Impact of resistant melon cultivars on Sphaerotheca fuliginea

Effects of the resistant cultivars of melon on the powdery mildew fungus, Sphaerotheca fuliginea, were investigated in a melon-growing area of Japan. Powdery mildew fungi were isolated from four melon cultivars ('HN21','Quincy', 'Earl's Knight Natsu 2' and 'Earl's Miyabi Natsu 2') which are resistant to different races of S. fuliginea. 'HN21' is resistant to races 1 and N1, 'Quincy' and 'Earl's Knight Natsu 2' are resistant to races 1 and N2, and 'Earl's Miyabi Natsu 2' is resistant to races 1, N1 and N2. All 155 strains isolated from 'HN21' were identified as race N2. Seventy out of 81 strains isolated from 'Quincy' and 'Earl's Knight Natsu 2' were identified as race N1, while other strains were identified as races 1, 2US, N2 and N3. Nineteen out of 29 strains isolated from 'Earl's Miyabi Natsu 2' were identified as race N3, while other strains were identified as races 1, 2US, N1 and N4. These results suggest that, although the resistant cultivars are extremely effective for protecting the plants from their respective races, a race succession rapidly takes place, even in a small area, and other minor races become predominant.