Field incidence and Sporecaster prediction of Sclerotinia stem rot ascospores and apothecia in various crops and rotations
Sampling ten similar fields for Sclerotinia sclerotiorum ascospores and apothecia was done to determine if different crops and crop rotation history had an effect on incidence of Sclerotinia Stem Rot inoculum. All fields were near Livingston, Wisconsin with similar soils, were within five miles of each other, and had some recent history of Sclerotinia Stem Rot. During the previous five years, each field had corn, soybean, and small grains in varied rotations, utilizing different management strategies for each crop. The forecasting application, Sporecaster, also was used to record the Sclerotinia Stem Rot risk level for all fields during the 2019 growing season to evaluate if Sporecaster risk levels matched apothecia development and ascospore release in actual field conditions. To trap ascospores, eight plates of potato dextrose agar supplemented with bromophenol blue and antibiotics were placed in each field sample area weekly during July and August, 2019. At the same time each week, multiple visual observations were conducted in each sample area for apothecia. Throughout the four week study, Sporecaster showed “low” risk for Sclerotinia Stem Rot, zero apothecia were observed, and zero confirmed colonies of S. sclerotiorum were isolated. This study revealed that Sporecaster worked as designed, but we were unable to properly evaluate the effects of crop rotation on the incidence of S. sclerotiorum apothecia and ascospores because of the lack of disease inoculum.