Fungal and oomycete damping-off pathogens kill pre- and post-emergent seedlings and can regulate the abundance of plant species to help maintain diversity in natural ecosystems. However, damping-off pathogens may be detrimental to post-mining ecological restoration, a key hurdle in the process is the loss of pre- and post-emergent seedlings. Recently, several putatively native Phytophthora species, soil-borne oomycete plant pathogens, have been recovered from hyper-diverse kwongan vegetation in southwest Australia and may be damping-off pathogens. Damping-off pathogens may contribute to maintaining the diversity of natural kwongan plant communities and reduce seedling establishment within ecological restoration. Four experiments were designed to identify the distribution and role of Phytophthora and other potential damping-off oomycetes in natural and restored kwongan plant communities. Putatively native Phytophthora arenaria, introduced P. cinnamomi and Pythium irregulare were identified as damping-off pathogens with wide host ranges of native kwongan plant species through a glasshouse pathogenicity trial. Fungicide seed coat treatments improved seedling emergence for five of the 14 plant species studied in ecological restoration field trials, identifying low to moderate levels of pre-emergent damping-off caused by oomycetes and fungi. Natural kwongan soils collected from different plant species affected seedling emergence and survival in a glasshouse experiment. Damping-off caused conspecific negative plant-soil feedback for Jacksonia floribunda and Xanthorrhoea sp. Lesueur, a process that contributes to the maintenance of diversity in plant communities. Although, the presence and abundance of oomycetes detected using metabarcoding from the same natural kwongan soils were not associated with reduced seedling emergence or survival. In a regional metabarcoding survey of natural kwongan plant communities, the Phytophthora species richness and abundance were far lower than previously hypothesised due to the elimination of sources of sampling biases. Plant species and host age were strong drivers of the oomycete communities detected at a local scale using metabarcoding. These studies provided an insight into the distribution of important plant pathogens in a species rich Mediterranean shrubland and identified damping-off pathogens could be a mechanism contributing to maintaining the diversity of natural kwongan plant communities and low seedling establishment in ecological restoration
