Diseases caused by invasive Phytophthora pathogens have significantly affected forest and natural ecosystems worldwide. The introduction and spread of new and/or regulated species has had negative impacts on the environment and on timber industries. A recent study suggests that the invasive P. ramorum, introduced to Europe in the mid-1990s, has potential to cause direct damage costs of €117 million for larch and €130 million for beech if not managed (Kartakis et al., 2026). The role of the plant trade in introducing and spreading Phytophthoras has been well studied (Beales et al., 2004; Green et al., 2021, 2025; Sims & Garbelotto, 2021). Although public gardens may have a crucial role in the epidemiology of these pathogens due to high volume of incoming stock, plant collections of diverse origin and high volume of visitors, they have not been extensively studied.  Given the continuum between natural forests, gardens and the nursery trade, it is vital to better understand the role of public gardens, which may act as a reservoir of pathogens and assist the survival and spread of Phytophthora species.

In the framework of this project, shared protocols for sampling and detection of Phytophthora sp. and other oomycetes, were applied to public gardens and their nurseries across six participating countries (United Kingdom/ United States of America/ Slovenia/ Italy/ Greece/ Czech Republic). Each partner aimed to collect 40 samples from each garden and its nursery/or its supplier nurseries, over two years. Soil, plant tissue and water samples were collected and analysed using a validated metabarcoding protocol (Scibetta et al., 2012). All Phytophthora-positive samples were processed in the UK for Illumina sequencing of the ITS region. The Phytophthora classification tool  https://pypi.org/project/thapbi-pict/ (Cock et al., 2023; Green et al., 2021) was used for the bioinformatics analyses for species identification. Data from all the samples collected during the collaboration showed that Phytophthoras and other oomycetes were present in all sampled sites, with 51% of the samples being positive. Overall, 55 different taxa of Phytophthora were detected, the most frequently occurring included the complex P.aleatoria/ P. alpina/ P. cactorum and the species P. multivora, P. nicotiana, P. syringae. P. cryptogea and P. pseudocryptogea. There were differences between the top detected species among countries; however, most were present in at least two different countries. Nurseries accounted for up to 68.9% of positive samples and 46 different taxa of Phytophthora, confirming that nursery stock poses great risk for gardens. The important role of public gardens in the epidemiology of the pathogen was also confirmed by the 20 new Phytophthora spp. detections, which included new records for the country and new host associations. Furthermore, our results suggest that some plant genera may have a more important role for harboring Phytophthora pathogens than is currently recognised; some genera were associated with a higher number of Phytophthora spp. compared to the known high-risk hosts. This is alarming, as these ‘lesser’ hosts lie ‘under the radar’ but may pose risks as new species’ associations and should be considered during risk analysis. Also, we identified 37 plant genera to be asymptomatic, although they were carrying more than one Phytophthora spp., including the regulated P. ramorum. Further study on the interaction between these host species and their associated Phytophthora species may enable us to understand if there are any patterns which could assist disease surveillance.