Microbial Antagonism at the Root Level Is Involved in the Suppression of Fusarium Wilt by the Combination of Nonpathogenic Fusarium oxysporum Fo47 and Pseudomonas putida WCS358

ABSTRACT Two biological control agents, nonpathogenic Fusarium oxysporum Fo47 and Pseudomonas putida WCS358, were evaluated for suppression of Fusarium wilt of flax grown in nutrient solution and for suppression of the population density and metabolic activity of the causal organism F. oxysporum f. sp. lini strain Foln3GUS on root surfaces. Due to the presence of an introduced gusA reporter gene construct in Foln3GUS, the pathogen expressed beta-glucuronidase activity that was related to its carbon metabolism. At a Fo47 to Foln3GUS inoculum ratio of 100:1, both the population density of the pathogen and the beta-glucuronidase activity on and in flax roots were reduced by the nonpathogenic strain, and Fusarium wilt was suppressed. At a Fo47 to Foln3GUS inoculum ratio of 10:1, Fo47 decreased the severity of Fusarium wilt to a smaller extent and it also reduced beta-glucuronidase activity without reducing the density of Foln3GUS on flax roots. At a nonpathogenic to pathogenic Fusarium strains ratio of 10:1, the addition of P. putida WCS358 further suppressed Fusarium wilt and the density of the pathogen at the root level, whereas a mutant of WCS358 deficient in pseudobactin production had no significant effect. Iron availability to WCS358 on flax roots, assessed by ice-nucleation activity conferred from a transcriptional fusion (pvd-inaZ) of an ice-nucleation reporter gene to an iron-regulated promoter, was sufficiently low to allow pseudobactin production. P. putida WCS358 did not reduce the severity of Fusarium wilt of flax when inoculated without Fo47, and it did not improve disease suppression achieved by high inoculum doses of Fo47 (a Fo47 to Foln3GUS ratio of 100:1). Together, these data provide evidence that (i) suppression of Fusarium wilt of flax by Fo47 is related to reductions in the population density and metabolic activity of the pathogen on the root surface; (ii) WCS358 can enhance the biological control activity of Fo47, but this enhancement depends on the population of Fo47 relative to the pathogen; and (iii) pseudobactin contributes to suppression of Fusarium wilt by the combination of Fo47 and WCS358 on roots in which conditions are conducive to pseudobactin production by the bacterium.