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New Zealand Plant Protection 66 (2013): 284-292

Resistance of Venturia inaequalis to quinone outside inhibitor (QoI) fungicides in New Zealand apple orchards

S.L.H. Viljanen-Rollinson, S.M. Thompson, S. Keenan, S.R. Bulman, P.J. Wright, P.N. Wood, N.M. Park and R.M. Beresford

ABSTRACT

DNA sequencing and optimisation of allele-specific primers targeting the G143A mutation, which confers resistance in Venturia inaequalis (apple black spot) to quinone outside inhibitor (QoI) fungicides, was used to develop a resistance testing method for orchard surveys. The method confirmed the resistance status of 15 V. inaequalis isolates that were classified as sensitive or resistant to trifloxystrobin using a mycelial growth assay. Disease caused by four isolates carrying the G143A mutation was not controlled by the QoI fungicide trifloxystrobin in an inoculated potted tree experiment. In a survey of 41 apple orchards in Hawke's Bay, Nelson, Otago and Waikato during 2011-12 the G143A mutation occurred in 54% of 802 V. inaequalis isolates, and 59% of orchards had more than 50% of isolates with the mutation present. The results indicate that orchards with a high G143A mutation frequency can be expected to experience loss of black spot control where QoI fungicides are used.

Keywords: apple scab, fungicide resistance, Quinone outside Inhibitor, resistance management.

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