Marcone, Carmine

Bermuda grass white leaf (BGWL) is a destructive, phytoplasmal disease of Bermuda grass (Cynodon dactylon). The causal pathogen, the BGWL agent, differs from other phytoplasmas that cluster in the same major branch of the phytoplasma phylogenetic clade in <2·5 % of 16S rDNA nucleotide positions, the threshold for assigning species rank to phytoplasmas under the provisional status ‘Candidatus’. Thus, the objective of this work was to examine homogeneity of BGWL isolates and to determine whether there are, in addition to 16S rDNA, other markers that support delineation of the BGWL agent at the putative species level. Phylogenetic analyses revealed that the 16S rDNA sequences of BGWL strains were identical or nearly identical. Clear differences that support separation of the BGWL agent from related phytoplasmas were observed within the 16S–23S rDNA spacer sequence, by serological comparisons, in vector transmission and in host-range specificity. From these results, it can be concluded that the BGWL phytoplasma is a discrete taxon at the putative species level, for which the name ‘Candidatus Phytoplasma cynodontis' is proposed. Strain BGWL-C1 was selected as the reference strain. Phytoplasmas that are associated with brachiaria white leaf, carpet grass white leaf and diseases of date palms showed 16S rDNA and/or 16S–23S rDNA spacer sequences that were identical or nearly identical to those of the BGWL phytoplasmas. However, the data available do not seem to be sufficient for a proper taxonomic assignment of these phytoplasmas.

Elm yellows group (16SrV) phytoplasmas, which are associated with devastating diseases in elm, grapevine, blackberry, cherry, peach and several other plant species in America, Europe and Asia, represent one of the most diverse phytoplasma clusters. On the basis of phylogenetic analysis of 16S rDNA sequences, elm yellows group phytoplasmas form a discrete subclade within the phytoplasma clade. Three phylogenetic parameters, namely 16S rRNA, ribosomal protein and secY genes, have been evaluated for their usefulness in differentiating elm yellows group phytoplasmas. RFLP analysis of 16S rRNA sequences differentiated the elm yellows group phytoplasmas into five subgroups. Twelve RFLP subgroups were differentiated on the basis of ribosomal protein and 13 were differentiated using secY gene sequences. Phylogenetic analysis of the ribosomal protein genes and secY gene alone or in combination indicated that the subgroups constitute 12 genetically distinct lineages, each of which appears to have evolved under different ecological constraints such as specific vector or plant hosts. On the basis of unique DNA and biological properties, it is proposed that the elm yellows phytoplasma EY1T represents a novel taxon, ‘Candidatus Phytoplasma ulmi’.