Mixed infection of grapevines in northern Italy by phytoplasmas including 16S rRNA RFLP subgroup 16SrI-B strains previously unreported in this host

Grapevine plants of cv. Chardonnay showing symptoms like those of flavescence doree disease in the field in Piemonte, Italy, contained phytoplasmas affiliated with two phylogenetically different 16S rRNA restriction fragment length polymorphism (RFLP) groups. These phytoplas mas were detected and identified by polymerase chain reaction (PCR) amplification of 16S rDNA and by RFLP analysis of amplified DNA as strains belonging to group 16SrI (aster yellows and related phytoplasmas) and group 16SrV (elm yellows and related phytoplasmas). Thirteen of 16 tested plants contained group 16SrI strains. Twelve contained strains belonging to subgroup 16SrI-G (Italian periwinkle virescence and related phytoplasmas), and one contained only a strain belonging to subgroup 16SrI-B (Maryland aster yellows and related phytoplasmas). One plant that contained a subgroup 16SrI-G phytoplasma strains also contained a strain belonging to group 16SrV. Three plants were doubly infected by subgroup 16SrI-G strains and strains belonging to subgroup 16SrI-B. These results indicate susceptibility of grapevines to infection by three distinct phytoplasmas, and reveal for the first time grapevine infection by subgroup 16SrI-B phytoplasmas and mixed infection of single grapevine plants by strains in two different subgroups in group 16SrI.