In June 2025, a declining wych elm (Ulmus glabra) with branch and twig browning and dieback was detected in Helsinki, Finland. Five shoots were sampled from the transition zone between living and dead tissue. Similar symptoms were later found on U. glabra in Vantaa, where four branches were collected in August (Figure 1). After surface sterilisation, samples were cultured on malt extract agar (MEA) and modified Tchernoff medium. Emerging fungal colonies were grouped by morphology, and the internal transcribed spacer (ITS) and β-tubulin were sequenced from Helsinki isolates. Isolate ELM1-4 from the Helsinki site showed 100% (443/443 bp) ITS identity and 99.3% (425/428 bp) β-tubulin identity to the ex-type strain of Phaeobotryon ulmi (KX464232, KX465068; Zhang et al. 2021). Phaeobotryon isolates from the Vantaa site had an identical ITS sequence to ELM1-4. The sequences were deposited in the European Nucleotide Archive under accessions OZ373025 and OZ373026. The observed conidia were ellipsoid, smooth, and predominantly hyaline and aseptate; only a few were brownish, and only a single 1-septate conidiospore was observed, indicating that they were mostly immature but morphologically consistent with Zhang et al. (2021). The pathogenicity of isolate ELM1-4 was tested in August 2025 by inoculating four 50 cm long U. glabra branches that were kept in water. Mycelial plugs (3 mm) from 24-day-old cultures were placed on exposed sapwood and wrapped with plastic film. After 7 days at room temperature, inoculated branches developed clear necrosis (Figure 2A), while controls remained symptomless (Figure 2B). Phaeobotryon ulmi was reisolated from the inoculated branches, but was not recovered from controls. Species identity was confirmed by ITS sequencing. This is the first report of P. ulmi causing decline in elm in Finland. The pathogen was first recorded in Europe in Poland as the causal agent of Botryodiplodia canker in U. glabra (Bartnik et al. 2019). In Estonia, it has been found; under the name Sphaeropsis ulmicola; in both symptomatic and asymptomatic elms, with the highest prevalence and severity in symptomatic U. laevis. It was also common in U. glabra, but less frequent in symptomatic hybrid elms (Jürisoo et al. 2025). Since it also occurs in healthy tissues (Jürisoo et al. 2025), environmental factors such as increased summer temperatures may trigger symptom development. Although first formally reported in Europe in 2019 (Bartnik et al. 2019), the earliest known European record of P. ulmi is a 1963 Finnish specimen from U. glabra preserved in the CBS-KNAW culture collection (CBS114123), later sequenced by Zhu et al. (2025). However, no additional information is available regarding the isolate or the host's condition. Phaeobotryon ulmi is absent from records of the Finnish Biodiversity Information Facility and from historical Finnish literature (Liro 1924; Rauhala 1958; Karsten 1873). This study provides the first report of shoot symptoms and dieback of U. glabra caused by P. ulmi. The authors would like to thank Linda Mutanen, Tuija Hytönen and Katri Leino for their excellent technical assistance.
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