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

G.H. Yang¹, X.Q. Chen¹*, H.R. Chen¹, S. Natio ³, A. Ogoshi ² and J.F Zhao¹

¹ Phytopathology Lab of Yunnan Province, Yunnan Agricultural University, Kumming 650201, China
² Hokkaido Starch Industry Society, Sapporo, Japan. ³ Faculty of Agriculture, Hokkaido University, Sapporo, Japan

*cxqzb@163.com

Accepted for publication 28/11/03

Brassica rapa subsp. chinensis (bok choy) is a vegetable popular in Asia. In July 2003, foliar blight symptoms were first observed on leaves of B. rapa subsp. 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 (Fig. 1). Under high moisture conditions, white hyphae appeared on the stems, however no lesions were observed. Sections (1 by 2 cm) cut from infected stems were surface-sterilised and plated onto 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).

Figure 1. Symptoms of foliar blight on bok choy (Brassica rapa subsp. chinensis)

Pathogenicity tests with a hyphal-tip isolate were carried out using B. rapa subsp. 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, 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 one week emerging seedlings exhibited both leaf and stem necrosis near the head. 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 subsp. chinensis) caused by R. solani AG4.

Acknowledgements

The Yunnan Nature Science Fund supported this study.

References

Abawi GS, Martin SB, 1985. Rhizoctonia foliar blight of cabbage in New York State. Plant disease 69, 158-161.

Sneh B, Ogoshi A, 1998. Identification of Rhizoctonia species. Third Edition. St. Paul, Minnesota, USA: The American Phytopathological Society.

The British Society for Plant Pathology