Case study: Apples

The QoI fungicides, kresomim-methyl (Sovran) and trifloxystrobin (Flint), are used for the management of apple diseases, including scab caused by Venturia inaequalis, which is a pathogen prone to developing fungicide resistance. Risk assessments predicted that alternative respiration and a G143A target site mutation were likely mechanisms of QoI resistance. Resistance developed initially as a quantitative population response involving alternative respiration, followed by the selection of immune target site mutants. In Europe, the transition occurred after a total of approximately 25 QoI applications. Current management strategies include reductions in the seasonal number of QoI applications, protective modes of application, high application rates, and periodic monitorings of orchard sensitivities. In addition to apple scab, QoIs are active in the control of powdery mildew, rust, fly speck, sooty blotch and Alternaria blotch. For Alternaria blotch caused by Alternaria mali, strong shifts toward QoI resistance have been reported after four to five years of QoI use, while for other apple diseases, resistance has not become a problem. The new fungicide Pristine contains the QoI pyraclostrobin and the unrelated respiration inhibitor boscalid. Resistance to Pristine has not been reported as yet but must be considered as high in QoI-resistant orchards.