A Witches’-Broom Disease of Cultivated Strawberry Associated with ‘Candidatus Phytoplasma Rubi’-Related Strains in Southern Italy

In the Campania region of southern Italy, a formerly undescribed witches'-broom disease of cultivated strawberry characterized by symptoms similar to those of strawberry witches'-broom and multiplier diseases occurring in North America, has been observed. Strawberry witches'-broom and multiplier diseases are not known to occur in Europe. To elucidate the etiology of the new strawberry disease occurring in southern Italy and to determine the taxonomic position of the presumable causal agent, field observations and PCR assays using universal and group-specific phytoplasma primers followed by multigene sequence analysis were carried out. All of the symptomatic strawberry plants examined tested phytoplasma positive with universal primers and primers specific to the elm yellows (EY) phytoplasma group or 16SrV group. The percentage of diseased plants in the fields was about 30%. Data obtained from sequence and phylogenetic and virtual RFLP analyses of PCR-amplified rDNA (16S rDNA and 16S/23S rDNA spacer region), <i>rpsV</i> (<i>rpl22</i>) and <i>rpsC</i> (<i>rps3</i>), <i>map</i>, <i>imp</i> and <i>groEL</i> gene sequences, showed that the diseased strawberry plants harbored phytoplasma strains which were identical or nearly identical to each other and to strains of the rubus stunt (RuS) agent '<i>Ca</i>. Phytoplasma rubi', a member of the 16SrV group, subgroup 16SrV-E. The 16S rDNA sequence similarity among the strawberry-infecting phytoplasma strains ranged from 99.1 to 99.9%. These strains shared the same range of 16S rDNA sequence similarity with RuS phytoplasma strains including the reference strain RUS of '<i>Ca</i>. Phytoplasma rubi'. This is the first report on the occurrence of RuS phytoplasma in naturally affected strawberry plants.