Saturday, 15 July 2006

Survival and Reproduction of Phytophthora ramorum in Forest Soils.

Elizabeth J. Fichtner, Shannon C. Lynch, and David Rizzo. UC Davis, Dept of Plant Pathology, 1 Shields Ave., Davis, CA 95616

Pathogen-infested soil may aid in long-distance transmission of regulated plant diseases, in part due to federal regulation of the disease and not the pathogen itself. Phytophthora ramorum, the pathogen causing sudden oak death, may be disseminated under the regulatory radar through infested forest soils and soilless potting media used in the nursery industry. This study assesses the seasonal distribution, winter survival, and reproduction of soilborne inoculum in a redwood-tanoak forest in California. Because sporulation varies between host plants, the distribution of soilborne inoculum under host species has been monitored from 2003 until present. Soil and leaf litter samples were collected monthly from 15 trees each of coast redwood (Sequoia sempervirens), tanoak (Lithocarpus densiflorus), and California bay laurel (Umbellularia californica) and pathogen presence was determined by baiting. P. ramorum was generally recovered from soils during the winter rainy season, with the highest frequency of recovery occurring under bay laurel. To assess survival potential of soilborne inoculum, ten colonized rhododendron leaf disks were placed in each of 360 mesh sachets before transfer to the field in January 2005 and 2006. The sachets were dispersed under 10 trees each of tanoak, bay laurel, and redwood, and at three vertical locations: i) leaf litter surface, ii) litter/soil interface, and iii) below the soil surface. Sachets were retrieved 4, 8, 20, and 52 weeks after introduction to the field. After 20 weeks, > 50% pathogen recovery was observed in both subsurface treatments and subsequent hydration of P.r.-negative leaf disks enhanced recovery in these treatments to > 95%. Hydration-stimulated recovery was associated with high populations of chlamydospores, suggesting a role of moisture in mediating chlamydospore germination and fungistasis. Leaf disks incubated in organic soils exhibited 60% fewer chlamydospores than those incubated in mineral soils. Ongoing studies focus on the role of soil microbial activity in chlamydospore production and pathogen survival, incorporating quantitative PCR to assess pathogen populations. The heightened recovery of subsurface inoculum suggests that pedoturbation processes resulting in propagule burial may enhance pathogen survival, allowing the soil to serve as an inoculum reservoir. Some soils, however, may suppress the production of survival propagules, potentially limiting the long-term survival of the pathogen.

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Back to The 18th World Congress of Soil Science (July 9-15, 2006)