Phytophthora ramorum and Phytophthora gonapodyides differently colonize and contribute to decay of California bay laurel ( Umbellularia californica ) leaf litter in stream ecosystems

The prevalence of Phytophthora species in surface waters has earned increasing attention in the past decades, in great part as a result of “stream monitoring” programs for detection and monitoring of Phytophthora ramorum and other invasive species. The potential for Phytophthora to survive and reproduce in streams has significant implications for evaluating and managing the risk of spread of pathogenic species. Therefore, it is important to understand the ecology of both introduced and endemic species in aquatic environments. Leaf litter is a potential substrate for the persistence and propagation of Phytophthora in streams. Our previous work showed that P. gonapodyides and related ITS clade 6 taxa are effective saprotrophs, colonizing dead leaf tissue; whereas, P. ramorum more effectively colonized fresh green leaves, but its colonization is limited in dead leaves. Therefore, P. ramorum’s prevalence on stream leaf litter may be limited by direct competition with other taxa as well as by the substrate becoming increasingly unsuitable due to decay. We conducted a field and laboratory study to determine how well P. ramorum and “clade 6” Phytophthora could colonize, persist, and sporulate on increasingly decayed leaves of California bay, the primary host for P. ramorum and a common riparian species in California coastal woodlands.