First Report of <i>Fusarium oxysporum</i> f. sp. <i>canariensis</i> on <i>Phoenix canariensis</i> (Canary Island Date Palm) in the United Kingdom

In September 2024, severe dieback and wilt symptoms (Figure 1) were observed on a single mature (over 25-year-old) Phoenix canariensis tree in a private garden in Devon, UK. Vascular necrosis and internal discolouration were observed within the petioles and rachises (Figure 2). Small pieces of internal diseased petiole tissue were excised and surface sterilised in 10% bleach solution for 2 min, plated onto potato dextrose agar and incubated at 21°C for 9 days with a 12-h white fluorescent light/12-h near-ultraviolet light cycle. Following incubation, fungal colonies with floccose white to pale violet mycelia (Figure 3) were consistently observed. On Spezieller Nährstoffarmer agar, microconidia were observed forming on short monophialides in the aerial mycelia. The colony morphology was characteristic of Fusarium oxysporum (Leslie and Summerell 2006). Genomic DNA was extracted from a pure isolate using a NucleoSpin Plant II mini kit (Macherey-Nagel, Germany) following the manufacturer's instructions. The 5.8S nrRNA gene with the flanking internal transcribed spacer (ITS) region was amplified with primers ITS5/ITS4. The partial translational elongation factor 1α gene (tef1) was amplified using EF728/EF2 (Carbone and Kohn 1999; O'Donnell et al. 1998). The partial RNA Polymerase II gene (rpb2) was amplified using fRPB2-5F/fRPB2-7CR (Liu et al. 1999). The obtained sequences were deposited in GenBank: ITS (Accession No. PV171105), tef1 (PV175755) and rpb2 (PV175754). A BLASTn search of these sequences showed 100% identity to F. oxysporum f. sp. canariensis. To confirm this relationship phylogenetically, the partial tef1 sequence was aligned with reference sequences from the FUSARIUM-ID database (Torres-Cruz et al. 2021) using Geneious Prime 2019.2.1 (https://www.geneious.com). The resulting analysis confirmed the isolate's identity, showing that it clustered within the F. oxysporum f. sp. canariensis clade (Figure 4). The symptoms observed were typical of F. oxysporum f. sp. canariensis, which is the causal agent of Fusarium wilt, a lethal disease of Canary Island date palm. The host range of F. oxysporum f. sp. canariensis is limited to a few species of Phoenix and Washingtonia (Laurence et al. 2015). To our knowledge, this is the first confirmed report of F. oxysporum f. sp. canariensis in the UK. Until now, the known distribution of F. oxysporum f. sp. canariensis within Europe was limited to several Mediterranean countries. F. oxysporum f. sp. canariensis is not regulated in the UK, and no statutory action was required. However, the infected tree was removed. This diagnostic work was funded through the Defra-Fera Long Term Service Agreement.