First Report of Neocosmospora vasinfecta var. vasinfecta Causing Soybean Stem Rot in South Korea

In September 2010, stem rot symptoms were observed on soybean plants (cv. Daepungkong) growing in a field located at Daegu (35.52° N, 128.35° E), South Korea. The first noticeable symptoms, observed on the top leaves, were difficult to distinguish from those of sudden death syndrome (SDS). However, after splitting the stems of symptomatic plants, typical stem rot symptoms appeared as reddish-brown to dark-brown discoloration of the pith. Stem lesions extended 15 to 20 cm upward from the soil surface. To isolate the causal agent, sections of diseased stems were surface disinfected with 1% sodium hypochlorite, placed on potato dextrose agar (PDA) containing streptomycin sulfate, and incubated at 25°C with a 12-h light regime. Two isolates were obtained (SSLNV17 and SSLNV18). Mycelia were white and floccose. Conidia (4.5 to 11.2 × 2.2 to 3.4 μm) were cylindrical to oblong-ellipsoidal, hyaline, and one-celled. Both isolates produced abundant perithecia after 3 to 4 weeks. Perithecia (205 to 331 mm in diameter) were orange to red, globose and ostiolate, with a short neck (80 to 126 mm in diameter). Unitunicate asci (88.6 to 115.3 × 14.5 to 17.3 mm) were cylindrical to clavate, with a short stalk (6.0 to 9.5 × 5.0 to 6.8 mm), and eight spores. Ascospores (13.3 to 17.5 × 10.7 to 12.7 mm) were uniseriately arranged, globose to oval, one-celled, and hyaline to pale brown, with walls with a rugose ornamentation. These morphological features are consistent with those of Neocosmospora vasinfecta var. vasinfecta (1). The internal transcribed spacer (ITS) region, partial translation elongation factor 1-alpha (EF1-α), and β-tubulin genes of rDNA of the two isolates were sequenced using primers ITS4/ITS5 (GenBank Accession Nos. KF662732 and KF662733), EF1-728F/EF1-986R (KF758839 and KF758840), and Bt2a/Bt2b (KF771004 and KF771005), respectively. Sequences of the ITS region, EF1-α, and β-tubulin genes of both isolates showed 99% similarity with several reported N. vasinfecta strains by BLAST analysis. Both morphological and sequence analyses confirmed that the two isolates were N. vasinfecta var. vasinfecta. Pathogenicity tests of both isolates were performed on 15 three-week-old seedlings of soybean cv. Williams inoculated with a spore suspension containing 1.0 × 10⁶ spores/ml, using stem puncture inoculation procedure under controlled conditions (4). Control plants were inoculated in the same way with sterile water. The results were observed by splitting the stem longitudinally and checking for discoloration of the pith 4 to 5 weeks after inoculation. Reddish-brown to dark-brown discoloration was observed in the stem pith of inoculated plants, with occasional chlorosis of the leaves. Moreover, numerous orange-red perithecia were produced on the inoculated stems. However, no symptoms were visible on control plants. The pathogen was re-isolated from the diseased plants, confirming Koch's postulates. Neocosmospora stem rot of soybean was first discovered in Japan and since then it has been reported in the United States and China (2,3,4). To our knowledge, this is the first record of soybean stem rot caused by N. vasinfecta var. vasinfecta in Korea. Our report indicates that Neocosmospora stem rot is a new threat to soybean production in Korea. References: (1) P. F. Cannon and D. L. Hawksworth. Trans. Br. Mycol. Soc. 82:673, 1984. (2) Y. Gai et al. Plant Dis. 95:1031, 2011. (3) F. A. Gray et al. Plant Dis. 64:321, 1980. (4) D. V. Phillips. Phytopathology 62:612, 1972.