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First Report of the Soybean Frogeye Leaf Spot Fungus (Cercospora sojina) Resistant to Quinone Outside Inhibitor Fungicides in North America

Quinone outside inhibitor (QoI; also known as strobilurin) fungicides sometimes are applied to soybean (Glycine max) fields to help manage frogeye leaf spot of soybean (caused by Cercospora sojina) in the United States. In August 2010, soybean leaflets exhibiting severe frogeye leaf spot symptoms were collected from a field in Lauderdale County, TN that had been treated twice with pyraclostrobin during that growing season. The field had been planted into soybean annually since at least 2008, and a QoI fungicide had been applied to the field in each of those years. Fifteen single-spore isolates of C. sojina were recovered from the affected soybean leaflets. These isolates were identified as C. sojina based on the observed symptoms on the soybean leaflets and the morphology and size of conidiophores and conidia (3). In addition, DNA was extracted from the cultures, PCR amplification of the small subunit rDNA and internal transcribed spacer (ITS) region was conducted (2), and the resulting PCR product was sequenced at the Keck Biotechnology Center at the University of Illinois, Urbana. The resulting nucleotide sequences were compared with sequences deposited in the nucleotide database (http://www.ncbi.nlm.nih.gov) and showed highest homology to sequences of C. sojina. The isolates were tested for their sensitivity to technical-grade formulations of the QoI fungicides azoxystrobin, pyraclostrobin, and trifloxystrobin with an in vitro conidial germination assay with fungicide + salicylhydroxamic acid (SHAM)-amended potato dextrose agar as described by Bradley and Pedersen (1). The effective concentration at which 50% conidial germination was inhibited (EC₅₀) was determined for all 15 C. sojina isolates, with mean values of 3.1644 (2.7826 to 4.5409), 0.3297 (0.2818 to 0.6404), and 0.8573 (0.3665 to 2.5119) μg/ml for azoxystrobin, pyraclostrobin, and trifloxystrobin, respectively. When compared with previously established mean EC₅₀ values of C. sojina baseline isolates (4), EC₅₀ values of the C. sojina isolates collected from the Lauderdale County, TN soybean field were approximately 249- to 7,144-fold greater than the EC₅₀ values of the baseline isolates. These results indicate that all isolates recovered from the Lauderdale County, TN soybean field were highly resistant to QoI fungicides. To our knowledge, this is the first report of QoI fungicide resistance occurring in C. sojina, and surveys for additional QoI fungicide-resistant C. sojina isolates are needed to determine their prevalence and geographic distribution. In light of these findings, soybean growers in Tennessee and adjacent states should consider utilizing alternative frogeye leaf spot management practices such as planting resistant cultivars, rotating to nonhost crops, and tilling affected soybean residue (3).

References: (1) C. A. Bradley and D. K. Pedersen. Plant Dis. 95:189, 2011. (2) N. S. Lord et al. FEMS Microbiol. Ecol. 42:327, 2002. (3) D. V. Phillips. Page 20 in: Compendium of Soybean Diseases. 4th ed. G. L. Hartman et al., eds. The American Phytopathological Society, St. Paul, MN, 1999. (4) G. Zhang et al. Phytopathology (Abstr.) 100(suppl.):S145, 2010.