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Identification of Resistance to Multiple Fungicides in Field Populations of Venturia inaequalis

August 2011 , Volume 95 , Number  8
Pages  921 - 926

Kimberly S. Chapman, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907; George W. Sundin, Department of Plant Pathology, 103 CIPS, Michigan State University, East Lansing 48824; and Janna L. Beckerman, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907

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Accepted for publication 9 March 2011.

Venturia inaequalis, the causal agent of apple scab, is controlled primarily by fungicides. Long-term, extensive fungicide use has led to the development of resistance to multiple fungicides. To assess fungicide resistance, isolates of V. inaequalis were collected from Indiana and Michigan orchards. Single-spore derived isolates were evaluated by mycelium growth assays with previously determined discriminatory doses on media containing dodine, kresoxim-methyl, myclobutanil, or thiophanate-methyl. Of 195 isolates tested, 5.2, 0.7, 57.0, and 92.6% of isolates were found to be resistant to dodine, kresoxim-methyl, myclobutanil, and thiophanate-methyl, respectively. This is the first report of kresoxim-methyl field resistance in these states. Isolates resistant or shifted to a single fungicide were often found to have multiple fungicide resistance. Of all isolates tested, 38% were identified as resistant or shifted to two fungicides, and 12% were resistant or shifted to all four fungicides tested. No fitness penalty was found for isolates resistant to multiple fungicides based on a statistical analysis of mycelial growth and conidial production.

© 2011 The American Phytopathological Society