Induced resistance and phytoalexin accumulation in biological control of Fusarium wilt of carnation by Pseudomonas sp. strain WCS417r

The involvement of mechanisms other than competition for iron in biological control of Fusarium oxysporum f. sp. dianthi of carnation by Pseudomonas sp. strain WCS417r was investigated. Pathogen and antagonist were spatially separated by bacterizing the roots while inoculating the stem with the pathogen. When the roots were bacterized with strain WCS417r 1 wk prior to stem inoculation with F. o. dianthi, the number of diseased plants in all experiments with cultivar Pallas was reduced from about 50 to 20%, and in one experiment with cultivar Lena from 69 to 38%. Strain WCS417r could not be isolated from stem tissue and control of Fusarium wilt by strain WCS417r, therefore, is not due to competition between F. o. dianthi and strain WCS417r, but has to be ascribed to induced resistance. Along with induced resistance, there was an increased accumulation of phytoalexins in stems of bacterized and inoculated plants compared with nonbacterized, fungus-infected plants. No accumulation of these compounds was found when the plants were bacterized but were noninfected. We concluded that signals, provided by strain WCS417r at the root system, induce in the stem sensitization of defense responses against F. o. dianthi, such as synthesis and accumulation of phytoalexins