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    Author(s): Elizabeth Fichtner; David Rizzo; Susan Kirk; A. Whybrow; J. Webber.
    Date: 2009
    Source: In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 281
    Publication Series: General Technical Report (GTR)
    Station: Pacific Southwest Research Station
    PDF: View PDF  (13.0 KB)

    Description

    Phytophthora kernoviae and Phytophthora ramorum, two pathogens recently introduced to the U.K., incite foliar lesions, shoot necrosis, and death of Rhododendron ponticum, an invasive weed pervading U.K. woodlands. In infested woodlands, R. ponticum serves as an epidemiologically important host, supporting sporulation of both pathogens. Bleeding cankers on trunks of European beech (Fagus sylvatica) caused by either P. ramorum or P. kernoviae are often associated with neighboring infected R. ponticum.

    Rhododendron ponticum has been removed from several woodlands as an inoculum management strategy, but the long-term efficacy of plant removal is unknown, in part due to lack of knowledge of pathogen persistence in roots and in emerging seedlings.

    The potential for P. ramorum and P. kernoviae to infect roots of R. ponticum in U.K. woodlands is unknown. To assess pathogen association with rhododendron roots, roots initiated from natural layering were excavated from two sites infested with P. kernoviae and one site infested with P. ramorum. At each site, four sets of layered roots were sampled, in addition to the associated leaf litter, rhizosphere soil, and symptomatic leaves. In the laboratory, soil and leaf litter were individually baited using leaf disks of Rhododendron catawbiense ‘Cunninghams White.’ Tissue from symptomatic leaves was embedded in SMA agar for isolation of Phytophthora spp. Neither pathogen was baited from rhizosphere soil, but both were routinely recovered from leaf litter. Phytophthora ramorum was baited from one set of layered roots; P. kernoviae was baited from three sets of roots at one site and from two sets at another site.

    A second objective focused on investigating the potential for infection of R. ponticum seedlings in a woodland cleared of R. ponticum in 2005 for management of P. kernoviae. Nineteen seedlings were excavated from the woodland and all foliar lesions were sampled for pathogen isolation. Rhizosphere soil and roots were independently baited with rhododendron leaf disks. Phytophthora kernoviae was recovered from foliar tissue on 2 seedlings, from roots of 5 seedlings, and from two samples of rhizosphere soil.

    The results suggest that both P. ramorum and P. kernoviae are associated with R. ponticum roots in infested U.K. woodlands. Furthermore, the presence of inoculum in litter but rarely in soil suggests that the pathogens may infect the roots, rather than simply persist on the rhizoplane. Further research is needed to assess the frequency of root associations and to histologically visualize root infections of thesetwo pathogens. These preliminary data suggest that the potential persistence of these pathogens in roots and litter should be considered when managing the diseases in infested woodlands.

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    Citation

    Fichtner; Elizabeth; Rizzo, David M.; Kirk, Susan A.; Whybrow, A.; Webber, J. 2009. Root sssociations of Phytophthora ramorum and Phytophthora kernoviae in U.K. woodlands. In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 281.

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