Abstract
Breeding of alfalfa (Medicago sativa L.) for resistance to sclerotinia crown and stem rot (SCR, causal agent: Sclerotinia trifoliorum Eriks.), which is a serious disease in Japan, was initiated in 1983. Increase of resistance to this disease has been examined following repeated selection of surviving plants from artificially inoculated field plots to the 9th generation. The strains selected for SCR resistance showed a higher resistance than Natsuwakaba and Tachiwakaba which were used as breeding materials and control cultivars. The effectiveness of selection based on the survival rate of the progenies increased gradually as generations in which the recurrent selection was performed advanced. Realized heritability was low in the 1st and 2nd generations in which selection was performed but was high in the 3rd to 9th generations. It is considered that the selection effect on the SCR resistance was enhanced in the subsequent generations. The results of these studies may indicate that recurrent selection is effective for the accumulation of resistance genes with minor polygenic effect, especially in the case of breeding work for the improvement of characters, which had been considered almost impossible by short-term selection trials.
