Pseudomonas spp. vs. tumorigenic Rhizobium sp. – biocontrol of crown gall disease

BACKGROUND Atypical Rhizobium sp. were identified as a causative agent of crown gall on rhododendron and blueberry in Germany. Considering the lack of efficient measures against crown gall disease, evaluation of innovative biocontrol measures would greatly contribute to sustainable agriculture. OBJECTIVES Identification of candidate bacterial strains that could be employed for biological control of the novel crown gall bacteria belonging to the ″tumorigenes″ clade. METHODS Antimicrobial activity of six antagonistic Pseudomonas and Bacillus strains were tested in vitro and in vivo against pathogenic strain Rhizobium sp. rho-6.2. The in vivo experiment, using co-inoculation and preventive inoculation strategies, was performed in controlled greenhouse conditions on tomato plants as a model system in four replicas and randomized. RESULTS The whole cultures of two antagonistic Pseudomonas strains were the most efficient against pathogenic Rhizobium sp. rho-6.2 in co-inoculation strategy with reducing tumor size of 92.86%. Contrary, in the preventive treatment same Pseudomonas strains were less efficient (15.38 and 30.77%). Despite the high in vitro antimicrobial activity of Bacillus strains their in vivo activity was less pronounced in preventive treatment (up to 15.38%), while in co-inoculation strategy was more prominent (42.86%). Consortium based on Bacillus and Pseudomonas strains, was increased biocontrol activity up to 38.6% of tumor’s reduction. The selected Pseudomonas strains could be further tested as an alternative strategy for the biocontrol of crown gall disease and potential involvement of quorum quenching mechanism will be determined.