Multilocus typing for characterization of ‘Candidatus Phytoplasma asteris’-related strains in several ornamental species in Italy

Different ornamental plants showing symptoms indicating phytoplasma presence were collected between 1993 and 2016 in various floricultural areas in north and south of Italy, including Sicily. These samples were determined to be infected by ‘Candidatus Phytoplasma asteris’-related strains, and after PCR/RFLP identification based on the 16S rRNA gene were assigned to the 16SrI-B subgroup. These infected samples were employed for phytoplasma strain differentiation on tuf, groel, rp and amp genes. Phytoplasma strains were from hydrangea, primula, Centaurium erythraea, petunia and gerbera samples showing flower virescence; from gladiolus samples both in vivo and in micropropagation showing the “germs fins” symptomatology, from statice with stunting and lack of flower production, from ranunculus and carnation with virescence and malformation of flowers. All the genes were amplified in nested PCR except the amp gene. For the tuf gene all samples resulted amplified, and Tru1I RFLP analyses confirmed identical profiles with those of 16SrI group phytoplasmas. However, for the other genes only samples from ranunculus, gladiolus in vivo, statice and hydrangea were amplified. For these genes the phytoplasmas were identical to each other and to reference strains belonging to 16SrI-B subgroups; RFLP analyses with Tru1I and AluI further indicated placement in the rpI-B and GroELI-III groups. Considering that these samples have been collected in different Italian regions during 23 years, the relevant conservation in the studied genotypes can perhaps be linked to the presence of common leafhopper vectors, not always identified nor detected in the cultivation areas where the diseased plants were collected. It is important to highlight that ‘Ca. P. asteris’ is the prevalent phytoplasma reported in flower cultivations worldwide, and its lack of genetic polymorphisms may also indicate a globalized trading of the pathogen together with its propagation material.