Wood composition, rather than microbial communities, underpins varietal differences in wood degradation and esca foliar symptom expression in grapevine
Deciphering the interplay between microbial communities and host defence mechanisms is key to understanding plant health. In perennial plants, the balance between endophytes and wood (i.e. secondary xylem) defence responses governs wood degradation and vascular disease expression. Esca is a complex vascular disease contributing to grapevine decline, with an incidence variable across cultivars but the mechanisms underlying its expression and varietal susceptibility remain unclear. We assessed relationships between internal wood degradation and esca foliar symptoms in Vitis vinifera L. cultivars grown in a common garden, and, at the cultivar level, we tested for correlations between (i) susceptibility to wood decay and esca expression (n = 16 cultivars) and (ii) wood biochemical traits, and healthy wood endophytic microbial communities (n = 23 cultivars). Unlike other types of necrosis, white-rot necrotic wood was significantly more abundant in plants that had expressed esca leaf symptoms in previous years, particularly in the most susceptible cultivars. These cultivars also contained significantly lower levels of constitutive wood extractives. However, glycosylated phenylpropanoids accumulated in the wood of esca-symptomatic plants, especially in highly susceptible cultivars. By contrast, esca expression and varietal susceptibility had only a marginal effect on the diversity, composition and putative functions of microbial communities in healthy wood. They did not influence either the relative abundance of Fomitiporia mediterranea, the putative causal agent of white-rot in grapevine. Esca susceptibility appears primarily linked to wood degradability and metabolic responses, rather than healthy wood's microbial communities, suggesting that the use of less susceptible varieties together with white-rot removal might attenuate grapevine decline.