Efficacy of native antagonistic bacterial isolates in biological control of crown gall disease in Egypt

In vitro analyzing the antagonistic activity of seventy native bacterial isolates towards plant tumorigenic Agrobacterium tumefaciens resulted in a selection of eight potential biocontrol agents. These isolates were screened for their antagonistic effect in vitro as well as their efficacy in reducing gall formation in planta. They were identified using Biolog microplates system as Bacillus megaterium, Paenibacillus polymyxa, Pseudomonas fragi (two isolates), Pseudomonas viridilivd, Pseudomonas asplenii, Curtobacterium flaccumfaciens and Curtobacterum sp.All antagonists tested exhibited considerable inhibitory activity in vitro and significantly reduced incidence and size of galls in rose shoots, kalanchoe leaves and squash fruits with variable degrees on the tested hosts. C. flaccumfaciens reduced the incidence of crown gall up to 100% in the case of rose shoots and kalanchoe leaves whereas the same antagonist reduced galling of squash fruits to 75%. Likewise, P. asplenii, P. viridilivd and P. polymyxa reduced the incidence of crown gall up to 100% in the case of kalanchoe leaves and squash fruits, whereas they reduced galling of rose shoots to 66.7%, 55.6% and 44.5% respectively. In the same manner, the two isolates of P. fragi reduced galling up to 100% in squash fruits, while it was 88.9% in rose shoots and kalanchoe leaves. Interestingly, B. megaterium isolate completely suppressed the gall development in rose shoots, whereas the gall incidence was 100% in kalanchoe leaves and 25% in squash fruits. Bacterial isolates characterized in this study may be considered as potential sources of novel bioactive metabolites as well as promising candidates to develop new biocontrol agents for controlling crown gall disease.