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Decaying leaf litter supports Phytophthora ramorum and endemic Phytophthora species in streams.
K. ARAM (1), D. M. Rizzo (2). (1) Univ of California, Davis, CA, U.S.A.; (2) University of California, Davis, CA, U.S.A.

In surface waters, leaf litter is a potential substrate for persistence and propagation of introduced tree pathogen <i>Phytophthora ramorum</i> and putatively endemic and saprobic species grouped in ITS clade 6. Previous research showed that <i>P. ramorum</i> readily colonizes fresh leaf tissue in streams, but is not effective at colonizing leaves killed by freezing or drying in the lab before exposure in streams. In contrast, <i>P. ramorum</i> was recovered abundantly from significantly degraded natural leaf litter. “Clade 6” species occurred on all leaf types. We asked how these species might compete for fresh leaf tissue introduced into streams, how long they would persist and whether they could sporulate from progressively more decayed leaf tissue. <i>P. ramorum</i>-infected and uninfected bay laurel (<i>Umbellularia californica</i>) leaves were incubated in two forest streams and sampled at 8 intervals over 16 weeks. Leaves were evaluated for Phytophthora colonization through isolations and morphological identification. <i>P. ramorum</i> and “clade 6” taxa quickly colonized leaves in streams and persisted throughout the full duration of in-stream incubation despite loss of as much as 70% of leaf mass due to decay. Both could be baited from leaf samples over the entire 16 weeks, demonstrating sporulation potential despite substantial decay and biomass loss. This indicates that bay laurel leaf litter can serve as a persistent source of <i>P. ramorum</i> and “clade 6” Phytophthora inoculum in infested streams.

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