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In April 2020, decline symptoms were observed on a single mature London plane (Platanus × acerifolia) tree in south-east England. A rapid decline of one of the major scaffolds, at 18 m height, was observed and the limb died within one year. The area below the dieback produced extensive epicormic growth. The removed limb measured 44 cm in diameter at its base. Bark and wood samples were initially sent to Forest Research, then a basal cross section of the limb and rhizosphere soil samples of the tree were collected for testing. The bark showed a chocolate-brown, tongue-shaped lesion (Fig. 1), whereas the underlying wood had orange-brown to pink discolourations (Fig. 2). Tissue from the margins of the bark lesion (Fig. 3) was plated onto Phytophthora-selective medium (SMA) (Brasier et al., 2005). Plates were incubated at 20°C in the dark. Green apples were used as bait for isolating Phytophthora from the soil sample (Pérez-Sierra et al., 2022). Phytophthora was isolated from bark, wood and the soil baits associated with the diseased tree (2020/272). DNA was extracted from three isolates, one each obtained from bark, wood and soil, for amplification and sequencing of the ITS and cox1 regions (Jung et al., 2020). The GenBank accession numbers for ITS are OR518906, OR518907 and OR518908, and for cox I are OR526110, OR526111 and OR526112. BLASTn analysis of the sequences revealed 100% identity with a new Phytophthora species from Clade 2c recently described as P. platani (Jung et al., 2024). GenBank accession numbers of 13 gene regions of the three UK and nine Italian isolates are given in Jung et al. (2024). An underbark inoculation test was done in 2021 on two-year-old London plane trees using P. platani strains from the UK (2020/272) and Italy (ex-type CBS 149638), and P. plurivora (2020/1628) as a positive control. For each isolate, a wound was made using a 4-mm diameter cork-borer on each of eight trees and a carrot agar (CA) plug colonised with the respective isolate was placed on the wound upside down, covered with sterile moistened cotton wool and wrapped in plastic film and aluminium foil. Control trees were treated in the same way but received sterile CA. The trees were incubated in a growth chamber with a 12-hour photoperiod, a 25/18°C day/night temperature cycle, and watered as necessary. After one month, lesion lengths were measured and subjected to an ANOVA F test with Tukey's post hoc test (R version 3.5.1; R Core Team, 2018). Phytophthora platani isolates 2020/272 (44.1 ± 9.8 mm) and CBS 149638 (34.8 ± 13.0 mm), and P. plurivora (30.9 ± 6.5 mm) differed significantly from the controls, which developed no lesions (F = 65.6, p<0.0001). There were no statistical differences between the isolates. However, while both P. platani isolates caused chocolate- to dark-brown external lesions, P. plurivora lesions were not visible externally (Fig. 4). All Phytophthora species were reisolated from the lesions confirming Koch's postulates. Despite a thorough survey, P. platani was only recovered from one plane tree in the UK. It is still unknown how this pathogen infected this single tree. This species has only been reported previously in Italy where it was first recorded in 2013 (Jung et al., 2024). To our knowledge this is the first report of P. platani in the UK. Funding from Tree Health & Biosecurity Research Programme of the Science and Innovation Strategy for Forestry in Great Britain and the Project Phytophthora Research Centre Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000453 co-financed by the European Regional Development Fund.