C. L. Trueman, Ridgetown Campus, University of Guelph, 120 Main Street East, Ridgetown, ON, Canada N0P 2C0;
L. E. Hanson, USDA-ARS, 1066 Bogue St., East Lansing, MI 48824;
N. Rosenzweig, Department of Plant, Soil, and Microbial Science, Michigan State University, 612 Wilson Rd., East Lansing 48824;
Q. W. Jiang, Department of Plant, Soil, and Microbial Science, Michigan State University, 1066 Bogue St., East Lansing 48824; and
W. W. Kirk, Department of Plant, Soil, and Microbial Science, Michigan State University, 612 Wilson Rd., East Lansing 48824
Cercospora beticola Sacc. causes Cercospora leaf spot (CLS) of sugar beet (Beta vulgaris L.) and is the most destructive foliar disease of sugar beet worldwide (1). The QoI fungicide pyraclostrobin has been an important management tool for CLS in Canada since 2003. Beginning in 2010, some growers reported poor disease control after applying pyraclostrobin. Leaf disk samples with CLS lesions were collected in September 2012 from 16 commercial fields located in Kent and Lambton Counties, Ontario, Canada. These counties (ca. 300,000 ha) encompass the major commercial sugar beet production area in Ontario (ca. 3,925 ha). CLS severity ranged from low to severe among the sampling sites. Leaf discs with a single leaf spot were cut from leaves using a hole punch. Spots were up to 5 mm in diameter with tan, light brown, or sometimes gray centers. DNA was extracted from leaf discs using a Qiagen DNeasy Plant Mini Kit (Germantown, MD) according to the manufacturer's instructions. PCR was used to amplify a fragment of the C. beticola cytochrome b (CYTB) gene (4). Pure cultures were obtained by placing plant tissue in a moist chamber and transferring single spores to V8 juice agar. PCR products were sequenced for 32 samples at the Genomics Technology Support Facility (Michigan State University, East Lansing, MI) and 25 were confirmed to have 100% identity with the sequence of QoI-resistant C. beticola from Michigan (2) and to QoI-resistant isolates from GenBank (Accession Nos. JQ619933 and JQ360628). The remaining seven had 100% identity with a sensitive isolate (EF176921.1). Each resistant isolate contained a change in codon 143 that is predicted to lead to a substitution of G143A in the cytochrome b gene. This G143A mutation has been associated with QoI resistance in a number of fungi (3). To confirm the result, a conidium germination bioassay was carried out using nine isolates with the G143A mutation on sugar beet leaf agar covered with water agar amended with pyraclostrobin at concentrations ranging from 0 to 54.3 μg/ml and distributed on a spiral gradient using an Eddyjet II spiral plater. The medium was supplemented with salicylhydroxamic acid (SHAM) to block the alternate oxidation pathway. Following incubation at 25°C for 2 days, the distance between the center of the plate at which conidial germination was 50% of the maximum observed growth (EC) and the point at which conidial germination terminated were measured (TEC). The EC50 values were determined from the SGE software for each isolate by entering the EC and TEC values, respectively. The estimated EC50 for a representative wild type (sensitive) isolate was 0.03 μg/ml, while the value for the resistant isolate could not be calculated because it was greater than the highest concentration tested (54.3 μg/ml). Additionally, in the controls with no SHAM or fungicide, the resistant isolate showed a consistent reduced germination rate compared to the sensitive isolate (30.0% and 93.5% germination, respectively). Confirmation of fungicide insensitivity will require a re-evaluation of current management practices in Ontario to minimize economic losses due to CLS.
References: (1) B. J. Jacobsen and G. D. Franc. Compendium of Beet Diseases and Pests, 2nd ed, APS Press, St. Paul, MN, 2009. (2) W. Kirk et al. New Dis. Rep. 26:3, 2012. (3) Z. Ma and T. J. Michailides. Crop Prot. 24:853, 2005. (4) A. Malandrakis et al. Pestic. Biochem. Physiol. 100:87092, 2011.