Dally, E. L.

X-disease is one of the most serious diseases known in peach (Prunus persica). Based on RFLP analysis of 16S rRNA gene sequences, peach X-disease phytoplasma strains from eastern and western United States and eastern Canada were classified in 16S rRNA gene RFLP group 16SrIII, subgroup A. Phylogenetic analyses of 16S rRNA gene sequences revealed that the X-disease phytoplasma strains formed a distinct subclade within the phytoplasma clade, supporting the hypothesis that they represented a lineage distinct from those of previously described ‘Candidatus Phytoplasma ’ species. Nucleotide sequence alignments revealed that all studied X-disease phytoplasma strains shared less than 97.5 % 16S rRNA gene sequence similarity with previously described ‘Candidatus Phytoplasma ’ species. Based on unique properties of the DNA, we propose recognition of X-disease phytoplasma strain PX11CT1R as representative of a novel taxon, ‘Candidatus Phytoplasma pruni’. Results from nucleotide and phylogenetic analyses of secY and ribosomal protein (rp) gene sequences provided additional molecular markers of the ‘Ca. Phytoplasma pruni’ lineage. We propose that the term ‘Ca. Phytoplasma pruni’ be applied to phytoplasma strains whose 16S rRNA gene sequences contain the oligonucleotide sequences of unique regions that are designated in the formally published description of the taxon. Such strains include X-disease phytoplasma and - within the tolerance of a single base difference in one unique sequence - peach rosette, peach red suture, and little peach phytoplasmas. Although not employed for taxon delineation in this work, we further propose that secY, rp, and other genetic loci from the reference strain of a taxon, and where possible oligonucleotide sequences of unique regions of those genes that distinguish taxa within a given 16Sr group, be incorporated in emended descriptions and as part of future descriptions of ‘Candidatus Phytoplasma ’ taxa.

Symptoms of abnormal proliferation of shoots resulting in formation of witches’-broom growths were observed on diseased plants of passion fruit (Passiflora edulis f. flavicarpa Deg.) in Brazil. RFLP analysis of 16S rRNA gene sequences amplified in PCRs containing template DNAs extracted from diseased plants collected in Bonito (Pernambuco) and Viçosa (Minas Gerais) Brazil, indicated that such symptoms were associated with infections by two mutually distinct phytoplasmas. One phytoplasma, PassWB-Br4 from Bonito, represents a new subgroup, 16SrIII-V, in the X-disease phytoplasma group (‘Candidatus Phytoplasma pruni’-related strains). The second phytoplasma, PassWB-Br3 from Viçosa, represents a previously undescribed subgroup in group 16SrVI. Phylogenetic analyses of 16S rRNA gene sequences were consistent with the hypothesis that strain PassWB-Br3 is distinct from previously described ‘Ca. Phytoplasma ’ species. Nucleotide sequence alignments revealed that strain PassWB-Br3 shared less than 97.5 % 16S rRNA gene sequence similarity with previously described ‘Ca. Phytoplasma ’ species. The unique properties of its DNA, in addition to natural host and geographical occurrence, support the recognition of strain PassWB-Br3 as a representative of a novel taxon, ‘Candidatus Phytoplasma sudamericanum’.

Catharanthus roseus (L.) G. Don (periwinkle) is well known as an experimental host for diverse phytoplasmas that are artificially transmitted to it through the use of dodder (Cuscuta sp.), laboratory vector insects, or grafting. However, few phytoplasma taxa have been reported in natural infections of C. roseus, and the role of C. roseus in phytoplasma dissemination and natural disease spread is not clear. In this study, naturally diseased plants of C. roseus exhibiting yellowing and witches' broom symptoms indicative of phytoplasma infection were observed throughout the year in the state of Rio de Janeiro, Brazil. Shoots and leaves of four diseased plants were assayed for the presence of phytoplasma DNA sequences by nested polymerase chain reactions (PCR) as previously described (2,3). Phytoplasma rDNA was amplified from diseased periwinkle plants in PCR primed by primer pair P1/P7 and was reamplified in nested PCR primed by primer pair R16F2n/R16R2 (F2n/R2). The results indicated the presence of phytoplasma in all four diseased plants. Phytoplasma identification was accomplished by restriction fragment length polymorphism (RFLP) analysis, using 11 restriction enzymes, of 16S rDNA amplified in PCR primed by F2n/R2. Phytoplasmas were classified according to the system of Lee et al. (1). On the basis of collective RFLP patterns of 16S rDNA, the phytoplasma infections in the four periwinkle plants could not be distinguished from one another. Furthermore, the collective RFLP patterns were indistinguishable from those reported previously for hibiscus witches' broom phytoplasma, “Candidatus Phytoplasma brasiliense” (2). The phytoplasma found in C. roseus, designated strain HibWB-Cr, was classified in group 16SrXV (hibiscus witches' broom phytoplasma group). HibWB-Cr is tentatively considered a new strain of “Ca. P. brasiliense”. C. roseus is the first known, naturally diseased alternate plant host of “Ca. P. brasiliense”. The present study identified strain HibWB-Cr in Rio de Janeiro State, where hibiscus witches' broom disease is prevalent (2). How this economically important disease of hibiscus spreads is not known. Our findings raise the possibility that a polyphagous insect vector is involved in the natural transmission of “Ca. P. brasiliense” and that C. roseus or other plant species serve as reservoirs for the spread of this phytoplasma taxon. References: (1) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (2) H. G. Montano et al. Int. J. Syst. Evol. Microbiol. 51:1109, 2001. (3) H. G. Montano et al. Plant Dis. 84:429, 1999.