and a check of our field trials under inoculated conditions within the past day is also showing symptoms of SDS. Weather conditions in 2010 have been very favorable for the development of SDS in the state with wet soil conditions during periods of planting followed by extensive rainfall during the flowering period.

Figure 1. Soybean plants expressing symptoms of Brown stem rot.

Figure 2. Early symptoms of Sudden death syndrome observed on July 29 at the West Madison ARS. The timing of scouting in soybean can help determine if symptoms are due to SDS or BSR.

What is SDS? Sudden death syndrome is caused by the fungus, Fusarium virguliforme. Foliar symptoms of SDS are similar to BSR so careful examination of plants is needed to differentiate the two diseases. It is also possible that both can occur in a soybean plant. Symptoms of SDS include a yellow to brown discoloration of the leaves around veins. Initially, these begin as small, circular spots. Examine the roots also since SDS can lead to a root root and these may be black in color. Also, there may be evidence of the pathogen on the root if you see a blue coloration (this is growth of the fungus). SDS does not lead to a brown discoloration of the vascular and pith tissues that is typical with BSR.

What are the risk factors that lead to SDS? The pathogen overwinters in soybean debris as chlamydospores, which as resistant fungal structures. Disease is favored by high soil moisture during vegetative growth and wet and cooler conditions around flowering.

What if I have SDS...what should I do? First of all, make sure to get a proper diagnosis. If you see evidence of SDS in the field, take a sample (including roots) and send it to the Plant Disease Diagnostic Clinic. After proper identification and if yield was impacted by SDS, consider the use of cultivars with increased resistance to SDS. Also, monitor conditions at planting to avoid cool soil temperatures that are favorable for infection by the pathogen and consider tillage to help increase soil temperature and drainage.

Further information about SDS is available here.