Biological control of Verticillium wilt on cotton by the use of fluorescent Pseudomonas spp. under field conditions

Four out of 59 fluorescent Pseudomonas spp. strains collected from cotton and weed rhizosphere were selected based on the following criteria: (1) inhibition of Verticillium dahliaein vitro, (2) disease suppression on two cotton cultivars grown from bacterized seeds using stem-injection with the conidia of V. dahliae, and (3) seedling vigor test (dry weight) under greenhouse conditions. Four selected Pseudomonas strains isolated from Xanthium strumarium (FP22), Portulaca sp. (FP23), Gossypium hirsitum (FP30), and Convolvulus arvensis (FP35), as well as the known biocontrol agent Serratia plymuthica (HRO-C48), were further tested for the impact on Verticillium wilt, growth parameters of cotton, and yield in a naturally infested field. The reduction of AUDPC by the seed bacterization with FP22, FP23, FP30, FP35, and HRO-C48 compared to non-bacterized control ranged from 39.2% to 50.9% and 22.1% to 36.8% in trials done in 2005 and 2006, respectively. The growth parameters (plant height, number of nodes on main stem, and NAWF-nodes above white flower) were significantly higher in seed bacterized plants compared to the untreated control. In the 2005 field trial, the increase of seed cotton yield by the treatment with four Pseudomonas strains and HRO-C48 ranged from 13.1% to 22.3% in Sayar 314 and 4.2% to 12.8% in Acala Maxxa. Seed cotton yield was not significantly influenced by the 2006 treatments. Our results indicate that seed treatment of cotton plants with our Pseudomonas spp. strains and the known strain Serratia plymuthica can help in the biocontrol of V. dahliae and improve growth parameters in cotton fields.