Xylem Sap Mycobiota in Grapevine Naturally Infected with <i>Xylella fastidiosa</i>: A Case Study: Interaction of <i>Xylella fastidiosa</i> with <i>Sclerotinia sclerotiorum</i>

Grapevine (<i>Vitis vinifera</i>) is a key crop in Mediterranean agriculture, now increasingly threatened by <i>Xylella fastidiosa</i> subsp. <i>Fastidiosa</i> (<i>Xff</i>), the causal agent of Pierce’s disease. This study investigated: (1) the diversity of culturable fungal endophytes in the xylem sap of naturally <i>Xff</i>-infected grapevines, and (2) the interaction between <i>Xff</i> and the pathogenic fungus <i>Sclerotinia sclerotiorum</i> identified in the sap. The xylem sap was collected from Cabernet Sauvignon vines in Mallorca, Spain, and fungal communities were characterized using culture-dependent methods. Both beneficial fungi (e.g., <i>Aureobasidium pullulans</i>, <i>Rhodotorula mucilaginosa</i>) and pathogenic species (e.g., <i>S. sclerotiorum</i>, <i>Cladosporium</i> sp., <i>Alternaria alternata</i>, and the <i>Phoma</i> complex) were isolated from both <i>Xff</i>-positive and <i>Xff</i>-negative plants, indicating similar community profiles. Although limited by small sample size, these findings offer preliminary evidence of complex ecological interactions between <i>Xff</i> and the xylem-associated mycobiota, with potential implications for grapevine health and disease development under varying environmental and management conditions. Further experiments under controlled conditions revealed that grapevines co-inoculated with <i>Xff</i> and <i>S. sclerotiorum</i> showed increased disease severity, suggesting a synergistic interaction. These preliminary results highlight the complex interplay between <i>Xff</i> and the fungal endophytic microbiome, which may modulate grapevine susceptibility depending on environmental and management conditions.