First report of foliar blight in Brassica rapa ssp. chinensis caused by Rhizoctonia solani AG-4

Abstract

Brassica rapa ssp. chinensis (bok choy) is a vegetable popular in Asia. In July 2003, foliar blight symptoms were first observed on leaves of B. rapa ssp. chinensis at the six- to eight-leaf stage in a field at Jialize Farm, Kunming, Yunnan Province, China. The symptoms appeared as primary lesions consisting of small, circular necrotic spots, 1–3 mm in diameter, accompanied by secondary lesions with large irregular borders. Under high moisture conditions white hyphae appeared on the stems; however, no lesions were observed. Sections (1 × 2 cm) cut from infected stems were surface-sterilized and transferred to potato dextrose agar. Rhizoctonia solani was recovered consistently from this material. Anastomosis group testing revealed that all 13 isolates belonged to AG4 (Sneh & Ogoshi, 1998). Pathogenicity tests with a hyphal-tip isolate were carried out using B. rapa ssp. chinensis in two controlled-environment cabinets (one set at 25°C, 16-h day and the other at 16°C, 8-h night cycle), one utilizing seeds and the other utilizing plants at the six-leaf stage. Both were planted in 3-week-old inoculated cornmeal-sand medium. A high level of humidity was maintained by placing a pan of water in the bottom of the incubators. Control plants were planted in uninoculated cornmeal-sand medium. After 1 week emerging seedlings exhibited both leaf and stem necrosis near the apex. On the 6-week-old plants, foliar blight of young leaves was more severe than those observed on the older leaves. After 2 weeks, grey-white hyphae appeared on the stem but no lesions were present. Control plants did not develop symptoms. Rhizoctonia solani AG4 was re-isolated from the inoculated plants. Rhizoctonia foliar blight of cabbage (Brassica oleracea var. capitata) has been reported in New York State, USA (Abawi & Martin, 1985). However, this is the first published report of foliar blight of bok choy (Brassica rapa ssp. chinensis) caused by R. solani AG4. The Yunnan Nature Science Fund supported this study.