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First report of mandipropamid resistance of grapevine downy mildew in North America

Anton Baudoin: Virginia Tech

<div>Carboxylic acid amide (CAA) fungicides constitute a group of fungicides highly effective against downy mildews. Two members are registered for use against grape downy mildew (<em>Plasmopara viticola</em>) in the United States, mandipropamid since 2008 and dimethomorph since 2009. Resistance of <em>P. viticola</em> against CAA fungicides, first detected in Europe in 1994, has gradually spread and is now considered widespread in most of Europe’s viticultural regions. CAA resistance has been attributed to a mutation in the cellulose synthase gene (<em>PvCesA3</em>). Since 2011, control failures and the presence of this mutation have also been reported from India, Japan and China, but not from North America. In 2016, we received a report of suspected control failure of mandipropamid in a west-central Virginia vineyard. All eight isolates collected from this location grew well on leaves and potted plants treated with a field rate of mandipropamid (156 mg/liter) whereas an older isolate produced no growth under these conditions. The 144 bp <em>PvCesA3</em> gene of two resistant <em>P. viticola</em> isolates was amplified and sequenced. The sequenced PCR products revealed a GGC-to-AGC substitution at codon 1105 of the <em>PvCesA3 </em>gene, the same G1105S mutation previously associated with CAA resistance of <em>P. viticola</em> in other regions.</div>