First report of head rot of cabbage and web blight of snap bean caused by Rhizoctonia solani AG‐4 HGI

During July 2004, head rot of cabbage (Brassica oleracea var. capitata) and web blight of snap bean (Phaseolus vulgaris) were frequently observed in Dehong district in southern Yunnan Province, China. Affected cabbages had a dark, sometimes wet decay at the bases of outer leaves and on emerging cabbage heads. The outer leaves later collapse. A brown mycelium appeared on affected parts after damp weather with occasional small brown sclerotia on the cabbage head. The first symptoms on snap bean were small, circular, watersoaked spots on stems, pods, and foliage, later tan-brown with a dark border, up to 2 cm across. Irregular, light brown sclerotia and a fine mycelium develop as plants become seriously blighted. Affected plant parts from cabbage and snap bean were surface-sterilized and incubated on potato dextrose agar (PDA). Rhizoctonia solani was recovered consistently. All cabbage and snap bean isolates anastomosed with tester isolates of subgroups HGI, HGII, and HGIII within AG-4, giving a C2 hyphal fusion reaction (Carling, 1996). The 5·8 s rDNA-ITS of four cabbage and snap bean isolates (CA-04–1, CA-04–2, SB-04–1 and SB-04–2 respectively) matched isolates of R. solani AG-4, subgroup HGI (Kuninaga et al., 1997). Cabbage heads and snap bean green pods were inoculated with two of their respective isolates grown for 4 days on PDA. After covering with moist cotton to avoid drying out, test plants were held in a greenhouse with ∼28°C-16 h day and 15°C-8 h night. Plugs of sterile PDA were placed on cabbage and snap bean plants as controls. These remained healthy while, within one week, inoculated cabbage and snap bean plants showed similar symptoms to those seen naturally. Rhizoctonia solani was reisolated from these plants, confirming its pathogenicity. This is first report of Rhizoctonia solani AG-4 HGI in China causing head rot of cabbage and web blight of snap bean. AG-A infects snap bean in China (Yang et al., 2005) and AG-4 in Iran (Balali & Kowsari, 2004), but through root infections. Rhizoctonia solani routinely affects cabbages and further study is needed to compare the pathogenicity of isolates in relation to their sub-groups. The research is supported by the 973 program (2006CB100200) and the National Nature Science Fund (30660006).