Sensitivity of Venturia inaequalis to myclobutanil penconazole and dodine in relation to fungicide use in Hawkes Bay apple orchards

Abstract

A survey of Integrated Fruit Production and organic apple orchards in Hawkes Bay in 201011 investigated sensitivity of Venturia inaequalis isolates to demethylation inhibitor (DMI) and dodine fungicides EC50 values from mycelial growth assays for two DMIs showed continued loss of sensitivity since the 1990s Sensitivity was lower in orchards where DMI usage exceeded resistance management guidelines The most DMIsensitive isolates were found in one organic orchard Plant inoculations showed that black spot control of myclobutanilresistant V inaequalis strains by one application of myclobutanil was only 55 compared with 99 for myclobutanilsensitive strains The study suggests that resistance to DMI fungicides has continued to develop that loss of black spot control could be expected in some orchards and that DMI resistance management guidelines need reviewing There was high sensitivity to dodine in all orchards (mean EC50 of 024 mg/litre) and no evidence of increased resistance since the 1990s