Severe decline was observed on a mature stand of western hemlock (Tsuga heterophylla) in late August 2021 during the Forestry Commission annual aerial surveillance for Phytophthora ramorum in south-west England (UK). Site visits followed in September to investigate the cause of the decline. Affected trees showed crown dieback, needle drop, branch and stem cankers. Samples were collected and sent for analysis to the Tree Health Diagnostic and Advisory Service at Forest Research, UK.
Phytophthora pluvialis was confirmed by isolation and sequencing of the ITS and COXII regions. The results were also confirmed by real-time PCR. Pathogenicity tests were carried out on 7–8-year-old western hemlock. After three months, cankers formed on inoculated trees were similar to those observed in the field confirming Koch’s postulates. This is the first report of P. pluvialis in Europe. This is also the first published report of P. pluvialis causing resinous cankers on western hemlock worldwide.