Powdery mildew (Sphaerotheca fuliginea) resistance in melon is selectable at the haploid level

The major cause of powdery mildew in melons (Cucumis melo L.) is the fungus Sphaerotheca fuliginea. There are several cultivar- and season-specific races of this fungus. In order to control powdery mildew, it is important to introduce resistance to fungal infection into new cultivars during melon breeding. Haploid breeding is a powerful tool for the production of pure lines. In this study, it was investigated whether powdery mildew resistance could be manifested at the haploid level from two disease-resistant melon lines, PMR 45 and WMR 29. the effects of various races of S. fuliginea on diploid and haploid plants of PMR 45 and WMR 29 and of a disease-susceptible line, Fuyu 3 were measured. The responses of haploid and diploid plants to powdery mildew were identical. In addition, haploids that were generated from hybrids between Fuyu 3 and disease-resistant lines were examined. Seven out of 13 haploids from a Fuyu 3xPMR 45 cross and 10 out of 12 haploids from a Fuyu 3xWMR 29 cross were classified as resistant plants because they showed the same responses as their disease-resistant diploid parents to the various fungal races. These results indicate that resistance in PMR 45 and WMR 29 is selectable at the haploid level. All of the plant responses were observed by microscopy. A possible mechanism for generating powdery mildew resistance in two different melon lines is discussed.