Study of Bacillus velezensis OEE1 potentialities in the biocontrol against Erwinia amylovora, causal agent of fire blight disease of rosaceous plants

Erwinia amylovora is the causal agent of fire blight disease affecting rosaceous plants. The present study focuses on the potential of Bacillus velezensis as a promising biological control agent toward this phytopathogenic bacterium. B. velezensis OEE1 showed the most effective antibacterial activity (80 %) among 165 tested bacterial strains under in vitro conditions against E. amylovora. Following the extraction of the active molecules, the antibacterial activity was detected in butanol extract. A separation of the active compounds using thin layer chromatography (TLC) of the butanol extract of the culture filtrate of B. velezensis OEE1 indicate the presence of amino acids by the presence of a spot with a frontal ratio (Rf) of 0.73. B. velezensis OEE1 also showed high levels of multi-resistance to penicillin, ampicillin, kanamycin, ciprofloxacin, amoxicillin and streptomycin. Biological and chemical treatments (by B. subtilis Y1336 and copper oxide Cu₂O respectively) were used to compare the efficacy of the treatment with the antagonistic bacterium OEE1. The results indicated that both protective and curative treatments with antagonistic bacterium were effective to reduce the disease progression. The curative method was generally significantly more efficient than the protective method for tests on detached tissues. The efficacy of B. velezensis OEE1 to reduce E. amylovora was evaluated also in greenhouse and under field conditions. B. velezensis OEE1 was able to improve the collar diameter and the number of leaves of healthy pear plants of the variety “Alexandrine” in greenhouse conditions. Experiments in greenhouse and under field conditions revealed that B. velezensis OEE1 significantly decreased the final mean disease severity index (FMS) and the percentage of dead plants (PDP). This study has demonstrated that B. velezensis is considered as a future potential new biocontrol agent for controlling fire blight disease.