VARIABILITY IN THE REACTION OF SQUASH (CUCURBITA-PEPO) TO INOCULATIONWITH SPHAEROTHECA-FULIGINEA AND METHODOLOGY OF BREEDING FOR RESISTANCE

Reaction to inoculation with powdery mildew caused by Sphaerotheca,ful iginea was observed on leaf discs and young plants of eleven represent atives of seven edible cultivar groups of Cucurbita pepo. Disease inte nsity (i.e. number of infections per leaf) was highly correlated (r2=0 .863, P<0.0001) with spore yield per leaf. Spore yield per leaf and fr equency of sporulation on leaf disks were moderately (r2=0.505), but s ignificantly correlated (P<0.01), suggesting that frequency of sporula tion can be used for initial screening against susceptibility in a bre eding programme. Spore yield per leaf and spore yield per artificially inoculated leaf disc were highly correlated (r2=0.87, P<0.0001); this suggests that counting of spores on leaf discs, a laborious but accur ate procedure, could be used on the remaining plants as a second step in selection for resistance. Of the variation in the response of edibl e C. pepo to the pathogen, 85.8% was attributed to differences between the edible groups and only 14.2% to individual cultivars within a gro up. Cultivars of the cocozelle and vegetable marrow groups were the mo st susceptible, whereas relatively resistant cultivars were found in t he scallop and straightneck groups.