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ISHS Acta Horticulturae 551: IX International Symposium on Small Fruit Virus Diseases

MARGINAL CHLOROSIS OF STRAWBERRY PLANTS CAN BE INDUCED BY TWO DIFFERENT PHLOEM-RESTRICTED BACTERIA: THE PROTEOBACTERIUM “CANDIDATUS PHLOMOBACTER FRAGARIAE” AND THE STOLBUR PHYTOPLASMA

Authors:   L. Zreik, J.L. Danet X Fossiac, J. Gandar, E. Verdin, J.M. Bové, M. Garnier, J.G. Nourrisseau
Keywords:   PCR detection, Homopteran insects, Phytoplasma, Proteobacteria
Abstract:
Marginal chlorosis has been observed on strawberry plants in France, since 1988. A phloem-restricted bacterium was found associated with the disease (8). 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” (10).
From the 16S ribosomal RNA sequence, two primers Fra4 and Fra5 were defined for specific amplification of “Candidatus Phlomobacter 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 “Phlomobacter 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 (4) and PCR with primers rStol and fStol (7). “Phlomobacter 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 “Phlomobacter 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 “Phlomobacter fragariae” was not suitable for its detection in insects as these are known to harbour enterobacteria as well as parasistes and/or symbionts also belonging to the gamma 3 subgroup of Proteobacteria.
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