A bacterial disease of artichoke (Cynara scolymus L.) was first observed in 1954 in Brittany and the Loire Valley, France. This disease causes water-soaked spots on bracts and depreciates marketability of the harvest. Ten strains of the pathogen causing bacterial spot of artichoke, previously identified as a member of the genus Xanthomonas, were characterized and compared with type and pathotype strains of the 20 Xanthomonas species using a polyphasic study including both phenotypic and genomic methods. The ten strains presented general morphological, biochemical and physiological traits and GMC content characteristic of the genus Xanthomonas. Sequencing of the 16S rRNA gene confirmed that this bacterium belongs to the genus Xanthomonas, and more precisely to the Xanthomonas campestris core. DNA-DNA hybridization results showed that the strains that cause bacterial spot of artichoke were 92-100 % related to the proposed type strain CFBP 4188 T and constituted a discrete DNA homology group that was distinct from the 20 previously described Xanthomonas species. The results of numerical analysis were in accordance with DNA-DNA hybridization data. Strains causing the bacterial bract spot of artichoke exhibited consistent determinative biochemical characteristics, which distinguished them from the 20 other Xanthomonas species previously described. Furthermore, pathogenicity tests allowed specific identification of this new phytopathogenic bacterium. Thus, it is concluded that this bacterium is a new species belonging to the genus Xanthomonas, for which the name Xanthomonas cynarae is proposed. The type strain, CFBP 4188 T , has been deposited in the Collection Franc: aise des Bacte! ries Phytopathoge ' nes (CFBP).
The randomly amplified polymorphic DNA (RAPD) method was used to investigate the genetic diversity in Xanthomonas cynarae, which causes bacterial bract spot disease of artichoke. This RAPD analysis was also intended to identify molecular markers characteristic of this species, in order to develop PCR-based markers which can be used to detect this pathogenic bacterium in artichoke fields. Among the 340 RAPD primers tested, 40 were selected on their ability to produce reproducible and reliable fingerprints in our genetic background. These 40 primers produced almost similar patterns for the 37 X. cynarae strains studied, different from the fingerprints obtained for other Xanthomonas species and other xanthomonad-like bacteria isolated from artichoke leaves. Therefore, X. cynarae strains form a homogeneous genetic group. However, a little DNA polymorphism within this species was observed and the collection of X. cynarae isolates was divided into two groups (one containing three strains, the second one including all other strains). Out of seven RAPD markers characteristic of X. cynarae that were cloned, four did not hybridize to the genomic DNA of strains belonging to other Xanthomonas species. These four RAPD markers were converted into PCR markers (specific characterized amplified regions [SCARs]); they were sequenced, and a PCR primer pair was designed for each of them. Three derived SCARs are good candidates to develop PCR-based tests to detect X. cynarae in artichoke fields.Water-soaked and dark green spots on capitulum bracts of artichoke (Cynara scolymus L.) were observed for the first time in 1954 in Brittany and near Angers (France). Warm and humid periods are favorable for development of this disease, which has been observed in numerous artichoke crops in Brittany and has resulted in substantial economic losses for the last decade. The causal agent of this disease was first isolated by Ridé (25) from such bract spots and was identified as a phytopathogenic bacterium belonging to the genus Xanthomonas (25). It was recently classified at the species level as Xanthomonas cynarae (33). An identification test of X. cynarae was described based on biochemical, physiological, and pathogenicity tests (33). Colonies of X. cynarae are yellow and surrounded with a white halo when grown on a Tween medium (17) and produce the typical symptoms of this disease when inoculated to detached scarified bracts of artichoke. A pathogenicity test is required, since other Xanthomonas species look like X. cynarae when grown on Tween medium. This identification procedure, including pathogenicity tests, was used to monitor bacterial populations in artichoke fields and showed that X. cynarae is present on the leaf surface of artichoke before the capitulum development.A rapid and specific identification test would be very useful to monitor the contamination of artichoke plants in order to develop strategies to control the disease in fields. Since the diagnostic test described above is time consuming and requires, for the pathogenicity t...
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