RELATIVE SUSCEPTIBILITY OF SOME COMMERCIAL AND local APPLE CULTIVARS TO POWDERY MILDEW(Podosphaera leucotricha) UNDER NATURAL CONDITION

Apple powdery mildew (Podosphaera leucotricha) is one of the most important diseases in this crop causing weakness and yield reduction in Semirom, Isfahan Province. In order to evaluate the reaction of 14 commercial cultivars budded trees (on apple rootstock)were planted in a completely randomised block design, with 3 replicates(3 plants in every plot) 2.5×1m apart, at Kahangan village in Semirom.Sampling were carried out at the time of matimum lesion size. Relative resistance was assessed on the basis of lesion size in natural conditions. The results showed that all of the cultivars infected with different intensity. Fugi, Golden delicious, Gala, Azayesh, Braborn and Delbar Stival cultivars, with Severity Index(SI): 30.12, 37.92, 31.71, 33.2, 42.91 and 25.78 were susceptible. Janatan, Golab Isfahan, Golab Kermanshah, Golab Khorasan and Golab Kohanz cultivars with the SI: 54.96, 55.57, 58.71, 53.68 and 59.42 were highly susceptible while Red Delicious and Ingrid Mary with the SI:9.04 and 11.44 were resistance respectively.