Impact of the "Flavescence Dorée" Phytoplasma on Xylem Growth and Anatomical Characteristics in Trunks of 'Chardonnay' Grapevines (Vitis vinifera)

Flavescence dorée (FD) is a grapevine disease caused by 'Candidatus Phytoplasma vitis' (FDp), which is epidemically transmitted by the Nearctic leafhopper Scaphoideus titanus. In this study, we applied dendrochronological techniques to analyse the response to FDp infections in terms of wood ring widths and anatomical structures of the xylem and phloem tissues of the trunk of the susceptible grapevine cultivar 'Chardonnay.' As a rule, grapevines are susceptible to water shortage and reduce their growth in diameter in case of summer drought. In the season of the external expression of FD symptoms, however, the ring width reductions are extreme and supersede any drought-induced effects. In addition, the anatomy of the phloem tissue in the year of the FD symptom expression appears heavily disarranged. Moreover, in the most suffering individuals, the xylem formation remains incomplete and mostly limited to the early wood tissue. In conclusion, even though the FD phytoplasma does not inhabit and replicate inside the xylem tissue, our results confirm existing indirect inhibiting effects on the ring growth and the xylem tissue formation in FDp-infected grapevines.