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Incubation in soil reduces sporulation and risk of epidemic development from leaf disks infested by Phytophthora ramorum
E. K. PETERSON (1), J. . Parke (1), N. J. Grünwald (2). (1) Oregon State University, Corvallis, OR, U.S.A.; (2) USDA-ARS Horticultural Crops Research Laboratory, Corvallis, OR, U.S.A.

Soilborne inoculum has been proposed as an important contributor to the local persistence of the sudden oak death pathogen <i>Phytophthora ramorum</i>, however current research has only assessed recovery of this pathogen from infested leaf debris. Two additional aspects of the disease cycle are being investigated at the National Ornamental Research Site at Dominican University of California: the infection of leaf baits at the soil surface, and sporangia production from inoculum incubated in soil. Rhododendron leaf disk inoculum was buried in soil at 5 and 15 cm in June 2014. Subplots were baited with non-infested leaf disks, and buried inoculum was recovered and placed in water to induce sporulation. Isolation of <i>P. ramorum </i>from soil-incubated disks remained stable after an initial decline observed within the first two weeks. Sporangia production was greatest from inoculum at 15 cm, but was reduced for inoculum at both depths. With the onset of cooler temperatures in autumn we observed a moderate increase in sporangia production, but have not yet isolated <i>P. ramorum</i> from baits placed above inoculum introduced in June. Our results indicate recovery of <i>P. ramorum</i> from infested leaf debris may overestimate the risk soilborne inoculum poses to new infections due to decreases in sporulation capacity. While cooler and more constant temperatures and moisture levels at lower depths may contribute to greater survival and sporulation, dispersal from these depths may be limited.

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