Survey on phytoplasmas associated with grapevine yellows in Eastern Georgia, Caucasus region

During field surveys conducted from July to October 2018–2020 in the eastern part of Georgia (Caucasus region), 145 out of 8000 (1.8%) and 147 out of 6600 (2.2%) grapevine plants, respectively, from mother stock/collection fields and commercial vineyards, were found exhibiting typical or suspicious grapevine yellows (GY) symptoms. Most of the symptomatic grapevine plants of Georgian cultivars showed mild symptoms with no berry alterations. Leaf samples from symptomatic plants were analyzed by serological (DAS--ELISA) and molecular (two previously published protocols of TaqMan triplex real-time PCR, here named Set I and Set II) tools for detecting GY-associated phytoplasmas. The presence of GY phytoplasmas was not detected in any examined grapevines by a serological method. GY phytoplasmas were identified in 22.41% and 6.9% symptomatic plants from mother stock and collection vineyards and in 48.3 and 19.0% symptomatic plants from commercial vineyards by Set I and Set II PCRs, respectively. As expected from previous studies reporting the wide presence of bois noir (BN) in Georgian vineyards, ‘Candidatus Phytoplasma solani’(CaPsol) was detected in most phytoplasma-infected plants (47.6%), with the highest infection rate in Chardonnay. Phytoplasmas belonging to taxonomic group 16SrV were detected in 45.6% of the phytoplasma-infected grapevines. To the best of our knowledge, this is the first report of 16SrV phytoplasmas in Georgia and in the Caucasus region. Further molecular typing of 16SrV phytoplasma strains is necessary to determine if such strains are associated with flavescence dorée (FD). The knowledge of typical GY symptoms and the utilization of accurate diagnostic tools are crucial for preventing pathogen spread and producing healthy planting material. Based on the results obtained in this study, the presence of BN and 16SrV phytoplasmas should be monitored in the next years using triplex real-time PCR.