カンキツかいよう病抵抗性の遺伝

Abstract

Inheritance of resistance to bacterial canker disease caused by Xanthomonas campestris pv. citri in citrus was studied using 4, 523 F1 seedlings obtained from 61 crosses.When greenhouse-grown F1 seedlings were 4 months old, the leaves were inoculated with bacteria in August in a greenhouse. Resistance to bacterial canker of each seedling was evaluated by an early evaluation method developed by the authors (Matsumoto and Okudai, 1988); 100 to 120 days after inoculation the size of the lesion on the leaves was measured and the seedlings were classified as either resistant or susceptible, according to the size of the lesion being smaller or larger than that on nucellar seedlings of Natsudaidai which shows an intermediate degree of resistance.The F1 segregation ratios on 61 crosses showed that the resistance was conferred by a single dominant allele. The genotype of Hyuganatsu, ponkan, satsuma and Tosa-buntan was dominant homozygous, that of Yuzu, Sweet Spring, Kiyomi, Hassaku, Natsudaidai, Clementine and Hirado-buntan was heterozygous, and the recessive homozygous cultivars were Fukuhara orange, grapefruit and Clement.The dominant homozygous and heterozygous cultivars determined by the above-mentioned early evaluation method ranged from highly resistant to moderately susceptible to bacterial canker disease under field conditions. The moderately susceptible cultivars showed a degree of canker resistance similar to that of Natsudaidai, while the recessive homozygous cultivars were more susceptible, and were classified as susceptible to highly susceptible in the field. Thus, the results obtained from the early evaluation method using young seedlings were in good accordance with those from field observation. This confirms the validity of using Natsudaidai to classify the seedlings into R (resistant) and S (susceptible). The possible relationship between this resistance gene and endogeneous resistance-related substances in citrus was discussed.