Exploring the Role of Fomitiporia mediterranea in the Development of Grapevine Leaf Stripe Disease Symptoms

The adverse effects of Esca on the world’s viticultural heritage are widely known. Authors such as Ravaz and Viala meticulously described the effects of this disease on the vineyards of the Mediterranean basin at the beginning of the 20th century and contributed to its definition. The evolution of “Esca” into the “Esca complex” stems from the efforts of scientists worldwide. Over the last century, disease management strategies have evolved to include the phasing out of the substances that are most dangerous to human and environmental health. The arsenic-based products initially used in the vineyard for the control of moths have been fundamental in limiting the foliar symptoms of Esca. The banning of these products resulted in a lack of Esca control strategies; consequently, abandoned techniques have been re-evaluated in more modern times, aided by the availability of more efficient equipment. Trunk surgery was historically applied for the removal of decayed wood on many tree species as well as grapevine. The first study in this thesis consolidates data on the efficacy of trunk surgery by testing three levels of decayed wood removal. Here, the published results of two years of survey (Annex I) are augmented by the findings of a third year of survey. A parallel study on the effect of the technique on the vine microbiota confirmed that Fomitiporia mediterranea (Fmed) is the species most affected by the removal of decayed wood, as also highlighted by other approaches. The results reinforced the hypothesis, formulated at the beginning of the 20th century, that the presence of decayed wood can lead to the expression of foliar symptoms. Using a multidisciplinary approach, a study was subsequently conducted on Fmed-associated wood degradation processes (Annex III) focusing on the enzymatic pathway of the fungus. Wood cell structure degradation was initially assessed by microscopy. Then, vine wood degradation was reproduced in lignum and the levels of each structural polymer were quantified. Finally, the activity of the main ligninolytic enzymes was measured and their molecular modulation was assessed. Our findings helped to consolidate on-field data that were not present in the literature when the experiments were designed and highlighted the importance of the complete removal of decayed wood in trunk surgery. Analysis of the Fmed enzymatic pathway contributed to the knowledge of the fungus, which has recently gained considerable research interest for its role in the Esca complex. The results of the analysis suggested that, as reported for other pathosystems that include Basidiomycota species, a distinct pathway which acts synergistically with the enzymes could be necessary for wood degradation to occur, thus opening up a new line of research on grapevine wood decay mechanisms.