First Report of <i>Ophiostoma clavatum</i> and <i>Fusarium verticillioides</i> Associated With <i>Ips acuminatus</i> ‐Infested Scots Pine in Western Ukraine

Over the past two decades, dieback of <i>Pinus sylvestris</i> L. stands has increased across Europe, largely due to mass outbreaks of the bark beetle, in particular, <i>Ips acuminatus</i> Gyll. (Coleoptera: Curculionidae). This beetle causes mechanical damage and vectors pathogenic fungi, including ophiostomatoid species that induce blue stain. <i>Ophiostoma clavatum</i> Math.-Käärik is the most frequently reported fungal associate, yet its occurrence has not been documented in Ukraine. While ophiostomatoid fungi are well studied in pine pathogenesis, the role of fast-growing co-occurring associates such as <i>Fusarium</i> spp. remains poorly understood. This study aimed to identify the dominant <i>Ophiostoma</i> and <i>Fusarium</i> species associated with <i>I. acuminatus</i> in western Ukraine and to evaluate their pathogenicity and in vitro interactions. Isolates from surface-sterilized beetle abdomens and blue-stained wood were identified as <i>O. clavatum</i> based on morphology and multi-locus molecular markers (ITS, TUB, TEF1-α). Pathogenicity tests showed that <i>O. clavatum</i> acts as a weak phytopathogen, primarily inducing localized lesions. The dominant <i>Fusarium</i> morphotype from blue-stained wood was identified as <i>Fusarium verticillioides</i> (Sac) Nirenberg, which induced severe necrosis and tissue maceration on pine seedlings. In dual culture, <i>F. verticillioides</i> displayed strong asymmetric competitive dominance over <i>O. clavatum</i>, reducing its growth by more than 45%. This study provides the first record of <i>O. clavatum</i> associated with <i>I. acuminatus</i> in Ukraine, extending its known European distribution. The observed pathogenicity and competitive ability of <i>F. verticillioides</i> suggest it may synergistically contribute to Scots pine decline, warranting further investigation into its role within the beetle-fungus complex.