INCREASE OF RESISTANCE OF ALFALFA TO SCLEROTINIA CROWN AND STEM ROT THROUGH RECURRENT SELECTION

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 started in 1983. Increase o f resistance to this disease has been examined following repeated sele ction of surviving plants from artificially inoculated field plots to the 9th generation. The strains selected for SCR resistance showed a h igher resistance than Natsuwakaba and Tachiwakaba which were used as b reeding materials and control cultivars. The effectiveness of selectio n based on the survival rate of progenies increased gradually as gener ations in which the recurrent selection was performed advanced, Realiz ed heritability was low (h(2)=0.078) in the 1st and the 2nd generation s in which selection was performed but was high (h(2)=0.364) in the 3r d to 9th generations. It is considered that the selection effect on SC R resistance became higher in the later generations, probably due to t he accumulation of genes which have minor or polygenic effects on resi stance.