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New Zealand Plant Protection 65 (2012): 106-113

Sensitivity of Venturia inaequalis to myclobutanil, penconazole and dodine in relation to fungicide use in Hawke's Bay apple orchards

R.M. Beresford, P.J. Wright, P.N. Wood and N.M. Park

ABSTRACT

A survey of Integrated Fruit Production and organic apple orchards in Hawke's Bay in 2010-11 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 DMI-sensitive isolates were found in one organic orchard. Plant inoculations showed that black spot control of myclobutanil-resistant V. inaequalis strains by one application of myclobutanil was only 55%, compared with 99% for myclobutanil-sensitive 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 0.24 mg/litre) and no evidence of increased resistance since the 1990s.

Keywords: fungicide resistance, apple scab, demethylation inhibitor, dodine, resistance management.

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