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First Report of Phomopsis Seed Decay of Soybean Caused by Phomopsis longicolla in South China

Phomopsis seed decay of soybean (Glycine max (L.) Merr.) causes poor seed quality and suppresses yield in most soybean-growing areas in the world. The disease is caused primarily by Phomopsis longicolla Hobbs. During the spring of 2010, soybean seeds without symptoms were planted in the fields but emergence was poor and the emergence rates ranged from 30 to 70% in south China. Approximately 3,000 symptomless seeds were randomly collected from seven fields at three locations in Guangzhou, Nanchang, and Wuhan. Seeds were surface disinfected with 1% sodium hypochlorite for 12 min, rinsed in sterile distilled water three to four times, and placed on 2% agar. Plates were then incubated at 26°C under 16/8-h photoperiod for 3 to 4 days. About 10 to 20% of the seeds produced white hyphae that spread rapidly and covered the whole seed. The hyphae from fungal isolates were transferred to potato dextrose agar (PDA) and incubated at 26°C in the dark. After 3 to 4 weeks, conidia were elliptical with two oil drops at both ends and hyaline (6.2 to 7.2 × 2.6 to 3.2 μm). The cultural and morphological characteristics of the isolates corresponded with the description of P. longicolla (2). Colonies on PDA were floccose, dense, and white. Stromata were large, black, and spreading. To confirm the morphological identification, the ribosomal internal transcribed spacers (ITS1-5.8S-ITS2) from three isolates were sequenced (GenBank Accession Nos. JQ899030, JQ899031, and JQ899032). BLAST analysis indicated that the isolates had 99% nucleotide sequence identity with P. longicolla (GenBank Accession Nos. AY857868.1, EF026104, and HQ130441.1). Pathogenicity tests were conducted on 2-week-old soybean seedlings (3). A mycelial plug (3 mm in diameter) from the margin of 1-week-old PDA culture of the Wuhan isolate was individually placed mycelial side down directly on the top of cut stem 1 to 2 cm above cotyledon node of the soybean seedling. PDA plugs without the fungus was used as the negative control. All seedlings were kept in a growth chamber at 26°C with 92 to 94% relative humidity. After 2 weeks, all inoculated seedlings showed browning, stem wilt, and the lesions were 0.3 to 2.0 cm long. No symptoms were observed in the control plants. P. longicolla was reisolated from the infected seedlings. The pathogenicity test was repeated three times. Soybean stem blight caused by P. longicolla has been reported in northeast China (1). To our knowledge, this is the first report of P. longicolla causing Phomopsis seed decay of soybean in south China. This report will establish a foundation for developing a program for screening germplasm for resistance to this disease in south China.

References: (1) Y. L. Cui et al. Plant Pathol. 58:799, 2009. (2) T. W. Hobbs et al. Mycologia 77:535,1985. (3) S. Li et al. Plant Dis. 85:1031, 2001.