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.

Evaluation of several commercial biocontrol products on European and North American populations ofPhytophthora ramorum

“…These are two hosts of P. ramorum that are commonly found in nurseries and landscapes and are considered to be important inoculum reservoirs for the disease (Tubajika, Bulluck, Shiel, Scott, and Sawyer 2006). In addition, variability in response to biocontrol agents among isolates of P. ramorum from North American and European populations was examined.…”

Susceptibility of Some Lilac Cultivars and Other Members of the Oleaceae to Phytophthora ramorum

“…Although lilac leaves were very susceptible in artificial inoculation and lesions produce large numbers of sporangia, it has not been one of the plants commonly found infected in nursery surveys of the west coast (15). This may have to do with the interaction of plant phenology and west coast climate.…”

“…Lilac is deciduous, and the buds and shoots do not appear to be very susceptible, so susceptible tissue may not be present when optimum conditions for infection occur on the west coast. The most commonly observed infected plants in nurseries on the west coast have been evergreens, Camellia and Rhododendron (15). During wet springs on the east coast, however, lilacs in nurseries might come into contact with infected stock and themselves become infected.…”

“…Although the distribution of P. ramorum in US forests is currently mainly limited to parts of California and Oregon, there is potential for spread with the movement of nursery stock. By the end of 2004, infected containerized ornamentals were inadvertently shipped from California to at least 40 states (15) leading to the instigation of an Emergency Federal Order, which placed restrictions on the movement of host plants shipped out of California, Oregon, and Washington (1). In 2003, the first infected lilacs were found in nurseries located in the United Kingdom (2).…”

A Bayesian model identifying locations at risk from human‐transported exotic pathogens

“…The relevant material being moved by human activity is wholesale nursery stock comprised, in part, of possibly infected P. ramorum host plants. While P. ramorum is only established in forest tracts in northern California and southern Oregon, the pathogen has been found in numerous commercial nurseries in both states, as well as in Washington (Tubajika et al, 2006). Infected plants have also been found at nurseries in more than 20 other US states, in many cases definitely traced to shipments from West Coast nurseries (California Oak Mortality Task Force, 2008; Grünwald et al, 2019).…”