Conditions Favorable for Sclerotinia Stem Rot Development

 Paul Esker, Extension Field Crops Plant Pathologist 

In assessments made yesterday (11 August 2009) at a trial at the West Madison Agricultural Research Station, we noted symptoms associated with Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum (Figure 1). The level of incidence was still low in those plots. In spite of the dry weather during the early summer period in many parts of the state, the amount of dew has been high, thus increasing the risk for SSR. The critical period for infection was during flowering. Seasonal risk of SSR is a function of weather, early canopy closure, a field history of SSR, and soybean variety.

Figure 1. Symptoms of SSR, including the fluffy white mycelium.

Why have conditions been more favorable for SSR? As reported by NOAA, July temperatures were either the coldest, or in the top ten coldest months for the majority of the states in the region (Figure 2). The growth of the fungus is favorable by temperatures < 85ºC. Moisture conditions favorable for SSR development, especially during the flowering to early pod development period, can include normal to above normal rainfall, high soil moisture (field capacity or higher), and/or prolonged periods of fog and leaf wetness during the morning.

Figure 2. July 2009 air temperatures for the continental United States. In Wisconsin, this was the second coldest July on record (Source: NOAA).

In attempting to put things in perspective, the current weather situation reminds Dr. Craig Grau of conditions similar to 1992, when SSR was widely prevalent throughout the North Central Region. Additionally, there have also been increased reports of SSR throughout the North Central Region (for example, see Ohio C.O.R.N. Newsletter for August 10-17).

In 2009, we established new trials at the Arlington, Marshfield, and West Madison Agricultural Research Stations to improve our understanding of the efficacy of different foliar fungicides for control of SSR, including some potential new active ingredients, as well as the use of the biocontrol agent Coniothyrium minitans. In particular, our interest in C. minitans is to understand how well the organism establishes in the soil. Results from these different trials will be reported in later articles for Wisconsin Crop Manager and during winter meetings.

For further information about SSR, http://www.plantpath.wisc.edu/soyhealth/cause.htm.

For an overview of managing for SSR based on field history: http://www.plantpath.wisc.edu/soyhealth/wmmgmttab.htm.