Phytophthora in urban tree planting stock: are we managing the risk to the urban forest and natural ecosystems?

The oomycete genus Phytophthora includes plant pathogens that pose significant threats to agricultural systems, natural ecosystems and urban forests. Urban forests are increasingly recognized for their role in mitigating climate change impacts and urban greening initiatives are underway worldwide. However, research suggests that the urban forest is also a reservoir of Phytophthora diversity, acting as bridgeheads for the introduction of these pathogens into production and natural ecosystems. The source of Phytophthora in urban forests is linked to anthropogenic factors, with the live plant trade the primary pathway. This study focused on the incidence and diversity of Phytophthora in amenity tree nursery stock in eastern Australia, primarily New South Wales, with one nursery from Victoria. The findings revealed a high incidence of Phytophthora in nursery stock destined for urban greening projects, having an overall positive rate of 22.1%, with incidence in individual nurseries ranging from 2.5% to 32%. The study detected 13 described Phytophthora species that are considered polyphagous on woody hosts and three lineages that potentially represent novel species. In addition, the detection of P. mediterranea and P. heterospora represent first reports for Australia, with P. mediterranea considered a biosecurity concern. The study highlights the substantial risk associated with the live plant trade as a source of Phytophthora introduction into urban forests and beyond. These findings underscore the urgent need to strictly implement clean nursery management practices to reduce the economic and environmental risks associated with Phytophthora in urban greening programmes.