Planting Date, Fungicide, and Cultivar Effects on Sclerotinia Crown and Stem Rot Severity in Alfalfa

This study was conducted to determine the effect of planting date, fungicide, and cultivar on severity of Sclerotinia crown and stem rot (SCSR) and subsequent productivity of alfalfa. The cultivars Armor and A9109 were seeded no-till in May, early August, mid August, and late August 1993 and 1994 in a sod uniformly infested with sclerotia of Sclerotinia trifoliorum. Four applications of the fungicide vinclozolin effectively controlled the disease. Averaged over years and cultivars, disease severity (percentage of plot area affected) in the no-fungicide treatment was 4, 12, 23, and 41% for the spring, early August, mid August, and late August plantings, respectively. Armor had higher disease severity than A9109 in the 1993 seeding, but not in the 1994 seeding. Forage yield the year after seeding reflected differences in disease severity ratings. The risk of severe SCSR damage in no-till summer seedings of alfalfa can be reduced dramatically if stands become established early enough so plants reach at least 10 weeks of age by the time apothecia emerge in the fall.