High diversity of Curtobacterium flaccumfaciens in poinsettia and detection of three pathogenicity plasmids for identification of C. f. pv. poinsettiae

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.