The Occurrence of Phytophthora ramorum in Nursery Stock in California, Oregon, and Washington States

Abstract

Data from nursery surveys conducted in 2003, 2004, and 2005 for ramorum blight and dieback caused by Phytophthora ramorum were analyzed to determine which plant cultivars were likely to be infected with P. ramorum. Susceptible plant host genera, species, and cultivars were identified using culture and nested-polymerase chain reaction analyses, and disease incidence was calculated. For the genus, species, or cultivar analyses at least 20 samples were utilized in this study. The results indicated that the genus Camellia had the highest disease incidence, followed by Rhododendron and Viburnum. Camellia cvs. Setsugekka, Jean May, April Remembered, Winter's Fire, Kramer's Supreme, Bonanza, Magnoliaeflora, Apple Blossom, Silver Waves, Yuletide, and Nuccio's Gem showed the highest risk of P. ramorum infection. Four Rhododendron cvs. Purple Splender, Cunningham's White, Unique, and Nova Zembla were also grouped into the high risk category. All other Rhododendron and Viburnum plant species were at medium or low risk of infection by the pathogen. This study indicated that there is a large variation in disease incidence among genera and specific cultivars within a genus. A series of controlled, replicated, and well designed experiments involving a large sample size of plant cultivars is essential in order to identify the underlying reasons for the observed variation in infection risks of the surveyed data. Accepted for publication 25 January 2006. Published 15 March 2006.