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Dominican University of California Researchers Study Spread of Sudden Oak Death in Nurseries

The same fungus-like organism that has infected thousands of trees with Sudden Oak Death is now threatening several species of popular plants throughout the United States, forcing nursery owners to take costly measures to ensure their stock remains disease free. Dominican University of California, in collaboration with the University of San Diego, the National Park Service, and Sunnyside Nursery, has started a yearlong project to determine how to control the spread of the deadly Phytophthora ramorum among camellia plants in nurseries.


In March 2004 camellias infected with P. ramorum were discovered in nurseries in Los Angeles and San Diego. By April infected plans from one wholesale nursery were turning up in Florida, Georgia, and Oregon. Many states restrict import of nursery stock from California (a $13.55 billion industry), along with wood and soil products. Dominican University of California has received a grant from the USDA Forest Service to support its research into P.ramorum.

“We now know that Phytophthora ramorum has been found to infect common ornamental plants including rhododendrons and camellias, resulting in a devastating impact on the nursery industry world wide, ” said Sibdas Ghosh, chair and professor of biology in the Department of Natural Sciences and Mathematics at Dominican University of California. “However, it is not known how the pathogen spreads from plant to plant in nurseries. ”

Ghosh and his team are setting up simulated nursery conditions to track the spread of SOD from infected to nearby non-infected plants. The experiments are designed to determine mode of pathogen transmission; the tissue damage at the infection site; and the minimum safe distance between potentially infectable plants.
The goal is to save nurseries money through efficient space utilization and reduced loss of stock.

“We could save governmental agencies money through more effective quarantine and destruction regulations and more focused inspections; and reduce the long-range spread of P. ramorum through infected nursery stock shipments, ” said Ghosh.

The findings, he added, will help determine whether or not current government regulations need to be revised for the handling and disposal of infected plants. Federal protocol requires nurseries to destroy all plants located next to an infected plant, as well as all host plants and associated plant species within 2 meters of the infected block. The 2-meter requirement, however, was adopted from European Union regulations, but Ghosh is unaware of any published studies that justify this distance.

“Wholesale nurseries do not have the space to accommodate large distances between potential Phytophthora ramorum hosts, ” said Ghosh. “The loss of an entire production block or more due to a single infected plant makes stocking host species an economic gamble for many nurseries.”

Leading this study are Ghosh, along with William King, assistant professor of mathematics at Dominican University of California; Peter Thut, laboratory and research manager at Dominican University of California; Mietek Kolipinski, National Park Service; and Lisa Baird, chair and professor of biology at the University of San Diego. Ross Perry of Sunnyside Nursery in Marin County is serving as an industry consultant.

The research team is completing a work area on Dominican’s San Rafael campus that will enable them to grow camellia plants and study how the fungus moves from plant to plant. This project will use1-gallon pots of three different cultivars of camellia, which may show different levels of resistance to the disease. Pots will be set up in a circle with an infected camellia in the center. Each plot will have six experimental plants: two of each cultivar. Three distances will be investigated: 0m, 1m, and 2m.

The researchers will gather samples from infected plants at regular intervals in order to determine the mode of pathogen entry to the plant and the progress of infection through the various tissues of a leaf or stem.

Dominican University of California students are assisting in all aspects of the project. “Our students are prime vectors for community education, and we are currently training future graduate students in plant pathology and forest management,” says Ghosh.


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