Resistance of Venturia inaequalis to demethylation inhibitor and dodine fungicides in four New Zealand apple-growing regions

A survey of 41 apple orchards in Hawke's Bay (25), Nelson (7), Otago (4) and Waikato (5) provided 796 isolates of inaequalis (black spot or scab), which were tested using an agar plate assay for sensitivity to two demethylation inhibitor (DMI) fungicides (myclobutanil and penconazole) and to dodine. Each fungicide was used at two concentrations to distinguish between highly sensitive, sensitive and resistant isolates. Sensitivity to DMIs in all regions was lower than baseline sensitivity in previous studies, particularly for myclobutanil. Waikato showed signiicantly lower DMI sensitivity than other regions. Dodine sensitivity was greater than in the 1990s, although Otago isolates were signiicantly less dodine-sensitive than those from other regions. In a plant inoculation assay to test control by ive DMI fungicides of disease caused by resistant isolates, lusilazol and difenoconazole gave signiicantly better disease control of resistant isolates than myclobutanil, penconazole or fenbuconazole, at standard ield rates.