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

Abstract

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