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Root and Stem Infection of Rhododendron from Potting Medium Infested with Phytophthora ramorum

October 2007 , Volume 91 , Number  10
Pages  1,265 - 1,270

J. L. Parke, Department of Crop and Soil Science and Department of Botany and Plant Pathology, and C. Lewis, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331



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Accepted for publication 28 April 2007.
ABSTRACT

Phytophthora ramorum has been detected in soil and potting media, but the potential for root infections is not fully understood. To determine whether the root system could become infected and transmit disease, rhododendron ‘Nova Zembla’ plants grown from rooted cuttings and native Pacific rhododendron (Rhododendron macrophyllum) plants grown from seed were transplanted into a potting medium artificially infested with P. ramorum. Inoculum consisted of V8-brothvermiculite cultures of P. ramorum, chopped infected leaves, or zoospores. Plants were watered from the bottom to prevent splash dispersal of inoculum onto stems and foliage. Both infested amendments and applications of zoospores resulted in plant mortality within 3 to 7 weeks. P. ramorum was isolated from hair roots, large roots, and stems above and below the potting medium surface. Noninoculated control plants remained healthy and did not yield P. ramorum. Epifluorescence microscopy of tissue culture plantlets inoculated in vitro revealed attraction of zoospores to wounds and root primordia, and colonization of the cortex and vascular tissues of roots and stems, including the xylem. Transmission of P. ramorum from infested potting media to stems via infected, symptomless root tissue demonstrates the need to monitor potting media for presence of the pathogen to prevent spread of P. ramorum on nursery stock.


Additional keywords:dose-response, ramorum shoot dieback, sudden oak death

© 2007 The American Phytopathological Society