Thyridium lauri sp. nov. (Thyridiaceae, Thyridiales): a new pathogenic fungal species of bay laurel from Italy

<i>Laurusnobilis</i> is an important Mediterranean tree and shrub native to Italy that is also commercially grown as spice and ornamental plant. Field surveys conducted since 2021 in Sicily (Italy) revealed that bay laurel plants in urban and private gardens and nurseries were severely affected by symptoms of stem blight and internal necrosis, which were associated with ambrosia beetle entry holes in the bark and internal wood galleries. The occurring ambrosia beetle was identified as <i>Xylosandruscompactus</i>, an invasive wood-boring pest previously reported from Sicily. Investigation of fungi from symptomatic tissues primarily resulted in the isolation of <i>Thyridium</i>-like colonies. The main symbiont of <i>X.compactus</i>, <i>Ambrosiellaxylebori</i>, was also isolated from infested plants. Phylogenetic analyses of a combined matrix of ITS, LSU, <i>act1</i>, <i>rpb2</i>, <i>tef1</i>, and <i>tub2</i> gene regions revealed that the isolated <i>Thyridium</i>-like colonies represent a new fungal species within the genus <i>Thyridium</i>. Based on both phylogeny and morphology, the new isolated fungus is described as <i>Thyridiumlauri</i> <b>sp. nov.</b> Moreover, two recently described species, <i>Phialemoniopsishipposidericola</i> and <i>Phialemoniopsisxishuangbannaensis</i>, are transferred to the genus <i>Thyridium</i> due to the confirmed synonymy of both genera, as supported by molecular phylogenies. Pathogenicity test conducted on potted plants demonstrated that <i>T.lauri</i> is pathogenic to bay laurel, causing internal necrosis and stem blight. The new species was consistently re-isolated from the symptomatic tissue beyond the inoculation point, thereby fulfilling Koch's postulates. This study represents the first report of a new pathogenic fungus, <i>T.lauri</i>, causing stem blight and internal necrosis of bay laurel plants and associated with infestation of the invasive ambrosia beetle <i>X.compactus</i>.