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

G.H. Yang^1*, J.Y. Chen² and W.Q. Pu¹

¹ Phytopathology Lab of Yunnan Province, Yunnan Agricultural University, Kunming 650201, China
² Yunnan Tobacco Company, Kunming 650000, China

*ghyang2000@yahoo.com

Accepted for publication 08/09/06

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).

Figure 1: Symptoms of head rot on cabbage (Brassica oleracea var. capitataare; left) and web-blight of snap bean (Phaseolus vulgaris; right)

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.

Figure 2: Homologic tree constructed by analysis 5.8s rDNA-ITS nucleotide sequences from 6 strains of Rhizoctonia solani.  The ITS sequences of AG-4HG-I (AY154307), AG-4HG-II (AY154308), AG-4HG-III (AY154309) were aligned with ones of CA-04-1, CA-04-2, SB-04-1 and SB-04-2.

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.

Acknowledgements

The research is supported by the 973 program (2006CB100200)

References

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Carling DE, 1996. Grouping in Rhizoctonia solani by hyphal anastomosis reaction. In: Sneh B, Jabaji-Hare S, Neat S, Dijst G, eds. Rhizoctonia species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control. Dordecht, The Netherlands: Kluwer Academic Publishers, 37-47.

Kuninaga ST, Nastsuaki T, Takeuchi R, Yokosawa, 1997. Sequence variation of the rDNA ITS regions within and between anastomosis groups in Rhizoctonia solani. Current Genetics 32, 237-243.

Yang GH, Chen HR, Naito S, Ogoshi A, Deng YL, 2005. First report of AG-A of binucleate Rhizoctonia in China, pathogenic to soya bean, pea, snap bean and pak choy. Journal of Phytopathology 153, 333-336.

The British Society for Plant Pathology