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First Report of Races 11 and 12 of Cercospora sojina, the Causal Agent of Soybean Frogeye Leaf Spot, in Argentina

During the growing seasons of 2008 to 2009 and 2009 to 2010, severe outbreaks of soybean (Glycine max (L.) Merr.) frogeye leaf spot, a disease caused by Cercospora sojina Hara, occurred in several areas in Argentina (1). Two surveys were conducted in soybean fields, one in 2008 that included the provinces of Buenos Aires, Córdoba, and Santa Fe, and another that was performed in 2009 in the same provinces plus three others: Entre Ríos, Santiago del Estero, and Tucumán. In both surveys, plants presented circular lesions with reddish brown-to-gray spots and bordered by typical, narrow, reddish purple margins (3). To promote sporulation and to enable identification of the causal agent, leaves of diseased plants were collected and placed in a moist chamber for 24 h with a 12-h light cycle at 25°C. Conidia were plated on potato dextrose agar medium amended with streptomycin and were incubated at 25°C and 12 h of fluorescent light. Isolated cultures sporulated in 10 days and, on the basis of their morphology, were identified as C. sojina. A total of 147 isolates were deposited at the Culture Collection of CEREMIC (Centro de Referencia de Micología). They produced one- to nine-septate hyaline, elongate to fusiform conidia that measured 54.9 ± 16.2 × 5.7 ± 1.0 μm. Six isolates of C. sojina, each representing a province, were inoculated on a set of 12 differential soybean cultivars: Lee, Davis, Hood, Richland, Lincoln, Kent, Tracy, S 100, Palmetto, Peking, CNS, and Blackhawk (2). Fifteen plants of each differential were sprayed at V3 growth stage with a suspension of 6 × 10⁴ conidia/ml. The test was conducted twice in a complete randomized design with three replicates. Control plants were sprayed with sterile distilled water. After inoculation, plants were placed in a greenhouse bench humidity chamber at 26 to 28°C for 72 h. Disease was rated 14 days after inoculation; plants with numerous lesions were considered susceptible and each of the 15 plants was given a score of 1. Plants with small or no lesions were classified as resistant and given a score of 0. Control plants remained healthy. The pathogen was reisolated from symptomatic plants and morphological characteristics were consistent with C. sojina. Based on the response of the differentials to each isolate and on the race designations, the isolates from Buenos Aires, Córdoba, Santa Fe, and Tucumán belong to race 11, while those from Santiago del Estero and Entre Ríos province to race 12. The finding of these two races threatening soybean cultivars in Argentina may be indicative of additional races. Thus, the incorporation of multiple resistance genes may reduce the impact of the disease on soybean. To our knowledge, this is the first report of the identification of races of C. sojina in Argentina.

References: (1) M. A. Carmona et al. Plant Dis. 93:966, 2009. (2) M. A. R. Mian et al. Crop Sci. 48:14, 2008. (3) D.V. Phillips. Page 20 in: Compendium of Soybean Diseases. 4th ed. APS Press, St. Paul, MN, 1999.