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

Abstract

DNA sequencing and optimisation of allele-speciic 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 conirmed the resistance status of 15 V. inaequalis isolates that were classiied as sensitive or resistant to triloxystrobin using a mycelial growth assay. Disease caused by four isolates carrying the G143A mutation was not controlled by the QoI fungicide triloxystrobin 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. © 2013 New Zealand Plant Protection Society.