Biological control of Pseudomonas syringae pv. glycinea by epiphytic bacteria under field conditions

The efficacy of a bacterial strain as a biocontrol agent in the field may b e related to the ecological similarity between the biocontrol agent and the target pathogen. Therefore, a number of different Pseudomonas syringae str ains were evaluated for their antagonistic activities in vitro (agar-diffus ion assay) and in planta (greenhouse assay) against the target pathogen, Ps eudomonas rsyringae pv. glycinea. Six strains of five different pathovars w ere found to be antagonistic in vitro as well as in planta. The epiphytic f itness of the antagonistic Pseudomonas syringae Strain 22d/93 and its two a ntibiotic-resistant mutants were examined on soybean plants in the fields. After adaptation the parental strain and its mutants had the ability to est ablish and maintain large epiphytic populations (about 10 degrees cfu/g FW) over the whole growing season after a single spray inoculation. The epiphy tic behaviors of the mutants and the parent were not significantly differen t. The introduced bacteria did not influence the total bacterial population size. When the antagonist was coinoculated with the pathogen, the developm ent of the pathogen was significantly reduced during the whole growing seas on. When the antagonistic strain was inoculated 4 weeks in advance of the p athogen, this antagonistic effect could be markedly enhanced. The final pop ulation size of the pathogen reached just 10(4) cfu/g FW and was significan tly reduced to 0.12% compared to the pathogen alone. This study demonstrate s that biological control of foliar pathogens through colonization of the h ost plants with near isogenic or ecologically similar antagonistical strain s seems to be a realistic goal.