Forests and woodlands worldwide are being severely impacted by invasive Phytophthora species, with initial outbreaks in some cases occurring on host trees located in public parks and gardens. These highly disturbed sites with diverse planting practices may indeed act as harbours for invasive Phytophthora pathogens which are particularly well adapted to surviving in soil. High throughput Illumina sequencing was used to analyse Phytophthora species diversity in soil samples collected from 14 public garden/amenity woodland sites in northern Britain. Bioinformatic analyses revealed some limitations to using internal transcribed spacer as the barcode region; namely reporting of false positives and ambiguous species matches. Taking this into account, 35 distinct sequences were amplified across the sites, corresponding to 23 known Phytophthora species as well as twelve oomycete sequences with no match to any known Phytophthora species. Phytophthora pseudosyringae and P. austrocedri, both of which cause serious damage to trees and are regarded as fairly recent introductions to Britain, were the two most abundant Phytophthora species detected. There was no evidence that any of the detected Phytophthora species were more associated with any one type of host, healthy or otherwise. This study has demonstrated the ubiquity and diversity of Phytophthora species endemic in highly managed, extensively planted soil environments in Britain. Suggested improvements to the methodology and the practical implications of the findings in terms of mitigating Phytophthora spread and impact are discussed.

The multidisciplinary ‘Phyto-threats’ project was initiated in 2016 to address the increasing risks to UK forest and woodland ecosystems from trade-disseminated Phytophthora. A major component of this project was to examine the risk of Phytophthora spread through nursery and trade practices. Close to 4000 water and root samples were collected from plant nurseries located across the UK over a three-year period. Approximately half of the samples tested positive for Phytophthora DNA using a metabarcoding approach with 63 Phytophthora species identified across nurseries, including quarantine-regulated pathogens and species not previously reported in the UK. Phytophthora diversity within nurseries was linked to high-risk management practices such as use of open rather than closed water sources. Analyses of global Phytophthora risks identified biological traits and trade pathways that explained global spread and host range, and which may be of value for horizon-scanning. Phytophthoras having a higher oospore wall index and faster growth rates had wider host ranges, whereas cold-tolerant species had broader geographic and latitudinal ranges. Annual workshops revealed how stakeholder and sector ‘appetite’ for nursery accreditation increased over three years, although an exploratory cost-benefit analysis indicated that the predicted benefits of introducing best practice expected by nurseries outweigh their costs only when a wider range of pests and diseases (for example, Xylella) is considered. However, scenario analyses demonstrated the significant potential carbon costs to society from the introduction and spread of a new tree-infecting Phytophthora: Thus, the overall net benefit to society from nurseries adopting best practice could be substantial.

The diversity of Phytophthora species in soils collected from 14 highly disturbed sites in northern Britain, including botanic gardens, arboreta, public parks and other amenity woodland sites, was analysed using a molecular technique known as DNA metabarcoding. This technique enables the identification of multiple species present in a single environmental sample based on a DNA ‘barcode’ unique to each species. The genus Phytophthora was targeted in this study due to its increasing impact on Britain’s forests and woodlands over thelast 20 years. The introduction and spread of new Phytophthora species into Britain has been strongly associated with the movement of traded containerised plants, with a number of Phytophthora outbreaks reported on host trees located in public gardens and parks that had recently undergone planting or landscape regeneration schemes. This study was undertaken to assess the extent to which these highly disturbed sites with extensive planting regimes act as harbours for woody-host infecting Phytophthora species. A total of 23 Phytophthora species, the majority of which are known to be pathogens of woody hosts, were detected across the 14 sites sampled. These included four quarantine-regulated pathogens and four species notpreviously recorded in Britain. Also detected were three as-yet undescribed Phytophthora species and nine oomycete sequences with no clear match to any known genus. There was no effect of geographical location, elevation, underlying soil type, host family or host health status on the Phytophthora assemblages at each site, suggesting that the Phytophthora communities detected are likely to comprise introduced species associated with planting programmes. P. austrocedri and P. pseudosyringae were two of the most abundant Phytophthoraspecies detected, both of which cause serious damage to trees and are regarded as fairly recent introductions to Britain. The practical implications of the findings in terms of mitigating Phytophthora introduction, spread and impact at botanic gardens, arboreta and urban parks are discussed.