The potential role of Aureobasidium pullulans in the development of foliar symptoms of Esca disease in grapevine

Esca belongs to the group of grapevine trunk diseases - fungal diseases present worldwide in all wine-growing regions. Some aspects of the disease, like the development of external symptoms, have still not been completely discovered and are believed to be affected by several factors, including interactions within the vine microbiome. The examination of the occurrence of the yeast-like fungus Aureobasidium pullulans in the healthy wood of Esca-diseased grapevines via both isolation and qPCR measurements showed a positive correlation between its abundance and the severity of foliar symptoms, suggesting the contribution of this fungus to Esca pathogenesis via an indirect action. In vitro confrontation tests revealed antagonistic interaction between A. pullulans and the Esca pathogen Phaeomoniella chlamydospora. Mutual growth inhibition and the induction of asexual sporogenesis were observed for both fungi without cytotoxic effects. In planta confrontation tests revealed that A. pullulans in combination with P. chlamydospora can lead to severe foliar damage in a strain-dependent manner. This phenomenon could be explained by the altered metabolism of the Esca pathogen in the presence of A. pullulans, or by the cumulative/synergistic effects of the secreted polysaccharides and/or proteins of the two fungi. The present study shows the importance of microbial interactions in the development of plant diseases, highlighting that even a non-pathogenic microorganism can act as a disease-enhancer.