Abstract
In the first and second reports, we confirmed varietal difference in resistance of alfalfa to southern blight, Corticium rolfsii, and discussed injury and evidence of the resistance for southern blight in selected strains for one generation. In the present paper we examined the increase of the resistance to southern blight based on the selection which was continued to 5th generation. Resistant plants of alfalfa to southern blight were selected in the inoculated field by maternal-line selection method and synthetic variety method. Selected resistant strains to southern blight showed more resistance than Natsuwakabe which was the main breeding stock. Synthetic strain CRSY 572-1 selected by maternal-line seletion method after passing four generations was the most resistant among all selected strains, and the strain CR 4871-5 selected by maternal-line selection method for five generations and the strain CRSY 541-1 selected by synthetic variety method after two generations followed them. As for the relation between the number of selection generation and the resistance to southern blight, the effectiveness of selection on the infected plant percentage increased slowly and linearly, but the effectiveness of the dead plant percentage was high in the early selection generations and low in the later ones. It becomes clear that the effect of selection for southern blight resistance was closely related to be the cumulative selection differential regardless of selection methods. And the heritability for southern blight resistance is relatively high (20.1〜58.4%) at the early and low (4.2〜12.0%) at the later selection generations. When rearranging these results, it is considered that effect of the selection for southern blight resistance is high at the early selection generations for fast gathering of the genes take part in the resistance. But, it is necessary that we select with a higher accuracy for more higher gathering of genes at the later selection generations.
