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

Abstract

DNA sequencing and optimisation of allelespecific 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 Hawkes Bay Nelson Otago and Waikato during 201112 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