Pathogenic Fusarium verticillioides and Ophiostoma clavatum Associated with Ips acuminatus in Ukraine

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