Baseline Sensitivities of Venturia inaequalis to Sterol Demethylation Inhibitors. Wolfram Köller, Assistant Professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Diana M. Parker, and Katherine L. Reynolds. Research Assistant, and Postdoctoral Research Associate, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. 75:726-728. Accepted for publication 15 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0726.

A set of monoconidial isolates of Venturia inaequalis representing a wild-type population was used to determine the baseline sensitivities and sensitivity distributions for several sterol demethylation inhibitors used in the control of apple scab. Baseline sensitivities expressed as ED₅₀ values for growth of mycelia were 0.008 ?g ml^–1 (flusilazole), 0.03 ?g ml^–1 (penconazole and pyrifenox), 0.04 ?g ml^–1 (fenarimol), 0.05 ?g ml^–1 (bitertanol and tebuconazole), and 0.07 ?g ml^–1 (myclobutanil). The resistance factors calculated from ED₅₀ values determined for a resistant baseline isolate of V. inaequalis were variable, ranging from 15 for flusilazole to 3 for pyrifenox.