First report of <i>Phytophthora pluvialis</i> in Europe causing resinous cankers on western hemlock

Severe decline was observed on a mature stand of western hemlock (Tsuga heterophylla) (Fig. 1) in late August 2021 during the Forestry Commission's annual aerial surveillance for Phytophthora ramorum south-west England. Site visits followed September to investigate cause decline. Affected trees showed crown dieback, needle drop, branch and stem cankers. Mortality young, regenerated understorey also observed. Cankered areas exuded copious resin 2) bark surface blackened cracked. The outer cankers removed, brown necrotic phloem which resembled lesions were Resin pockets detected below most lesions. Samples collected sent analysis Tree Health Diagnostic Advisory Service, Forest Research. Tissue from lesion margins plated onto selective medium (SMA) (amended as per Brasier et al., 2005) incubated at 20°C two weeks. Developing colonies transferred carrot agar (CA), grown 10 days DNA extracted amplification sequencing ITS coxII regions. Sequences edited Sequencher version 5.4.6 (Gene Codes Corporation, Ann Arbor, USA) all had 100% identity with pluvialis GenBank. representative isolate submitted GenBank (OK474970- OK490501). Identification P. confirmed by real-time PCR using protocol McDougal al. (2021). Pathogenicity tests done 7–8-year-old hemlock. A used trial six inoculated branches. wound made cork-borer (6 mm diameter) CA plug colonised placed wound, covered cotton wool soaked sterile distilled water wrapped plastic paraffin film then aluminium foil. Controls similarly but plug. Trees maintained growth chamber photoperiod temperature 17°C day (8 hours)/15°C night (16 hours) watered demand. After three months, chlorosis needles close inoculation points observed, more widespread necrosis cast. Cankers formed similar those field 3). recovered isolation fulfilling Koch's postulates. No controls. This is first report Europe. species described originally USA affecting tanoak Douglas-fir (Reeser 2013). In 2014, it reported New Zealand red cast disease Pinus radiata (Dick 2014). Zealand, acts mainly pathogen. published causing resinous worldwide. We thank Commission (FC), England Department Environment, Food & Rural Affairs (Defra) colleagues their support acknowledge funding work FC, Defra Biosecurity Research Programme Science Innovation Strategy Great Britain.