G. W. Moorman, Department of Plant Pathology, The Pennsylvania State University, University Park 16802;
A.-S. Walker, INRA, UR 1290 BIOGER-CPP, BP01, 78850 Thiverval-Grignon, France; and
S. May, Department of Plant Pathology, The Pennsylvania State University, University Park 16802
Greenhouse-grown Heuchera plants, treated with fenhexamid (Decree, SePRO, Carmel, IN; FRAC group 17 hydroxyanilide), with active gray mold were submitted to the Penn State Plant Disease Clinic in December 2010 from a commercial operation in north-central Pennsylvania. Genetic and phenotypic analyses identified the isolate as Botrytis cinerea Pers. (teleomorph Botryotinia fuckeliana (de Bary) Whetzel), HydR3 phenotype (2) and not B. pseudocinerea (previously Botrytis group I) (4), naturally resistant to fenhexamid (phenotype HydR1) (1). While 0.2 μg of fenhexamid per ml or less is required to slow mycelial growth and germ tube elongation of sensitive isolates by 50% (EC50), the radial growth EC50 of the Heuchera isolate was approximately 2,000 μg of fenhexamid per ml in culture. Five cucumber seedlings receiving 25 μl of 0.1 M dextrose containing the label rate of Decree (1,800 μg/ml) on the growing tip were inoculated with colonized agar in the drop. Five check plants received 25 μl of 0.1 M dextrose. B. cinerea from silica gel storage since 1988 was also tested. This experiment was repeated three times. The 1988 isolate killed all fungicide-free but no fenhexamid-treated plants. The Heuchera isolate killed all fungicide-free and fenhexamid-treated plants within 4 days. To our knowledge, this is the first report of B. cinerea from a greenhouse in North America with fenhexamid resistance. Resistance occurs in U.S. fields (3). The Heuchera isolate's HydR3 resistance phenotype (2) has been detected in Germany, Japan, and France and has mutations affecting the 3-keto reductase protein, encoded by the erg27 gene, the specific target of fenhexamid and involved in Botrytis sterol biosynthesis. The Decree label states that it is to be used only twice on a crop before switching to a different mode of action. Greenhouses have resident Botrytis populations that are likely to be exposed to any fungicide applied in the structure. Growers should consider using fenhexamid only twice in a particular greenhouse, rather than on a particular crop, before switching to a different mode of action.
References: (1) P. Leroux et al. Crop Prot. 18:687, 1999.(2) P. Leroux et al. Pest Manag. Sci. 58:876, 2002. (3) Z. Ma and T. J. Michailides. Plant Dis. 89:1083, 2005. (4) A.-S. Walker et al. Phytopathology 101:1433, 2011.