Inheritance of resistance to Podosphaera xanthii in melon accessions AM-55 and AC-15

Powdery mildew is a severe disease caused by the fungus Podosphaera xanthii (Px) that reduces the melon production around the world. Genetic resistance is available in melon, but the reported alleles do not confer total resistance to all the pathogen races. In this study, the inheritance of the resistance of two new resistant sources was characterized. The accessions AM-55 and AC-15 (exotic types from India and Brazil, respectively) were crossed with the susceptible cultivar 'Vendrantais'. Different derived populations (F-1, F-2, BC1 and BC2) and nine differential cultivars were screened under artificial inoculation conditions. Plants were artificially inoculated by depositing a small amount of conidia on the second true leaf of each plant. Fifteen days after inoculation, the plants were scored according to the level of sporulation of the fungus. Four phenotypic classes were defined (1-4). Plants in classes 1 and 2 were considered resistant, and those in classes 3 and 4 were considered susceptible. The observed resistance/susceptibility segregation ratios indicated a monogenic and dominant inheritance in AM-55 and two independent genes with epistatic interaction in AC-15.