Biocontrol of Pseudomonas savastanoi, causative agent of olive knot disease: antagonistic potential of non-pathogenic rhizosphere isolates of fluorescent Pseudomonas.

Olive knot, a major bacterial disease of olive plants (Olea europaea), is caused by Pseudomonas savastanoi. There is no cost-effective way to control olive knot using available chemical bactericides (Young 2004). Furthermore, development of resistance and environmental concerns urge the necessity for alternative, more selective control methods such as biological control. A potential resource for novel compounds of interest for use in biocontrol is constituted by the diverse biomolecules that mediate antagonism among plant-associated bacteria, including peptide or proteins (bacteriocins) and secondary metabolites frequently produced by pseudomonads (Parret et al. 2003, 2005). Lavermicocca et al. identified a bacteriocin inhibitory against P. savastanoi from the carob tree pathogen P. syringae pv. ciccaronei. (Lavermicocca et al. 1999, 2002). Notably, P. savastanoi was formerly regarded as a subspecies of P. syringae (Kennelly et al. 2007), indicating that this interaction represents an example of bacteriocin-mediated antagonism by a closely related bacterium. Antibacterial activity targeting P. savastanoi by phylogenetically more distant, non-phytopathogenic Pseudomonas isolates has not been reported. In the present work we explored the potential of in vitro P. savastanoi antagonism for a collection of such Pseudomonas isolates from different plant rhizospheres and geographical origins.