Abstract
Mechanisms of resistance to fusarium wilt(Fusarium udum)were investigated in pigeon pea cultivars from Malawi. Wilt‐susceptible (Malawi local) and wilt resistant (ICP 9145) plants were stem‐inoculated with a spore suspension containing 2·106conidia/ml of the pathogen. Occlusion of a small proportion of infected vessels was observed, but the resistant reaction appeared to depend mainly on rapid phytoalexin synthesis. Four fungitoxic isoflavonoid phytoalexins—hydroxygenistein, genistein, cajanin and cajanol‐were isolated from plants 15 days after inoculation. Cajanol was identified as the main antifungal compound. The concentration of cajanol was 329·4 μg/g in the resistant cultivar as against 88·6 μg/g in the susceptible cultivar 15 days after inoculation. Crude extract from the resistant plants sampled at 24 h after inoculation contained 34·8 μg ml of cajanol. The LD50value of cajanol for spore germination was determined as 35μg/ml. The cajanol content of fungus‐infected ICP 9145 10 days after inoculation totally inhibited conidial germination ofF. udum.
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