Variability in the reaction of squash (Cucurbita pepo) to inoculation with Sphaerotheca fuliginea and methodology of breeding for resistance

Reaction to inoculation with powdery mildew caused by Sphaerotheca fuliginea was observed on leaf discs and young plants of eleven representatives of seven edible cultivar groups of Cucurbita pepo. Disease intensity (i.e. number of infections per leaf) was highly correlated ( r ²=0·863, P <0·0001) with spore yield per leaf. Spore yield per leaf and frequency of sporulation on leaf disks were moderately ( r ²=0·505), but significantly correlated ( P <0·01), suggesting that frequency of sporulation can be used for initial screening against susceptibility in a breeding programme. Spore yield per leaf and spore yield per artificially inoculated leaf disc were highly correlated ( r ²=0·87, P <0·0001); this suggests that counting of spores on leaf discs, a laborious but accurate procedure, could be used on the remaining plants as a second step in selection for resistance of the variation in the response of edible C. pepo to the pathogen, 85·8% was attributed to differences between the edible groups and only 14·2% to individual cultivars within a group. Cultivars of the cocozelle and vegetable marrow groups were the most susceptible, whereas relatively resistant cultivars were found in the scallop and straightneck groups.