First report of wilt disease caused by Pythium myriotylum on peanut plants in Korea.

Abstract

Peanut (Arachis hypogaea L.) has long been cultivated worldwide as an important crop for oil and protein production. Among the various diseases in peanut plants, wilt diseases caused by soil-borne pathogens such as Ralstonia solanacearum and Verticillium dahliae are especially destructive and substantially diminish both quantity and quality in peanut production (Kokalis-Burelle et al., 1997; Thiessen et al., 2012). In July 2022, wilt symptoms were observed in 1 to 3% of the area of peanut fields in Yeoju-si, Korea (37°23´04.0˝N; 127°33´43.0˝E). The xylem in the stems of the wilted plants was dark brown at the soil-surface, which is a representative symptom of vascular wilt pathogens (Yadeta et al. 2013). To isolate the causative pathogens, the stems exhibiting dark lesions were disinfected with 1% NaOCl for 1 min, rinsed with sterile distilled water, and placed on potato dextrose agar medium. The plates were incubated at 25℃ for 2 days, and white hyphae that grew out from the tissues were subcultured twice on V8 juice agar (V8A) medium. Among the 3 isolates, morphological characteristics of the representative strain YJ1-2 were observed under a microscope. The sporangia were terminal intercalary, filamentous, inflated lobulate, and ranging from 37.4 to 73.6 μm in diameter. The antheridia were diclinous, with clavate, elongate, and crook-necked shapes. The oogonia were mostly globose, with an average of 27.1 μm (range from 20.2 to 35.2 μm, n = 50) in diameter, and mated with one to several antheridia. Both plerotic or aplerotic oospores were observed. Overall, the morphological characteristics of the sporangia, antheridia, oogonia, and oospores indicated that YJ1-2 belongs to the genus Pythium. To genetically characterize YJ1-2, genomic DNA was extracted using cetyltrimethylammonium bromide buffer, and the internal transcribed spacer (ITS) region and cytochrome c oxidase subunit I (cox1) gene were amplified by PCR using primer sets ITS4/ITS5 and OomCoxI-Levlo/ OomCoxI-Levup, respectively (White et al., 1990; Robideau et al. 2011), sequenced, and identified using BLASTN (NCBI, National Center for Biotechnology Information). The ITS sequence (NCBI Acc. No. OR125595) of YJ1-2 has 99% similarity with that of P. myriotylum isolate PY39 (NCBI Acc. No. KX671096). A neighbor-joining phylogenetic tree was constructed from aligned cox1 sequence (NCBI Acc. No. OR224334) of the 10 Pythium species strains including YJ1-2 by CLUSTALW method was used as an outgroup. The YJ1-2 was most closely related to P. myriotylum isolate PM30 (NCBI Acc. No. MT823167). To substantiate the pathogenicity of YJ1-2, the crown roots of peanut plants grown in pots for 4 weeks were wounded using a sterile tweezer, and the mycelial plugs of YJ1-2 cultured for 5 days on V8A were inoculated on the wounds. The inoculated plants were cultivated in a growth chamber at 30℃ and 70% relative humidity with a 12-h photoperiod. The infected peanut plants exhibited wilt symptoms 11 days after inoculation, consistent with the initial observation, while uninoculated plants remained healthy. To satisfy Koch's postulates, white mycelia were re-isolated from the stems of inoculated plants and axenically cultured in V8A. The morphologies and ITS sequences of the re-isolates were consistent with those of YJ1-2. P. myriotylum has been reported as a causal pathogen of peanut pod rot in the United States and China. However, to the best of our knowledge, this is the first report of wilt disease in peanut plants caused by P. myriotylum in Korea. To prevent the incidence of wilt disease, we will continue our investigations to develop control strategies, including the selection of appropriate agrochemicals.