Oomycetes as damping‐off pathogens and functional plant traits associated with the susceptibility of west Australian native plant species

Damping‐off oomycete and fungal plant pathogens reduce the germination, emergence, and survival of seedlings. In agricultural systems, this poses a significant problem as it may reduce productivity; in contrast, the impact is more subtle in natural ecosystems and may be essential for maintaining the structure and diversity of plant communities. The oomycete genus Phytophthora is frequently detected in natural plant communities causing disease in mature plants, but it is rarely assessed as a damping‐off pathogen. A pathogenicity experiment was established with five oomycete treatments, three putatively native Phytophthora species, the invasive P. cinnamomi, and Pythium irregulare, to determine the damping‐off host range in selected plant species from a hyper‐diverse Mediterranean‐type ecosystem. Additionally, plant and seed traits were analysed to determine if they were correlated with susceptibility to pre‐emergent damping‐off. Phytophthora arenaria was the only putatively native generalist damping‐off pathogen, causing 12 of 19 plant species to experience significantly reduced seedling emergence or survival relative to the control treatment. Pythium irregulare and the introduced P. cinnamomi caused significant damping‐off for 12 and 13 plant species, respectively. Plant species that were slowest to germinate and emerge were the most susceptible to pre‐emergence damping‐off caused by P. arenaria. These results suggest native and invasive Phytophthora may substantially influence the structure and diversity of natural plant communities through damping‐off. While mature plant species are the most common focus in Phytophthora research, our study indicates damping‐off should be considered to determine the full impact of these soil‐borne plant pathogens.