INDUCTION AND ACCUMULATION OF PHYTOALEXINS IN COWPEA ROOTS INFECTED WITH A MYCORRHIZAL FUNGUS GLOMUS-FASCICULATUM AND THEIR RESISTANCE TO FUSARIUM-WILT DISEASE

The interaction of a vesicular-arbuscular mycorrhizal fungus Glomus fasciculatum with a wilt-causing soil borne pathogen, Fusarium oxysporum, was studied in cowpea (Vigna unguiculata). It was found that pre-establishment by vesicular-arbuscular mycorrhizal fungus reduced the colonization of the pathogen and the severity of the disease, as determined by reduction in vascular discolouration index. In mycorrhizal plants, the production of phytoalexin compounds was always higher than in the non-mycorrhizal plants. There appeared to be a direct correlation between the concentration of the phytoalexins and the degree of mycorrhizal association. Three different compounds with R(f) values of 0.23 (I), 0.17 (II) and 0.11 (III) were obtained from mycorrhizal plants. Similar compounds were also found to be induced by an abiotic elicitor CuSO4. The first compound was identified as an isoflavonoid, daidzein and the other two remain to be identified. These compounds were checked for their antifungal activity in vitro. The germination of conidial spores of Fusarium oxysporum was strongly inhibited by the compound III than the other two. It is argued that the production of phytoalexin compounds in mycorrhizal plant could be one of the mechanisms imparting tolerance of the plants to wilt disease.