Biological control of plant bacterial pathogens by a beneficial Streptomyces sp.

In the present study a set of beneficial microorganisms were evaluated for their in vitro antimicrobial activity on various phytopathogenic bacteria, like Xanthomonas vesicatoria, Clavibacter michiganensis subsp. michiganensis (Cmm), Clavibacter michiganensis subsp. sepedonicus (Cms), Acidovorax citrulli and Ralstonia solanacearum. A prospective antagonist, strain SA51 (Streptomyces sp.) was selected based on its remarkable in vitro antibacterial activity and was further studied under greenhouse conditions on tomato (Solanum lycopersicum) seedlings as a model plant against X. vesicatoria. Fromin vitro results, it was clearly evident that SA51 was very active against bacteria like Cmm, Cms, R. solanacearum and X. vesicatoria compared to other antagonists. Tomato plantlets were inoculated with SA51 at the roots and, thereafter, they were sprayed with X. vesicatoria. Protection against X. vesicatoria by the bacterized tomato plants was confirmed in the greenhouse: disease was reduced by approximately 96%. Additionally, plants bacterized by strain SA51 showed significant plant growth, particularly in aerial parts as compared to un-bacterized controls. Finally, benefit was seen in inoculated healthy plants in terms of a significant increase in dry weight and length of roots and shoots, as compared to the uninoculated controls. A GFP mutant of strain SA51 was produced to study its endophytic colonisation in tomato plants: results confirmed that SA51 was able to efficiently colonise tomato endophytically, from the roots to the leaves. Field experiments confirmed the ability of strain SA51 to act as plant growth promoting agent: such promoting activity was also reflected into an increase of fruit production by approximately 7%.