Marginal chlorosis of strawberry plants can be induced by two different phloem-restricted bacteria: the proteobacterium 'Candidatus Phlomobacter fragariae' and the stolbur phytoplasma.

摘要

Marginal chlorosis has been observed on strawberry plants in France since 1988. A phloem-restricted bacterium was found associated with the disease. As the bacterium could not be obtained in culture, its characterization, was carried out after amplification, cloning and sequencing of the highly conserved 16S ribosomal RNA gene. The bacterium was shown to be a new Candidatus genus in the gamma3 subdivision of Proteobacteria and was named Candidatus Phlomobacter fragariae. From the 16S ribosomal RNAsequence, two primers, Fra4 and Fra5, were defined for specific amplification of Candidatus P. fragariae by PCR (16S-PCR). 16S-PCR was performed on DNA extracts of strawberry plants collected in the various strawberry-growing regions of France both in production fields and nurseries. Marginal chlorosis symptoms could be found on strawberry plants in which P. fragariae could not be detected. In these symptomatic strawberry plants the stolbur phytoplasma was shown to be present by serology with a stolbur-specific monoclonal antibody and PCR with primers rStol and fStol. P. fragariae was predominant in strawberry production fields, while the stolbur phytoplasma was essentially detected in nurseries. Phloem-restricted bacteria are vectored by phloem-feeding insects. Evidence for natural transmission of P. fragariae was obtained by exposing in vitro strawberry plants to contamination in strawberry production field. The 16S-PCR test used to identify insect carriers of P. fragariae was not suitable for its detection in insects as these are known to harbour enterobacteria as well as parasites and/or symbionts also belonging to the gamma 3 subgroup of Proteobacteria.