Plant defence reactions against fusarium wilt in chickpea induced by incompatible race 0 of Fusarium oxysporum f.sp ciceris and nonhost isolates of F-oxysporum

Germinated seeds of 'kabuli' chickpea cv. ICCV 4 were inoculated with a conidial suspension of the incompatible race 0 of Fusarium oxysporum f.sp. ciceris (Foc) or of nonhost F. oxysporum resistance 'inducers', and 3 days later were challenged by root dip with a conidial suspension of highly virulent Foc race 5. Prior inoculation with inducers delayed the onset of symptoms and/or significantly reduced the final amount of fusarium wilt caused by race 5. However, the extent of disease suppression varied with the nature of the inducing agent; the nonhost isolates of F. oxysporum were more effective at disease suppression than the incompatible Foc race 0. Inoculation with the inducers gave rise to synthesis of maackiain and medicarpin phytoalexins in inoculated seedlings; these did not accumulate in plant tissues but were released into the inoculum suspension. Inoculation with inducers also resulted in accumulation of chitinase, beta-1,3-glucanase and peroxidase activities in plant roots. These defence-related responses were induced more consistently and intensely by nonhost isolates of F. oxysporum than by incompatible Foc race 0. The phytoalexins and, to a lesser extent, the antifungal hydrolases, were also induced after challenge inoculation with Foc race 5. However, in this case the defence responses were induced in both preinduced and noninduced plants infected by the pathogen. It is concluded that the suppression of fusarium wilt in this study possibly involved an inhibitory effect on the pathogen of preinduced plant defences, rather than an increase in the expression of defence mechanisms of preinduced plants following a subsequent challenge inoculation.