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 fa sciculatum with a wilt-causing soil borne pathogen, Fusarium oxysporum , was studied in cowpea (Vigna unguiculata). It was found that pre-est ablishment by vesicular-arbuscular mycorrhizal fungus reduced the colo nization of the pathogen and the severity of the disease, as determine d by reduction in vascular discolouration index. In mycorrhizal plants , the production of phytoalexin compounds was always higher than in th e non-mycorrhizal plants. There appeared to be a direct correlation be tween the concentration of the phytoalexins and the degree of mycorrhi zal association. Three different compounds with R(f) values of 0.23 (I ), 0.17 (II) and 0.11 (III) were obtained from mycorrhizal plants. Sim ilar compounds were also found to be induced by an abiotic elicitor Cu SO4. The first compound was identified as an isoflavonoid, daidzein an d the other two remain to be identified. These compounds were checked for their antifungal activity in vitro. The germination of conidial sp ores of Fusarium oxysporum was strongly inhibited by the compound III than the other two. It is argued that the production of phytoalexin co mpounds in mycorrhizal plant could be one of the mechanisms imparting tolerance of the plants to wilt disease.