Diversity of woody-host infecting Phytophthora species in public parks and botanic gardens as revealed by metabarcoding, and opportunities for mitigation through best practice.
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 the last 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 not previously 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 Phytophthora species 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.