The bacterial wilt of common bean, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens(Cff) is one of the most severe diseases affecting Phaseolus vulgaris production worldwide. This study aimed at evaluating the biocontrol potential of strains of rhizobacteria against bacterial wilt of common bean. Sequence analysis of the 16S rRNA gene was used to identify Cff isolates and also the bacterial antagonists. A soft agar overlay assay was used to select three biocontrol isolates based on their antagonistic activity against Cff. Our findings demonstrate that seed treatment using rhizobacterial P. fluorescens, Bacillus cereus, and Paenibacillus polymyxa species coupled with foliar application significantly reduced Cff disease incidence and disease severity. Therefore, biocontrol methods are potentially a safe, effective, and sustainable alternative to chemicals for controlling bacterial wilt of beans.
Outbreaks in Europe have raised concerns about poinsettia bacterial canker caused by Curtobacterium flaccumfaciens pv. poinsettiae described in the USA. Using a semi-selective medium containing aztreonam and fosfomycin and selection during isolation based on MALDI-TOF MS spectra, 424 Curtobacterium strains were isolated from Belgian poinsettias of African and European origin. Different populations coexisted: 130 strains were identified as C. flaccumfaciens with scores ≥2.0, but 294 with scores <2.0 or as another Curtobacterium. MALDI-TOF MS libraries constructed using similar medium and extraction procedure and a pathogenicity test on poinsettia were used to screen collections from poinsettia and wheat, a possible alternative host, for C. f. pv. poinsettiae. The concatenated recA-gyrB partial sequences showed that 114 poinsettia or wheat strains belonged to different Curtobacterium species. Eighty-eight nonpathogenic strains and four U.S. strains from litter were intermingled with bean pathogenic C. f. pv. flaccumfaciens strains in the three genetic groups of C. flaccumfaciens, but lacked their pathogenicity markers. Four new European recA-gyrB sequatypes of C. f. pv. poinsettiae, obtained from symptomatic and asymptomatic poinsettias, were distinguished from three American sequatypes. Six sequatypes were pathogenic in tests, belonged to genetic groups related to two different genomospecies, and possessed a plasmid. Sequencing of six plasmids revealed three related plasmids containing proteases and a polygalacturonase found only in strains pathogenic in pathogenicity tests and specifically identified by polygalacturonase-based PCR and LAMP assays. Similarities between plant and litter Curtobacterium and the role of plasmids in pathogenicity and punctual transmissions of pathogenicity among heterogeneous C. flaccumfaciens are suggested.
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