The prevalence of Phytophthora in British plant nurseries; highrisk hosts and substrates and opportunities to implement best practice

Invasive Phytophthora species infect a very broad range of herbaceous and woody hosts globally. The UK alone has experienced a particularly damaging series of outbreaks and epidemics of new, invasive Phytophthora species affecting the nation's trees over the last 30 years. The link between Phytophthora outbreaks and the importation and spread of infected nursery stock is well established across many countries worldwide. To understand better the pathways of spread of Phytophthora in the nursery trade in Britain, we applied a standardized nursery sampling method combined with a refined metabarcoding detection method to capture the diversity of Phytophthora species at 134 British plant nurseries representing a range of biosecurity and trading practices over multiple sampling years between 2016 and 2022. This included root and water samples collected from 17 nurseries sampled seasonally and root samples collected from 117 nurseries sampled once as part of plant health inspections. Based on analyses of 1894 pooled samples, DNA barcodes of 85 Phytophthora species or complexes were detected, with variation in species' relative frequencies across nurseries. We present the top 20 host–Phytophthora associations ranked by relative frequency and report five novel Phytophthora records for the UK. We identified surprisingly high-risk hosts (such as Douglas fir) with the greatest number of Phytophthora associations and revealed Phytophthora nursery niche preferences for water or roots. We discuss the implications of our findings in terms of pathogen diversity and abundance, high-risk hosts, our information dissemination approach and resulting advice on nursery practices aimed at reducing risk.