Enhanced Detection of Phytophthora Species at P. pluvialis Outbreak Sites in Commercial Forests Across Britain

Invasive Phytophthora species are increasingly impacting UK landscapes. Most recently, cryptic outbreaks of P. pluvialis Reeser, Sutton &amp: E. Hansen have occurred on western hemlock and Douglas fir at several forest sites across Britain. To better understand the ubiquity and life cycle of this pathogen in British forests and the assemblages of co-inhabiting Phytophthora species, metabarcoding and baiting methodologies were applied to soil, stream water, and rainwater samples collected over a full calendar year from seventeen sites across Britain. Thirty-five Phytophthora species were detected across all sites, substrates, and detection methods, with most detections occurring in stream water by metabarcoding. The three most frequently detected species were (1) P. pluvialis, (2) P. gonapodyides H.E. Petersen &amp: Buisman and (3) P. ramorum Werres, De Cock &amp: Man in &lsquo:t&rsquo: veld. Other species detected included the regulated pathogens P. austrocedri Greslen &amp: Hansen, P. kernoviae Brasier, Beales &amp: S.A. Kirk and P. lateralis Tucker &amp: Milbraith, as well as P. ornamentata Scanu, Linald &amp: T. Jung, a new species record for the UK. Phytophthora pluvialis was most frequently detected in March, with rainfall trap metabarcoding data suggesting that aerial dissemination occurs predominantly in late winter/early spring. Consistent detections of P. pluvialis in soil by metabarcoding indicate the potential for soilborne transfer of this pathogen by animal or human vectors, including equipment or machinery in forest operations. The study&rsquo:s findings are discussed in relation to understanding how P. pluvialis spreads and the approaches needed to address key knowledge gaps in relation to inoculum sources. The results provide a baseline for Phytophthora diversity in British commercial forests, facilitating a greater understanding of typical and unusual trends in species assemblages. This study also consolidates the value of metabarcoding as an effective surveillance tool for Phytophthora in commercial forests.