ENHANCEMENT OF VERTICILLIUM WILT RESISTANCE IN TOMATO TRANSPLANTS BY IN-VITRO COCULTURE OF SEEDLINGS WITH A PLANT-GROWTH PROMOTING RHIZOBACTERIUM (PSEUDOMONAS SP. STRAIN PSJN)

The potential utilization of a plant growth promoting rhizobacterium, Pseudomonas sp, strain PsJN, to enhance the resistance of tomato trans plants to verticillium wilt was investigated. Plant growth and disease development were tested on the disease-susceptible cultivar Bonny Bes t after Verticillium dahliae infection of tissue culture plantlets bac terized in vitro (by co-culturing with the bacterium) and seedlings ba cterized in vivo (after 3 weeks growth in the greenhouse). Significant differences in both disease suppression and plant growth were obtaine d between in vitro bacterized and nonbacterized (control) plants. The degree of protection afforded by in vitro bacterization depended on th e inoculum density of V. dahliae; the best and worst protection occurr ed at the lowest (10(3) conidia.ml(-1)) and highest (10(6) conidia.ml( -1)) levels, respectively. In contrast, the in vivo bacterized tomatoe s did not show plant growth promotion when compared to the nonbacteriz ed control plants. When challenged with Verticillium, significant grow th differences between in vivo bacterized plants (26.8% for shoot heig ht) and nonbacterized controls were only seen at the 3rd week after in oculation. Compared with the in vitro inoculation, there was no delay in the verticillium wilt symptom expression, even at the lowest concen tration of V, dahliae, by in vivo PsJN inoculation. These results sugg est that endophytic colonization of tomato tissues is required for the Verticillium-resistance responses. Plant growth promotion preceeds th e disease-resistance responses and may depend on the colonization thre sholds and subsequent sensitization of hosts.