First report of head rot of cabbage and web-blight of snap bean caused by Rhizoctonia solani AG 4 HG-I

During July 2004, head rot of cabbage (Brassica oleracea var. capitataare) 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 (Fig. 1). 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 2cm across (Fig. 1). Irregular, light brown sclerotia and a fine mycelium develop as plants become seriously blighted. Affected plant parts from cabbage and snap bean were surface-sterilised and plated on potato dextrose agar. Rhizoctonia solani was recovered consistently. All cabbage and snap bean isolates anastomosed with tester isolates of subgroups HG-I, HG-II, and HG-III within AG-4, giving a C2 hyphal fusion reaction (Carling, 1996). The 5.8s 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 HG-I (Kuninaga et al., 1997; Fig. 2).

Cabbage heads and snap bean green pods were inoculated with two of their respective isolates grown on 4 days-old PDA. After covering with moist cotton to avoid drying out, test plants were held in a greenhouse with ca. 28°C-16h day and 15°C-8h 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. R. solani was reisolated from these plants, confirming its pathogenicity.

This is first report of Rhizoctonia solani AG 4 HG-I 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. R. solani routinely affects cabbages and further study is needed to compare the pathogenicity of isolates in relation to their sub-groups.