POTENTIAL EFFECT OF RHIZOBACTERIA IN THE MANAGEMENT OF CROWN GALL DISEASE CAUSED BY AGROBACTERIUM TUMEFACIENS BIOVAR 1

Crown gall disease caused by Agrobacterium tumefaciens is considered as the main bacterial disease of stone fruit rootstocks in Tunisia against which only prophylactic control measures are available. Therefore, twelve rhizobacterial strains and two reference antagonists Agrobacterium rhizogenes K84 and K1026 were investigated for their efficacy against the causal agent of the disease in vitro, in pots and in the field. In vitro and in pot experiments were carried out with three strains of A. tumefaciens (C58, B6 and AR125) whereas field trials were conducted in a nursery with a soil naturally contamined by the bacterium. In vitro experiments revealed that A. rhizogenes (K84 and K1026) and Agrobacterium radiobacter (O33, O34, MM8.2 and MM8.1) were effective only against nopaline-producing strains C58 and AR125. However, Bacillus subtilis and Pseudomonas fluorescens strains were effective also against nopaline- and octopine-producing agrobacteria. On the basis of pot tests, five rhizobacterial strains (O33, O34, BSCH14, BSCH15, BSCH16) and the two reference strains K84 and K1026, that significantly reduced the percentage of galled plants, were selected for field testing in a nursery. Apart from the reference antagonists K84 and K1026, only the rhizobacterial strains BSCH14 and O33 significantly reduced the percentage of galled plants to 3.85% and 5.19%, respectively. The preliminary characterization of the antibacterial compounds of B. subtilis BSCH14 showed that they were of proteinaceous nature, stable at 100°C for 60 min and resistant to pH variation. Thi study has shown that effective biological control using rhizobacterial strains can be obtained, thus suggesting their possible use in crown gall disease management.