D. Xu, and
C. Ji, Department of Plant Pathology, South China Agricultural University, Guangzhou 510642, China; and
M. Deng, Plant Protection Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China
In October 2011, a disease resembling stem blight of soybean was found in Zengcheng City, Guangdong Province, southern China. Symptoms began as a brown fusiform lesions on the stems, usually at the nodes. The lesions then darkened, elongated, and often girdled the stems, causing wilt of the above stems. The whole plant eventually died. There were many small, black, raised fruiting bodies in the lesions. The disease incidence was about 20%. Lesions with typical symptoms were sampled from diseased plants. Microscopic examination revealed that the fruiting bodies were pycnidia in which alpha-conidia were common but beta-conidia were rare. Alpha-conidia were hyaline, ellipsoidal to fusiform, guttulate, and measured 7.0 (4.3 to 10.0) × 3.0 (1.8 to 4.3) μm. The length/width ratio of alpha-conidia was 2.3 (1.4 to 4.5). Beta-conidia were hyaline, filiform, hamate, and measured 28.7 (18.2 to 35.7) × 1.8 (1.2 to 2.8) μm. A fungus was consistently isolated from the lesions on acidified potato dextrose agar (APDA, pH 4.5) at 25°C under intermittent fluorescent light (12 h daily). The colonies were floccose, dense, and white, with occasional green-yellow areas; the reverse was colorless with large, black stromata. To induce the production of fruiting bodies, autoclaved soybean stems were placed on the colonies growing on water agar at 25°C in darkness. Pycnidia with long beaks were observed on the stems 7 days later. The fungus was identified as Phomopsis longicolla (2). The rDNA internal transcribed spacer (ITS) region of the fungus was amplified with universal primers ITS4/ITS5 and sequenced (4). The sequences of two isolates were submitted to GenBank (Accession Nos. JX827608 and JX827609). BLASTn analysis showed that there was 99 to 100% similarity with sequences of P. longicolla deposited in GenBank (EF026104, AY857868, HQ130441, JF309198, JF309199, and AF132796). Pathogenicity tests were conducted on 14-day-old seedlings (cv. Huaxia 3) inoculated by placing mycelial plugs (5 mm in diameter and 4 days old) on slight cuts made on the lower stems (six replicates). The plugs were covered with a piece of wet cotton to maintain moisture. The control seedlings were treated the same but without mycelial plugs. All treated plants were incubated in 25°C in humid champers. Typical brown lesions with black raised pycnidia on the stems were observed 14 days after inoculation and P. longicolla was reisolated from these stem lesions. No disease was observed on control plants. To further verify that the fungus can cause seed decay, seeds were disinfected by 0.02% sodium hypochlorite and inoculated by putting them on the surface of the fungal colonies grown on APDA (pH 4.5) at 25°C. The control seeds were treated the same but without fungal colonies. All of the inoculated seeds decayed within 30 days whereas the control seeds maintained healthy. P. longicolla has been described as a pathogen causing serious stem blight and seed decay on soybean (3). The disease has been previously reported in Heilongjiang Province, northern China, but it was not known elsewhere in China (1). To our knowledge, this is the first report of P. longicolla on soybean in Guangdong Province, southern China. The pathogen may pose a serious threat to the production of soybean in this region of China.
References: (1) Y. Cui et al. Plant Pathol. 58:779, 2009. (2) T. W. Hobbs et al. Mycologia 77:535, 1985. (3) S. Li et al. Plant Dis. 94:1035, 2010. (4) A. W. Zhang et al. Plant Dis. 81:1143, 1997.