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First Report of Hypocotyl and Root Rot Disease Caused by Pythium deliense on Soybean Sprouts in Korea

Soybean (Glycine max Merr.) sprouts have been a traditional Korean food for at least 1,000 years. During the summers of 2000 and 2001, severe hypocotyl and root rot occurred on fully grown soybean sprouts, especially in commercial recirculating mass production systems. Brown rot on water-soaked hypocotyls and roots of soybean sprouts caused a 10 to 20% loss in production yield. To investigate the cause, 180 sections of tissue were surface sterilized for 30 s in 75% ethanol and plated on potato dextrose agar (PDA). A fungus with white, aerial mycelia and cultural characteristics of Pythium sp. was isolated consistently. The fungal isolates were identified as Pythium deliense Meurs based on various mycological characteristics (2) on corn meal agar (CMA) and sucrose-asparagine bentgrass leaf culture medium (1). P. deliense oogonia were spherical, smooth, 19 to 23 μm in diameter, and their stalks bent toward the antheridia. The antheridia were the shape of a straw hat, curved club-shaped, terminal or intercalary, monoclinous, occasionally diclinous, 12 to 15 × 8 to 11 μm, and 1 per oogonium (2). In pathogenicity tests, soybean sprouts and the fungus were cultured simultaneously in containers (30 × 30 × 50 cm [W × L × H]) with a daily 3-h showering period at 25°C. There were 8,000 to 10,000 seeds per container inoculated with four plugs of agar inoculum (2 × 2 cm). Inoculum was prepared from 5-day-old fungal cultures grown on PDA. After 6 days, the inoculated soybean sprouts showed the same symptoms as described above, whereas the noninoculated sprouts remained healthy. The fungal pathogen was reisolated from all the inoculated sprouts. P. deliense has never been reported as a pathogen of soybean sprouts. To our knowledge, this is the first report of hypocotyl and root rot disease caused by Pythium deliense on soybean sprout in Korea.

References: (1) J.-W. Kim and E.-W. Park. Kor. J. Mycology 25:276, 1997. (2) A. J. Van der Plaatis-Niterink. Monograph of the genus Pythium. Vol. 21, Studies in Mycology. Centraalbureau voor Schimmelcultures, Inst. R. Neth. Acad. Sci. Lett. The Netherlands, 1981.