育種学雑誌

Alfalfa varieties and strains were evaluated for their resistance to Sclerotileia trlfoliorum ERIKS., the casual agent of Sclerotinia crown and stem rot (SCR) which is a serious disease in areas characterized by a warm and wet climate. Three experiments were carried out from 1983 to 1985, one under natural conditions for the infection and two under artificial inoculation in test fields. For the tests under artificial inoculation, all the varieties and strains were sowed in the field in September and were inoculated with cultured hyphae of SCR on the basal part of the plants in December. The disease severity was observed after 4 - 6 weeks. The symptoms of the disease were enhanced by the use of covering treatments after inoculation, and varietal differences in the severity of the disease were recognized based on the percentages of the plants which survived (Tables 3, 4, 5). A high correlation coefficient (r = - 0.798^**) was obtained between the survival rate under artificial inoculation and the disease severity under natural infection (Fig. 1). It was suggested that this inoculation method which used hardening under natural conditions enabled to evaluate the resistance to SCR.

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Inheritance of ripening time of Japanese pear was investigated using twenty full-sib families. Frequency distribution in each family could be approximated to normal distribution, indicating that this character was inherited polygenically (Fig. 1). Significant regression coefficients and correlation coefficients were obtained between mid-parent and offspring; the regression was in the range of 0.867∼1.045, the correlation being 0.916∼0.985 (Table 3). Both regression coefficients and correlation coefficients, obtained between mid-parent and offspring in the full-sib population, are generally recognized as valuable genetic parameters. Especially, the regression of offspring on mid-parent in the random mating population is expected to be equal to the estimate of the heritability. Therefore the eritability on ripening time is considered to be extremely high (Table 3). Variance analysis indicated that variances between and within family were significant. Variances due to year and interaction between family and year were not significant (Table 7). The ratios of variance due to family, year, interaction between family and year, individual and error to the total variance were estimated to be 0.341, 0.003, 0.005, 0.612 and 0.039 respectively. Arnong the ratios of variance cornponent, those due to family and individual were both high (Table 8). It is concluded that ripening time in Japanese pear population has a heritability as high as 0.95 (Table 3), and environmental variance of this character is relatively small(Table 8).

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