Resistance to Dodine in Populations of Venturia inaequalis in Quebec, Canada

Abstract

Dodine was introduced in the USA and Canada in the early 1960s for the control of apple scab. Following control failure, growers stopped using dodine in the mid-1970s. Despite the curtailment of dodine use more than 30 years ago, persistent resistance to the fungicide was suspected in V. inaequalis populations. The dodine sensitivity was determined for two populations that were not exposed to dodine for at least 30 years – a wild type population (25 monoconidial isolates) and a population constructed with isolates collected in orchards managed for apple scab (156 isolates). The sensitivity to dodine was determined by monitoring growth of these isolates on agar Petri dishes amended with 0, 0.01, 0.1, 1.0, or 10.0 μg/ml of dodine. Sensitivity to fungicide was evaluated based on ED50 values. Both populations showed a lognormal distribution of ED50 values. The ED50 means were 0.525 μg/ml and 1.735 μg/ml for the wild type and managed orchards populations, respectively. In managed orchard, 31.4% of the isolates were resistant to dodine (ED50 > 1.0 μg/ml). Cross-resistance with myclobutanil and with kresoxim-methyl was tested and found not to be significant. The results of this study suggest that resistance to dodine is still present in the populations of V. inaequalis from Quebec and that reintroduction of dodine should only be done along with an appropriate resistance management strategy. Accepted for publication 27 April 2010. Published 14 June 2010.