Acetate Signalling Regulates Virulence-Associated Traits in the Esca Pathogen Phaeomoniella chlamydospora

Phaeomoniella chlamydospora (Pch) is a pioneer pathogen of esca, one of the most destructive grapevine trunk diseases worldwide. A recent work suggests that acetate may act as a quorum-sensing (QS) molecule in Pch, promoting biofilm formation in a concentration-dependent manner. However, the broader influence of acetate on virulence-associated traits remains unexplored. In this study, three Pch isolates were cultured under increasing sodium acetate concentrations (0–100 mM) and assessed for pigmentation, extracellular enzyme activities (amylase, cellulase, protease, esterase, and pectinase), phenolic compound-degrading capacity, and antibacterial activity against a grapevine-associated Pseudomonas sp. isolate. Pigmentation, as well as amylase and cellulase activities, were significantly increased at low acetate supplement levels (6.25–12.5 mM), while esterase activity was unaffected. The expression of these traits decreased above 25 mM acetate supplementation. Phenolic compound degradation capacity, antibacterial efficacy, as well as protease and pectinase activities progressively suppressed at all acetate concentrations. These results indicate that acetate concentration modulates multiple virulence-associated phenotypes of Pch in vitro. Based on these patterns, we propose a hypothetical model in which acetate-dependent phenotypic changes may reflect a shift between establishment-associated activities and reduced extracellular activity at higher acetate levels. This model remains to be validated by mechanistic and in planta infection-based studies.