Ecological and quantitative studies of occurrence of soybean Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, in the north-central region of the United States

Since the early 90's, Sclerotinia stem rot of soybeans (SSR), caused by Sclerotinia sclerotiorum (Lib.) de Bary, has emerged as a serious problem in the north-central soybean production region of the United States. Understanding the epidemiology of Sclerotinia sclerotiorum and developing models that explain, and eventually may forecast, the risk of SSR occurrence in the region may help extension specialists and growers manage the disease. Regional prevalence of soybean SSR was modeled using historical data collected between 1995 and 1998 from 4 states of the North-Central Region of the United States (Illinois, Iowa, Minnesota and Ohio). Tillage practices, soil texture, and summer weather variables from the National Oceanic and Atmospheric Administration (average monthly air temperature and total precipitation during July and August) were used as input variables. Logistic regression was used to estimate the probability of stem rot prevalence in the four states. A developed model had high explanatory power (77.8%). Bayesian analysis suggested that the four-year data set used in the analysis may not be informative enough to produce reliable estimates of the effect of some explanatory variables on SSR prevalence. The variable most sensitive to extra-sample incorporated information was precipitation of August. The relationships between management practices, weather variables and soybean yield were examined using multiple linear regression to investigate whether high potential yield environments are also high SSR risk environments. Occurrence of SSR was indeed found to be strongly associated with high attainable yield. Differences in soil temperature and water potential have been observed under different combinations of tillage regimes and planting row widths, with soil surface temperature and moisture fluctuating more under the combination of no tillage and wide rows than under other combinations. Experiments were conducted to determine the effects of soil temperature and water potential fluctuations on sclerotium germination, and apothecium production of Sclerotinia sclerotiorum. Our results showed that small temperature fluctuations increased sclerotia germination and apothecium production compared to large or no temperature fluctuations. Moisture fluctuations were detrimental to sclerotium germination and apothecium production, with the constant saturation treatment yielding the highest number of germinated sclerotia and apothecia.