Selection Effectiveness and Mechanism of Expression in Fusarium oxysporum f.sp. melonis Race 1,2y Resistance in a Breeding Process of Rootstock Cultivar ‘Dodai No. 1’

A rootstock cultivar of melon (Cucumis melo L.) ‘Dodai No. 1’, which shows remarkable quantitative resistance to race 1,2y of Fusarium oxysporum f.sp. melonis. was developed from a cross between ‘Melon Parental Line 1’ ((var. reticulatus Naud. × var. makuwa Makino) × var. reticulatus Naud.) and ‘Tokyo-wase (maruba)’ (var. conomon Makino). To clarify the selection effectiveness of resistance, both parents, F1 line (‘Melon Parental Line 1’ × ‘Tokyo-wase (maruba)’), F2 line and F3−F6 lines were evaluated for resistance by inoculation with F. oxysporum f.sp. melonis race 1,2y. Resistance was remarkably improved from F2 to F3, and moderately improved from F4 to F6. These findings indicate that F. oxysporum f.sp. melonis race 1,2y resistance is controlled by several additive genes derived from both parents. Observation using KOH-aniline blue technique for fluorescent staining of fungi demonstrated that the infection rate and extension of hyphae were suppressed in resistant cultivars and lines. These findings imply that the mechanisms in expression of additive genes are related to suppression of entrance and extension of F. oxysporum f.sp. melonis race 1,2y hyphae. Furthermore, it seemed that these mechanisms are common among C. melo cultivars that have different genetic backgrounds.