Surveys on grapevine yellows incidence on Georgian grapevine varieties

INTRODUCTION – Grapevine yellows (GY) are a phytoplasma-associated disease complex that induces severe crop losses in almost all varieties used for wine production in the Euro-Mediterranean area and in other continents. Typical GY symptoms include berry shrivel, desiccation of inflorescences, color alterations and curling of the leaves, reduction of growth, and irregular ripening of wood. Chemical treatments against insect vectors of GY-associated phytoplasmas are essential to contain the disease spreading. A strategy for phytoplasma disease control is based on the selection of resistant, tolerant or not susceptible plant varieties. Unfortunately, up to now none of the examined Vitis species and V. vinifera varieties have been found immune or resistant to the phytoplasma associated with GYs. AIMS AND SCOPES – The main objective of this study was to verify the presence of GY diseases in collections of Georgian autochthon grapevine varieties and to investigate the spreading of phytoplasma diseases in vineyards. MATERIALS AND METHODS – Three collections representing the autochthon Georgian germplasm, located in Saguramo (Shida Kartli), Shumi and Kindzmarauli (Kakheti province), were visited in order to looking for the presence of typical GY symptoms, clearly visible in September. Moreover, three Georgian vineyard areas [one in Mukhrani (Shida Kartli) and two in Khondoli (Kakheti)], in which both international and local varieties were cultivated, were also surveyed for GY disease. RESULTS AND DISCUSSIONS – In Saguramo and Shumi, in most cases, grapevines were symptomless. GY symptoms were observed only in plants of the varieties Mtredisphekha and Adznizhi in Saguramo, and in plants of the varieties Saperavi Pachkha and Grdzelmtevana in Shumi. In the collection of Kindzmarauli, GY symptoms were observed in 18 varieties among which Saperavi, Goruli Mtsvane, Khikhvi and Kisi are more cultivated in Georgia. In Mukhrani, GY largely affected Chardonnay and, with lower incidence, Muscat Blanc, while no evident symptoms were reported on white autochthon varieties (Rkatsiteli and Goruli Mtsvane). On the other hand, plants of the red local variety Saperavi showed GY symptoms. Interestingly, Convolvulus arvensis, widely reported as host plant of phytoplasmas associated with 'bois noir' (one of the diseases of GY complex) and of its insect vector Hyalesthes obsoletus, was largely covering the ground of these vineyards (both international and local varieties) in Mukhrani. Moreover, some plants of C. arvensis showed symptoms (yellowing, reddening, dwarfism, leaf malformation) related to infection by phytoplasmas. In Kakheti region, typical GY symptoms were observed not only on numerous plants of Chardonnay, but also on few plants of local varieties Rkatsiteli, Kisi, Saperavi and Mtsvane Kakhuri. In this region, the presence of C. arvensis was considerably lower than in Mukhrani, and the more spread weed was Artemisia vulgaris, recently reported in literature as host plant of the 'bois noir' phytoplasma vector H. obsoletus. CONCLUSIONS AND POSSIBLE APPLICATIONS – Further molecular analyses will be carried out to identify the phytoplasmas infecting grapevines in Georgia. This aspect is crucial for understanding the GY epidemiology and the application of strategies for disease containment. Evidences from this study highlighted that GY are present in Georgia, as showed by numerous diseased plants of international varieties (mainly Chardonnay). Intriguingly, the fact that some local autochthon varieties are symptomless in areas where GY strongly affects Chardonnay constitutes a preliminary indication that such Georgian varieties could be tolerant or resistant to phytoplasmas.