Quantification, Persistence, and Status of Dodine Resistance in New York and Michigan Orchard Populations of Venturia inaequalis

Abstract

Isolates of Venturia inaequalis were sampled from apple orchards during 1990 to 1996 and isolate sensitivities to dodine were determined by testing the relative growth (RG) of mycelial colonies at a discriminatory dose of 0.2 μg ml-1. Sensitivities were not significantly different for a wild-type population and several populations sampled from orchards never or rarely treated with dodine, and respective data were combined to provide a reference for baseline populations. The baseline sensitivity was compared with sensitivities determined for four orchards with evidence for practical dodine resistance. At these sites, increases of phenotype frequencies were most pronounced for isolates with RG values >90; they had increased from a baseline level of 0.9 to >30% and, therefore, were rated dodine-resistant. For two orchards with confirmed cases of previous dodine resistance, frequencies of resistant isolates had declined to 11 and 14% after dodine use was discontinued for 13 and 4 years, respectively. Sensitivities had not returned to baseline levels, and indirect evidence suggested that practical dodine resistance could recur rapidly in response to resumed dodine usage. Monitoring of commercial orchards in New York and Michigan revealed that dodine sensitivities were not uniform throughout regions where dodine resistance was widespread in the late 1970s. Sensitivities ranged from baseline to dodine-resistant and appeared to reflect the dodine use history at particular sites. Because the history of dodine use is not always known to growers, applications of dodine remain risky when accurate historical records are not available.