Sclerotinia stem rot (SSR), also known as white mold, is caused by the soilborne fungal pathogen Sclerotinia sclerotiorum and consistently ranks in the top ten diseases that reduce soybean yields in the United States (Bradley et al., 2021). Between the years of 2015 and 2019, yield losses from SSR amounted to 200,000,000 bushels of soybeans. In Minnesota, yield losses exceeded 37,000,000 bushels and approximately $334 million dollars were lost during these five years due to the disease (cropprotectionnetwork.com).
Managing SSR is complicated by a variety of factors. Crop rotation does not eradicate the pathogen since it can infect a wide range of crop plants. The hardened survival structures of the fungus, called sclerotia, can survive for many years in the soil and will initiate new infections when conditions are favorable. Chemical control can be effective, but there is a narrow range during which to apply fungicides (R1-R3 growth stages), and chemical control may not be economical if more tolerant varieties are planted (Dr. Damon Smith, badgercropdoc.com).
One of the most effective means to control any disease, including SSR, is the use of resistant cultivars. Scientific strides have been made in identifying genetic sources of SSR resistance through quantitative trait loci (QTL) mapping (Bastien et al. 2014; Vuong et al. 2008; Zhao et al. 2015). All resistance to SSR is partial (some degree of infection still occurs, even in tolerant cultivars), but disease severity can be significantly reduced when disease resistant cultivars are grown (McCaghey et al. 2017). Disease can also be reduced with wider row spacing and lower seeding rates, but these management strategies also result in lower yields (Webster et al. 2021).
Disease reduction with less canopy closure is likely due to a variety of factors including an altered micro-climate with less humidity and leaf moisture and a change in light quality and intensity. To cause disease in soybean, apothecia must first emerge. From the apothecia, infective ascospores are released. The formation of apothecia is very light dependent and requires exposure to wavelengths in the range of 276 and 319 nm (Thanning and Nilsson 2000). It is also likely that light dynamics in a closing canopy favor SSR development. The interaction of the soybean canopy with SSR development is an understudied area of research. If identified, plant architecture traits to reduce disease development may be useful for future resistance breeding efforts, especially when combined with physiological resistance.
To enhance disease resistance breeding in the in Minnesota, we propose foundational work to apply a panel of three Sclerotinia sclerotiorum isolates, collected throughout Minnesota, to comprehensively screen soybean line. We also propose developing and comparing field techniques for infesting research fields to conduct applied SSR research. Lastly, this project aims to define relationships between canopy architecture and S. sclerotiorum development, to provide another, underexplored consideration for disease resistance breeding to SSR.