Diallel analysis for resistance of melon (Cucumis melo) to Fusarium wilt caused by Fusarium oxysporum f.sp. melonis race 1,2y

Melon production in Hokkaido has been severely hampered since Fusarium wilt of melon caused by Fusarium oxysporum f.sp. melonis race 1,2y was first reported there in 1993. We immediately initiated a rootstock breeding project for race 1,2y resistance to meet farmers' demand. Diallel analysis of race 1,2y resistance was performed using six netted melon (C. melo var. reticulatus Naud.) varieties and one oriental pickling melon (C. melo var. conomon Makino) variety. At 11 days after seeding, seedlings were inoculated by dipping their roots in a conidial suspension (105 per ml) of the race 1,2y. Each seedling was transplanted individually in a 25 ml pot containing perlite. The inoculation test was arranged in a randomized block design with two blocks: 16 individuals (pots) of each parental or F1 line were provided for a block of the test. The disease severity index (0-no symptoms; 1-chlorosis of cotyledon; 2-moderate wilt with chlorosis; 3-severe wilt with chlorosis; 4-death of plant) was recorded at 10 or 11 days after inoculation, and the degree of disease severity (Σ (100 × disease severity index / maximum disease severity index × No. of individuals inoculated)) was calculated. The disease severity of the parental varieties ranged from 1.3 to 71.5. Since the netted melon variety "Charentais" showed a reciprocal difference in disease severity, 6 × 6 subdiallel analysis was performed, by excluding "Charentais" and its F1s. The 6 × 6 diallel analysis failed to reveal either a reciprocal difference or epistasis of the genes. Resistance was partially dominant over non-resistance and controlled by additive effects of genes. The dominant effect was also statistically significant. Heritability values in broad and narrow sense were 0.96 and 0.81, respectively. The (Wr + Vr) / Pr graph suggested that the oriental pickling melon variety "Tokyo wase (maruba)" displayed recessive resistance gene(s) to the race 1,2y.