Abstract
Sclerotinia crown and stem rot caused by Sclerotinia tri-foliorum Eriks. is a serious disease of alfalfa (Medicago sativa L.). Alfalfa plants of all ages are susceptible to this disease, but the damage is most severe at the seedling stage. Two experiments were carried out to determine the critical seedling stage and growth conditions under which the resistance was expressed most strongly among varieties and strains of alfalfa. Infected tissues were observed cytologically in individuals of selected SR-strains and commercial varieties. They were inocu-lated with the cultured hyphae of two isolates, one from the Aichi field and the other from the Tochigi field. When alfalfa varieties and strains grown under the low temperature/short day regime were inoculated with Sclerotinia trzfoliorum, all of them showed low survival rates in younger plants less than 6-wk-old, but variable rates in 10-wk-old plants, and the 10-wk-old plants of selected strains survived better than commercial varieties.
