Association ofPhytophthorawith Declining Vegetation in an Urban Forest Environment

Urban forests consist of various environments from intensely managed spaces to conservation areas and are often reservoirs of a diverse range of invasive pathogens due to their introduction through the nursery trade. Pathogens are likely to persist because the urban forest contains a mixture of native and exotic plant species, and the environmental conditions are often less than ideal for the trees. To test the impact of different land management approaches on thePhytophthoracommunity, 236 discrete soil and root samples were collected from declining trees in 91 parks and nature reserves in Joondalup, Western Australia (WA). Sampling targeted an extensive variety of declining native trees and shrubs, from families known to be susceptible toPhytophthora. A sub-sample was set aside and DNA extracted for metabarcoding usingPhytophthora-specific primers; the remaining soil and root sample was baited for the isolation ofPhytophthora.We considered the effect on thePhytophthoracommunity of park class and area, soil family, and the change in canopy cover or health as determined through sequential measurements using remote sensing. Of the 236 samples, baiting techniques detectedPhytophthoraspecies from 24 samples (18 parks), while metabarcoding detectedPhytophthorafrom 168 samples (64 parks). Overall, forty-fourPhytophthoraphylotypes were detected. Considering only sampling sites wherePhytophthorawas detected, species richness averaged 5.82 (range 1-21) for samples and 9.23 (range 2-24) for parks.Phytophthora multivorawas the most frequently found species followed byP. arenaria,P. amnicolaandP. cinnamomi.While park area and canopy cover had a significant effect onPhytophthoracommunity the R(2)values were very low, indicating they have had little effect in shaping the community.Phytophthora cinnamomiandP. multivora,the two most invasive species, often co-occurring (61% of samples); however, the communities withP. multivorawere more common than those withP. cinnamomi, reflecting observations over the past decade of the increasing importance ofP. multivoraas a pathogen in the urban environment.