Cylindrocladium crotalariae causing red crown rot of soybean in China

Soybean is one of the main crops grown in Yunnan Province, China, where the crop is grown in spring, summer and autumn. Occurrence of a disease was observed in this region in 2002. The disease symptom looked like red crown rot (Russin & Kuruppu, 1999). The top leaves of infected plants had interveinal necrosis, while small red spheres were scattered or concentrated on roots and stems bases, which also had black, streaky lesions. The outer surface of stems from infected plants was dark in colour. In some autumn and summer crops, the disease caused severe yield losses. Symptomatic plants were sampled from fields of autumn-season soybean crops. Red spherical fruiting bodies were isolated from diseased tissues. Microscopic examination revealed that these were perithecia, 310-510 µm diameters and 325-520 µm in length. Under gentle pressure, asci and ascospores were exuded from these bodies. The asci were curved, fusoid, shuttle-shaped, 10-23 µm in diameter and 101-150 µm in length. Ascospores were rod-shaped or falcate, with mostly rounded ends, double celled, and 4.5-5.6 µm in diameter and 30-54 µm in length. Isolation was made with stem tissues from the edge of disease lesions on potato dextrose agar (PDA) at 28° C. Hyphae were observed after incubation for 2 days and conidia were seen after 3-4 days. The conidia measured 4.2-6 µm in diameter and 30-65 µm in length, and were terminal budding by conidiophores. Conidia were cylindrical and had one or two cells. The fungus was identified as Cylindrocladium crotalariae (sexual form Calonectria crotalariae), which is an ascomycete in the Pyronomycete group. Greenhouse experiments were conducted to determine the pathogenicity of isolates. Isolates were increased on PDA and mixed with sterilised soil in pots. Soybean seeds (variety Dian Cang 2) were planted in the pots and placed in greenhouse at 25±2° C. Leaf and stem symptoms of red crown rot were observed in the plants and Cylindrocladium crotalariae was isolated from these diseased plants, according to Hwang's method (Hwang & Ko, 1976).

This is first finding of soybean red crown rot in Yunnan province, where outbreaks of this disease have been observed in counties in the central and eastern part of the province, where autumn soybeans are planted after summer rice. This pathogen has been previously reported in China in Jiangsu Province (Gai, 1992) but is not known elsewhere.