Quantification, persistence, and status of dodine resistance in New York and Michigan orchard populations of Venturia inaequalis

Isolates of Venturia inaequalis were sampled from apple orchards during 199 0 to 1996 and isolate sensitivities to dodine were determined by testing th e relative growth (RG) of mycelial colonies at a discriminatory dose of 0.2 mu g ml(-1). Sensitivities were not significantly different for a wild-typ e population and several populations sampled from orchards never or rarely treated with dodine, and respective data were combined to provide a referen ce for baseline populations. The baseline sensitivity was compared with sen sitivities determined for four orchards with evidence for practical dodine resistance. At these sites, increases of phenotype frequencies were most pr onounced for isolates with RG values >90; they had increased from a baselin e level of 0.9 to >30% and, therefore, were rated dodine-resistant. For two orchards with confirmed cases of previous dodine resistance, frequencies o f resistant isolates had declined to 11 and 14% after dodine use was discon tinued for 13 and 4 years, respectively. Sensitivities had not returned to baseline levels, and indirect evidence suggested that practical dodine resi stance could recur rapidly in response to resumed dodine usage. Monitoring of commercial orchards in New York and Michigan revealed that dodine sensit ivities were not uniform throughout regions where dodine resistance was wid espread in the late 1970s. Sensitivities ranged from baseline to dodine-res istant and appeared to reflect the dodine use history at particular sites. Because the history of dodine use is not always known to growers, applicati ons of dodine remain risky when accurate historical records are not availab le.