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Pythium species are one of the most important groups of seedling pathogens affecting soybean yield. In July 2023, eight soybean plants (cv. Daewon, V3 to V4 stage) that were wilted with browning at their lower stems were discovered in a field located in Suwon, Korea. The disease incidence was 0.1% in an area of 0.1 ha, and severity was 100%. The eight plants exhibited brown root rot when removed from the soil. Their main roots were almost completely rotten, with only a few rootlets remaining. Symptomatic stem tissues (approximately 0.5 × 1 to 1.5 cm 2 ) were sampled from the crown of each plant, disinfected with 75% ethanol for 30 sec and 1% NaOCl for 1 min, rinsed with sterile distilled water, and incubated on sterile filter paper for 3 days at 25°C. White hyphae emerging from the tissues were isolated and cultured on potato dextrose agar (PDA) plates, resulting in eight isolates. To further investigate their morphological features, the isolates were subcultured on PDA media at 25°C in the dark for 7 days. The colonies formed dense, white, fluffy aerial mycelia. The oogonia were globose with a smooth surface, typically appearing terminal, and had an average diameter of 31 μm (28 to 36 μm). The oospores were aplerotic, with an average diameter of 29 μm (26 to 31 μm). These morphological characteristics closely matched those of Pythium myriotylum as described in Van der Plaats-Niterink (1981) and Tomioka et al. (2013). The internal transcribed spacer (ITS) region and cytochrome c oxidase subunit I (COX1) were amplified using the primer pairs ITS1/ITS4 and OomCoxI-Levlo/OomCoxI-Levup (Robideau et al. 2011; White et al. 1990). Sequences from the eight isolates (SW2-4, SW-DF2, SW-DF3, SW-DF7, SW-DF9, SW-DF13, SW-DF14, and SW-DF16) were deposited in GenBank under following accession numbers: ITS (PP145893; PP913926 to PP913932) and COX1 (PP853484; PP977183 to PP977189). These sequences showed 100% homology with those of P. myriotylum strain CBS25470 (GenBank accession no. HQ643701, HQ708745). One isolate SW2-4 was selected to assess its pathogenicity using soil infestation method. The isolate SW2-4 was cultured on 20 ml of V8 juice agar medium per petri dish (9 cm in diameter) in the dark at 26°C for 10 days. Cultures collected from 4 petri dishes were homogenized with 40 ml of sterile distilled water, and then mixed with 1.1 liter of commercial potting soil that had been pre-soaked in 1 liter of sterile distilled water. Next, 450 ml of this Pythium-infested soil mixture was placed into pots (12 cm in diameter). Ten seeds of soybean (cv. Daewon) were then sown on top of the infested soil and covered with 2 cm of uninfested soil. Another ten seeds (cv. Daewon) were sown in the uninfested soil as controls. The pots were placed in a growth chamber (26°C, light: dark cycle of 12:12h). The experiments were repeated six times. The inoculated plants grew slowly, and dark-brown lesions appeared at the stem base 5 days after inoculation. Affected plants began to wilt 10 days after inoculation, whereas the control plants showed no symptoms and remained healthy throughout the experiments. An oomycete pathogen was re-isolated from the symptomatic stem tissue to fulfill Koch’s postulates, while none was isolated from the control plants. The pathogen’s morphological characteristics and DNA sequences (ITS and COX1) were confirmed to be identical to those of the inoculated isolate. To the best of our knowledge, this is the first report of P. myriotylum causing root rot and wilting in soybean in the Republic of Korea.
Kang et al. (Thu,) studied this question.