SHABI, E., Department of Plant Pathology, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan, 50 250, Israel
KOENRAADT, H., Laboratory of Phytopathology, Agricultural University, Binnenhaven 9, Wageningen, 6709 PD, Netherlands
KATAN, T., Department of Plant Pathology, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan, 50 250, Israel

Fourteen single‐spore cultures of benomyl‐resistant Venturia pirina were isolated from pear scab lesions at four sites in Israel. According to the ability of the isolates to germinate and grow at varying benomyl concentrations, four levels of resistance were determined in vitro: three isolates with low resistance (LR) grew at 0.5 but not at 5 μg/ml benomyl: five moderately resistant (MR) isolates grew at 5 but not at 50 μg/ml benomyl: five highly resistant (HR) isolates grew at 50 μg/ml but their hyphae were curled: and one isolate with very high resistance (VHR) grew unaffected at 50 μg/ml benomyl. The difference between the HR and the VHR phenotypes was clearly shown on medium amended with N‐(3.5‐dichlorophenyl) carbamate (MDPC): only the VHR isolate showed negative cross‐resistance to 1 μg/ml MDPC. whereas HR isolates grew unaffected. Crosses between resistant isolates and sensitive wild types, as well as between different resistant isolates, showed that the various levels of resistance are conferred by four allelic mutations that constitute a polymorphic series at a single locus. Copyright © 1986, Wiley Blackwell. All rights reserved