The best known Microdochium spp. are important pathogens of small-grain cereals and/or endophytes of diverse monocot hosts. This study is the first report of M. majus isolated from asymptomatic grapevine tissues. It was hypothesised that this M. majus strain, CBS 152328, was an endophyte and an antagonist of some fungal pathogens of grapevine. Microscopic examinations revealed that this strain was a necrotrophic mycoparasite of Botrytis cinerea. This was demonstrated in the confrontation zones of dual cultures of M. majus and B. cinerea, and also on the surface of co-inoculated grape leaf discs and germinated wheat grains. Pathogenicity tests indicated that M. majus can colonise both grape leaf discs and germinated wheat, but it only damaged wheat. When co-inoculated with B. cinerea onto grape leaf discs, the M. majus strain CBS 152328 suppressed its mycohost on grape tissues and prevented leaf necrosis caused by B. cinerea. In addition to the parasitism, M. majus also showed mild antibiosis against B. cinerea, as well as a defence elicitor effect on grape leaf discs. This work is the first report of the mycoparasitic behaviour of M. majus, in addition to its first isolation from a dicot host.

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.

An isolate of Peniophora lycii was obtained from grapevine, and its interactions with several grapevine-associated fungi and the plant host were examined. The fungus was not able to infect intact leaves, but necrotized the margins of foliar disks and caused necrosis and white rot in woody tissues. In dual cultures, P. lycii and Aureobasidium pullulans showed mutual antagonism. Mycoparsitism of P. lycii was observed on epiphytic Botrytis cinerea, Alternaria sp., and endophytic Botryosphaeria dothidea interaction partners. In contrast, P. lycii showed trophic growth towards endophytic Phaeomoniella chlamydospora without any signs of harmful interactions. Dual inoculations of foliar disks with epiphytic fungi revealed no effects of fungal interactions on necrosis development by pathogens and verified mycoparasitic interactions in planta. Co-infection assays of cuttings with endophytic pathogen fungi showed cumulative effects of fungal interactions on wood symptom expression, with the exclusive contribution of P. lycii to white rot development. In addition to providing the first isolation of P. lycii from grapevine and the description of its mycoparasitic behavior, the present study suggests that the fungus may act as an opportunistic grapevine pathogen, probably as a secondary colonizer in trunk diseases. The observed dual host preference may allow trunk disease pathogens to initially feed on fungi, followed by damage to the grapevine. This may be in connection with the exceptionally long latency of these syndromes.