Fungicides in the quinone outside inhibitor class (QoI, also referred to as strobilurins, FRAC group 11) are relied upon for management of a wide range of diseases, including anthracnose, black rot, downy mildew, and powdery mildew on grape. In June 2012, a grape grower from Anderson County in central Kentucky reported a planting of grapevines (Vitis vinifera cv. Vidal Blanc) with 90% downy mildew (Plasmopara viticola) incidence that would not respond to applications of the QoI fungicide Abound 2.08F (22.9% azoxystrobin, Syngenta Crop) or the QoI-containing fungicide Pristine (12.8% pyraclostrobin + 25.2% boscalid, BASF Corporation). Symptoms included yellow irregular lesions or brownish red angular lesions with necrosis on upper sides of leaves. Undersides of leaves contained dense white sporulation. The grower confirmed usage of 4 to 5 applications each of Abound and Pristine fungicides between 2011 and 2012, which exceeded the maximum number of applications allowed per season, as expressed on individual labels. Samples were collected from throughout the 0.4-ha vineyard, and the pathogen was identified as Plasmopara viticola based on morphology of sporangia and sporangiophores (2). Fungicide sensitivity was determined using methods published by Wong and Wilcox (3). Leaves were selected from the 3rd to 6th leaf position from disease-free plants. Nine-millimeter leaf discs were surface disinfested and treated with fungicide concentrations ranging from one-half of the lowest labeled rate to twice the highest label rate (850, 170, 270, and 540 mg/l azoxystrobin and 40, 80, 120, and 240 mg/l pyraclostrobin). Leaf discs were inoculated by placing 10-μl droplets of sporangial suspensions (1 × 104 sporangia per ml) and then incubated at room temperature (22 to 24°C) under fluorescent lights with a 12-h photoperiod (1,3). Dense white fungal growth developed within 10 days; discs treated with water did not develop signs of disease. Fungicide sensitivity was evaluated by determination of the effective concentration (EC50) (3). Leaf discs were examined under a dissecting microscope after 14 days to determine presence of fungal growth; those with visible sporangia and/or sporangiophores were considered diseased. Resulting EC50 concentrations were 420 and 390 mg a.i./l for Abound (azoxystrobin) and Pristine (pyraclostrobin), respectively. This was higher than EC50 ranges of resistant isolates reported by Baudoin et al. (100 and 25 mg/l for azoxystrobin and pyraclostrobin, respectively) (1). Additionally, ranges were higher than EC50 values of isolates not exposed to QoI fungicides reported by Baudoin et al. (1) and higher (14× and 39×, respectively) than those previously determined from P. viticola from Kentucky that were not exposed to QoI fungicides (Ward, unpublished). These EC50 ranges were also much higher than recommended label application rates, which ranged from 170 to 270 mg a.i./l for Abound and from 80 to 120 mg a.i./l for Pristine. Results indicated that P. viticola from this vineyard became insensitive to the fungicides Abound and Pristine. This will lead to future fungicide failures and increased incidences of downy mildew in vineyards. Although QoI-resistant P. viticola has been reported in Europe and elsewhere in the United States, this is the first documented report of QoI-resistant P. viticola in Kentucky. A complete survey is necessary to determine whether this phenomenon is widespread within the state.
References: (1) A. Baudoin et al. Plant Health Progress doi:10.1094/PHP-2008-0211-02-RS, 2008. (2) R. C. Pearson and A. C. Goheen, eds. Compendium of Grape Diseases, 4th ed. The American Phytopathological Society, St. Paul, MN, 1998. (3) F. P. Wong and W. F. Wilcox. Plant Dis. 84: 275, 2000.