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Predicting the Risk of Sclerotinia Stem Rot of Soybean
Mon - June 6, 2016
Apothecia of Sclerotinia sclerotiorum
Apothecia of Sclerotinia sclerotiorum
by Mehdi Kabbage and Damon Smith, Soybean Plant Pathologists, University of Wisconsin.

Sclerotinia stem rot (SSR), also called white mold, is a sporadic but yield-limiting disease in the north-central region. Sclerotinia stem rot develops when soybean flowering coincides with cooler temperatures (less than 82°F) and extended periods of high humidity and surface wetness from rain or dew. If these specific weather conditions are absent, then SSR is unlikely to occur.

The sporadic nature of Sclerotinia stem rot makes this disease difficult to control. A combination of management strategies is the best approach. These include reduced tillage, crop rotation (especially with small grains), canopy management to reduce leaf wetness (such as wide row spacing or lower plant populations), and fungicides. One of the objectives in our current work on Sclerotinia stem rot, funded by the North Central Soybean Research Program, is to more clearly define the effect of weather and application timing on the efficacy of fungicides.

This is our second year of fungicide trials in Iowa, Wisconsin, and Michigan soybean fields with a history of white mold. We will rate currently-labeled fungicides (see Fungicide Efficacy for Control of Soybean Foliar Diseases) and some recently-identified antifungal formulations for their effect on white mold development and soybean yield. We plan to develop return-on-investment estimates for the fungicides we test at the end of the 3-year project.

We are also validating a new spray prediction model that uses weather data to predict the amount of the spore-producing structures (apothecia) that must be present during the soybean flowering period in order for SSR to occur. By predicting when and if high spore numbers are present during the flowering period in soybean, more accurate timing of fungicide applications can be made to prevent SSR in years when conditions are favorable.

In the first year of validation (2015), conditions were favorable for the development of SSR and the model recommended the first fungicide spray at the time when apothecial numbers were increasing and just prior to heavy spore catches. This allowed adequate time to apply fungicide to protect plants prior to the conducive infection periods. The second application was also recommended with similar accuracy, during a second increase in apothecial numbers (Figure 1).

A beta version of an on-line risk map will be piloted in the 2016 field season. In subsequent seasons, we hope to provide a public online tool that growers and agronomists can use to make fungicide application decisions. A savings of just one SSR fungicide application could result in at least a $20/acre savings to a soybean farmer.

For the most current information on the management of white mold, please read White Mold,  and view the video Soybean White Mold (Sclerotinia Stem Rot), at the Plant Management Network website.