First Report of Red Crown Rot Caused by Cylindrocladium parasiticum on Soybean in Guangdong, Southern China.

Abstract

In October 2006, occurrence of a disease on soybean (Glycine max) was observed in Guangdong Province of southern China with symptoms resembling red crown rot (3,4). Reddish brown lesions girdled the basal stems with numerous reddish orange perithecia on the lesion surface. Roots became black and rotted and whole plants wilted and died. More recently, outbreaks of this disease were observed in several counties in Guangdong. Disease incidence reached as much as 80% on cv. Huaxia 3 in some fields, causing severe yield losses. Isolation was made from the edge of lesions on potato dextrose agar at 25°C. The fungus produced white, aerial mycelia and a burnt orange-to-dark brown submerged growth. Conidiophores were borne laterally on a stipe, terminating in a hyaline, globose vesicle measuring 4.0 to 13.0 μm in diameter. After branching from the stipe, the conidiophore continued to develop by forming two to three single-celled branches. These cells might give rise to two or three shorter branches (14.8 to 36.9 μm long) and phialides. The phialides were doliform, nonseptate, and measured 7.4 to 19.7 × 3.7 to 4.9 μm. Conidia were hyaline, cylindrical with one to three septa (mostly three septa), and measured 54.1 to 76.3 × 4.9 to 7.4 μm. Perithecia were orange to red, subglobose to oval or globose, 212.1 to 454.5 μm high, and 111.1 to 333.3 μm wide. Asci were hyaline, clavate, thin walled, long stalked, measured 121.0 to 200.8 × 11.5 to 25.6 μm, and each contained eight ascospores that became aggregated in the upper half of the ascus at maturity. The ascospores were hyaline, fusoid to falcate with one to three septa (mostly with one septum), constricted slightly at the septum, and measured 29.5 to 73.8 × 4.9 to 9.8 μm. The fungus was identified as Cylindrocladium parasiticum (teleomorph Calonectria ilicicola) (1,2). The beta-tubulin gene fragment sequences of three isolates were obtained (one sequence being GenBank Accession No. GU073284) and comparisons with GenBank showed 99 to 100% similarity with Calonectria ilicicola (EF159730 and AY725643). Pathogenicity was confirmed by inoculation of 2-week-old seedlings of cv. Huaxia 3 in plastic pots (10 × 9 cm) by drenching with a conidia suspension (105 conidia per ml). All inoculated plants showed similar red crown rot symptoms on stem bases and roots 1 week after inoculation. C. parasiticum was reisolated from the diseased plants, and many orange-to-red perithecia of Calonectria ilicicola were formed on the lesions 3 weeks after inoculation. This pathogen may pose a serious threat to >300,000 ha of soybean production as well as >300,000 ha of peanut production in Guangdong Province. It has been previously reported in Jiangsu Province in eastern China (3) and Yunnan Province in western China (4).