DOSE-RESPONSE RELATIONSHIPS IN BIOLOGICAL-CONTROL OF FUSARIUM-WILT OF RADISH BY PSEUDOMONAS SPP

The dose-response relationships in the suppression of Fusarium wilt of radish by Pseudomonas putida strain WCS358 and I! fluorescens strain WCS374 were investigated. The sole mechanism involved in the suppression of Fusarium wilt of radish by strain WCS358 is siderophore-mediated competition for iron(III), whereas strain WCS374 suppresses this disease by induction of systemic resistance. The efficacy of siderophore-mediated disease suppression and induced resistance was highly dependent on the level of disease incidence. Both mechanisms of biological control were effective over a wide range of disease incidences. The absolute disease reduction reached a maximum of approximately 30% at an average level of disease incidence of 50% for both mechanisms. The rhizosphere population density of strain WCS358 and of strain WCS374 was an important determinant of their efficacy to suppress Fusarium wilt of radish. Regression analysis demonstrated significant nonlinear asymptotic relationships between the rhizosphere population densities of strain WCS358 or strain WCS374 and the level of suppression of Fusarium wilt of radish. At moderate to relatively high levels of disease incidence for both mechanisms, a threshold population density of the bacterial strain of approximately 10(5) CFU per gram of root is required for a significant suppression of Fusarium wilt of radish. When rhizosphere population densities of both strains dropped below this threshold level, a relatively small decline in the population density had a major effect on their efficacy to suppress Fusarium wilt of radish. Increasing the rhizosphere population density of both strains to levels higher than the threshold level did not result in a significant improvement of disease suppression.