Colonization of different grapevine cultivars and rootstocks by controlled infection with Flavescence dorée phytoplasma

Flavescence dorée (FD) is a devastating disease of grapevine caused by phytoplasmas transmitted in vineyards by the leafhopper Scaphoideus titanus Ball. Different grapevine cultivars and rootstocks show varying levels of susceptibility to the disease. To better understand this point, the present study focused on investigating the phytoplasma colonization using four Vitis vinifera cultivars (Chardonnay, Glera, Corvina, and Tocai friulano) and three rootstocks (Rupestris du Lot, 140 Ru, and K5BB). In 2023 in a controlled greenhouse experiment, fifty 1-year old plants per cultivar were grown, then 30 were inoculated with S. titanus (using seven infective insects per plant), while 20 plants served as controls. Six months later, during the dormant phase, phloem samples from the first-year cane, scion, rootstock, and roots were collected. These samples were analysed for FD phytoplasma titre and distribution by RNA extraction and PCR. Moreover, in spring 2024 vegetative stage assessments and symptom observation were conducted. The results revealed significant differences in FD phytoplasma presence and distribution among the cultivars and the rootstocks. Chardonnay and Glera exhibited the highest susceptibility with extensive FD distribution in woody organs, including in some cases roots in Chardonnay. In contrast, Corvina and Tocai friulano displayed just a few symptoms and low phytoplasma titre, primarily restricted to the inoculation site on the first-year cane. The rootstocks displayed varying susceptibility as well. Rupestris du Lot showed FD phytoplasma presence in the wood of almost all the analyzed plants, with a limited presence of symptoms. In contrast, 140 Ru and K5BB remained asymptomatic, with FD phytoplasma detectable at low concentrations in some plants in winter. Further work is in progress. The results indicate that the varying susceptibility to FD among the grapevine cultivars and rootstocks is likely due to differences in their genetic defence mechanisms against the FD phytoplasma itself, rather than against the insect vector. This research underscores the need for further investigation into the specific genetic mechanisms responsible for resistance to FD phytoplasma in grapevine cultivars and rootstocks. Acknowledgments The present work was funded by the Veneto regional project FD.CONTROL (2022-2024). The authors are grateful to V. Forte, L. Filippin, S. Casarin, A. Di Muzio, C. Dumitrescu, F. Rampino, A. Saccol, A. De Zorzi, V. Benetti, A. Gerlin from CREA for the technical support in the trial.