Half diallel analysis for resistance of sugar beet (Beta vulgaris L.) to damping-off caused by Aphanomyces cochlioides

Damping-off, which is caused by Aphanomyces cochlioides Dreschsler, is known to be one of the major factors responsible for a poor stand of sugar beet. In the case of direct sowing culture, damping-off results in a lack of harvestable plants and severe loss of yield, therefore, the occurrence of this disease is a major problem. We initiated the breeding program for resistant cultivar to damping-off disease. In order to clarify the inheritance of this resistance trait, we performed a half diallel analysis. Five selfed O-type lines that differed in their resistance to damping-off, half diallel crossed Fsub(1), and three standard cultivars were used as materials. Two weeks after sowing, the seedlings were inoculated with zoospore suspension. Four to five weeks after sowing, each seedling was classified according to the severity of the disease into the 6 stages (0-no symptoms; 1-clear symptoms; 2-light; 3-intermediate; 4-severe; 5-dead). The disease index (DI) of each line was calculated as the average. Genetic analysis was carried out by the program “DIALL” developed by Ukai (1989). The DIs of the parental lines ranged from 2.0 (NK-237BRmm-O) to 4.2 (NK-195mm-O). The DIs of all the Fsub(1) progeny ranged from NK-237BRmm-O to NK-195mm-O. The results of analysis of variance, which was conducted according to Jones's formula (1965), indicated significant additive and dominant genetic effects. Among the dominant effect, b1 (refers to general dominance) was not significant but b2 (refers to further dominance due to a particular parent) was significant. The graphical analysis of Wr/Vr revealed that epistasis was not detected, and clarified that the additive-dominance model fitted to the disease resistance. NK-229BRmm-O had the most dominant genes. However, the DI of this line was 3.9, and it was classified as susceptible one. On the other hand, the resistant line NK-237BRmm-O had the most recessive genes. Moreover, none of the NK-237BRmm-O Fsub(1) progeny was classified as resistant. Therefore, in order to breed resistant Fsub(1) cultivar, both parent lines should be resistant. The average degree of dominance was 0.947, and the disease resistance was a completely dominant trait. The heritability values considered in the broad sense and narrow sense were 0.906 and 0.629, respectively. This result indicates that the resistance to damping-off may improve efficiently. Moreover, in order to increase the resistance of a line to the damping-off effect, it may be useful to select for genes with the additive effect that control disease resistance.