MIXTURES OF PLANT GROWTH-PROMOTING RHIZOBACTERIA ENHANCE BIOLOGICAL-CONTROL OF MULTIPLE CUCUMBER PATHOGENS

Plant growth-promoting rhizobacteria (PGPR) strains INR7 (Bacillus pum ilus), GB03 (Bacillus subtilis), and ME1 (Curtobacterium flaccumfacien s) were tested singly and in combinations for biological control again st multiple cucumber pathogens. Investigations under greenhouse condit ions were conducted with three cucumber pathogens-Colletotrichum orbic ulare (causing anthracnose), Pseudomonas syringae pv. lachrymans (caus ing angular leaf spot), and Erwinia tracheiphila (causing cucurbit wil t disease)-inoculated singly and in all possible combinations. There w as a general trend across all experiments toward greater suppression a nd enhanced consistency against multiple cucumber pathogens using stra in mixtures. The same three PGPR strains were evaluated as seed treatm ents in two field trials over two seasons, and two strains, IN26 (Burk holderia gladioli) and INR7 also were tested as foliar sprays in one o f the trials. In the field trials, the efficacy of induced systemic re sistance activity was determined against introduced cucumber pathogens naturally spread within plots through placement of infected plants in to the field to provide the pathogen inoculum. PGPR-mediated disease s uppression was observed against angular leaf spot in 1996 and against a mixed infection of angular leaf spot and anthracnose in 1997. The th ree-way mixture of PGPR strains (INR7 plus ME1 plus GB03) as a seed tr eatment showed intensive plant growth promotion and disease reduction to a level statistically equivalent to the synthetic elicitor Actigard applied as a spray.