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Practical and qualitative resistance to trifloxystrobin in populations of Venturia inaequalis in the northeastern United States.
Z. FREDERICK (1), K. D. Cox (2). (1) Cornell University, Trumansburg, NY, U.S.A.; (2) Cornell University, Geneva, NY, U.S.A.

The development of widespread resistance to demethylation inhibitor fungicides led to increased use of quinone outside inhibitor (QoI) fungicides to manage apple scab in the northeastern United States. We monitored the development of qualitative resistance to the QoI fungicide trifloxystrobin in regional populations of <i>Venturia inaequalis</i> from 2007 to 2012. 139 orchard populations (n > 25) were tested for quantitative and qualitative resistance to trifloxystrobin using microscopy-aided relative growth/germination assays. Out of 139 orchards, 35 were found to have members with qualitative resistance to trifloxystrobin, which comprised 2.5 to 95.8% of the population. In 2011, trials were conducted in an apple orchard in Geneva NY to develop a ground truth standard for determining practical resistance to trifloxystrobin. In this population, 49.5% of the members had qualitative resistance. When trifloxystrobin was applied under standard conditions, the incidence of apple scab on mature ‘Empire’ fruit was higher (49.5 ± 5.7%) than that of a succinate dehydrogenase inhibitor (SDHI) standard program (20.0 ± 4.6%). On ‘Jonagold’, a less susceptible cultivar, the incidence of apple scab was slightly higher for the trifloxystrobin program (18.5 ± 11.1%) compared to that of the SDHI standard program (5.0 ± 1.3%). This information provides the basis for ascertaining the development of practical resistance in orchard populations of <i>V. inaequalis</i>.<p><p>Keywords: Fungus, Fruits-Nuts, Apple

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