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First Report of Soybean Pod and Stem Blight Caused by Diaporthe phaseolorum var. sojae in Taiwan

Genera, Diaporthe and Phomopsis, from an important pathogenic complex of soybean (Glycine max) throughout the world, cause reductions in plant stands, yield, and seed health and quality (1). In a study of the Diaporthe/Phomopsis complex in Taiwan in March 2008, four stem samples with black fruiting structures in linear rows on senescent soybean were collected from the research fields at AVRDC, Shanhua, Tainan, Taiwan. Unidentified fungal isolates were obtained by surface disinfection of infected stems and plating excised tissues on potato dextrose agar (PDA). Colonies of the isolates showed ropelike white mycelia with yellowish tonalities. Small and scattered black stromata were observed frequently in the medium. Mutic pycnidia were found solitarily or aggregated in conidiomata on PDA plates after 10 days of incubation at 24°C with a 12-h photoperiod with near-UV light. All isolates produced α-conidia that measured 8.78 × 3.32 (7.00 to11.00 × 3.00 to 4.00) μm, and sporadically, β-conidia of 30.58 × 0.85 (26.00 to 33.00 × 0.60 to 1.20) μm. Perithecia were not observed in the collected stem samples or the fungal cultures on PDA. Restriction fragment length polymorphism patterns of the PCR products amplified by ITS₄ and ITS₅ primers for all isolates were identical to the patterns reported for Diaporthe phaseolorum var. sojae (3). Thus, all isolates were identified as D. phaseolorum var. sojae on the basis of morphologic and genetic characteristics (2,3). Pathogenicity was confirmed through inoculations during the V2 growth stage of soybean seedlings by atomizing conidial suspensions (1 × 10⁷ α-conidia/ml) of each isolate on soybean seedlings as well as by injecting the inoculum into soybean stems separately. Four plants in each of three replications were inoculated for each method and six noninoculated plants served as controls. Plants were incubated in a growth chamber at 25°C and maintaining relative humidity at 100% by a humidifier for 48 h in darkness; thereafter, plants were maintained in the greenhouse at temperatures ranging from 23 to 34°C. Seven days after inoculation, red-brown leaf spots and coalescent lesions were observed on seedlings atomized by inoculum suspensions, as well as brown lesions and black pycnida in linear rows observed on plants inoculated by stem injection. No symptoms were observed on noninoculated plants. The fungal isolates obtained from inoculated soybeans were morphologically identical to those used as inoculum. The pathogenicity test was repeated twice. To our knowledge, this is the first explicit report identifying the causal agent of soybean pod and stem blight in Taiwan. The vouchers of infected specimens are available at AVRDC-The World Vegetable Center.

Reference: (1) R. P. Mulrooney. Plant Dis. 70:600, 1988. (2) R. N. Pioli et al. Phytopathology 93:136, 2003. (3) A. W. Zhang et al. Phytopathology 88:1306, 1998.