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Researchers from ӣ����Ƶ of California’s federally funded ornamental plant nursery have identified cotoneaster, a common and popular plant sold by nurseries throughout the United States, as a new host plant for Phytophthora ramorum, the pathogen that causes Sudden Oak Death (SOD).

“This is the first report of P. ramorum occurring on Cotoneaster sp. in the United States,” says Wolfgang Schweigkofler, research associate professor and lead scientist at the National Ornamentals Research Site at ӣ����Ƶ of California (NORS-DUC). 

A collaboration between Dominican, the United States Department of Agriculture, and the California Department of Food and Agriculture, NORS-DUC studies emerging and invasive pathogens of ornamental (nursery) and native plants in an open, nursery-like environment.

“We focus on diseases of ornamental and forest plants, especially those caused by members of the genus Phytophthora, such as P. ramorum, which killed millions of native trees in coastal California, changed whole ecosystems, and increased fire hazard,” Schweigkofler says.

Identifying host plants and educating the nursery industry about ways to control the spread of P. ramorum is a critical step toward preventing the pathogen from spreading. 

Long-distance spread of P. ramorum occurs mainly through the trade of ornamental plants. These plants often show no or only very limited symptoms, which are difficult to identify to the untrained eye. Once established in a native forest, the disease spreads mainly through spores produced in huge numbers on leaves of the California Bay Laurel, a host plant which is not killed by the disease.

NORS-DUC will this month host its annual Marin County “SOD Blitz,” inviting the public to help monitor the spread of SOD and collect samples of potentially infected plants. Timely detection of the disease is key for a successful proactive attempt to slow down the SOD epidemic.

NORS-DUC scientists first detected symptomatic leaves on a Cotoneaster pannosus (silverleaf cotoneaster) plant in spring 2022 in an area in Marin already heavily infested by P. ramorum. A wider survey in 2023 detected more symptomatic cotoneaster plants in Marin, indicating cotoneaster might play a role in the epidemiology of sudden oak disease.

Studies conducted at NORS-DUC and recently published in the peer-reviewed journal Plant Disease confirmed cotoneaster as a host plant. The co-authors were Dr. Wolfgang Schweigkofler, lead scientist at NORS-DUC, and NORS scientists Dr. Tomas Pastalka and Dr. Nilwala Abeysekara.

Cotoneaster is a genus of woody plants native to the Palearctic region, which covers Europe, Central Asia, and Siberia. While they are now popular ornamental plants, some have also become invasive in parts of the United States. Many other common ornamental plants, such as rhododendrons, azaleas, and camellias, are also known hosts of P. ramorum.

“Our focus at NORS-DUC is applied research, such as validation and development of best management practices; development of remediation options for soil, water and infested plants; and development of monitoring and control strategies,” Schweigkofler says. “We share our research results with the public through a strong outreach program, as well as through scientific and technical publications.”

Recent work at NORS-DUC includes:

• Applying a chemical-free steaming technique developed by NORS-DUC scientists to control the spread of plant disease is now helping one of the world’s largest berry growers reduce the use of disposable plastic containers.
• Examining how SOD could adapt to drier and warmer conditions and spread to new environments, eventually threatening more species of native plants along the California coast. Early indicators and field monitoring show that the disease might become established among many of the more than 100 different species in the genus Arctostaphylos (manzanitas).

Read more information about ongoing research at NORS-DUC.

The NORS SOD Blitz will be held April 19-22. It is part of a larger “citizen science” event organized in partnership with Dr. Matteo Garbelotto and his team from UC Berkeley and scientists at different locations along coastal California.

“SOD-blitzes inform and educate the community about Sudden Oak Death, get locals involved in detecting the disease, and produce a detailed web-based map of disease distribution,” Schweigkofler says. “The map can then be used to identify those areas where the infestation may be mild enough to justify proactive management. The SOD Blitz is one of the biggest citizen science events in forest pathology worldwide.”

More information about the SOD Blitz, including a training video and sample collection . Participants can pick up sample collection packets outside the main entrance to the Dominican’s Joseph R. Fink Science Center, 155 Palm Avenue, San Rafael. Sampling packets can be picked up and returned anytime between April 18 at 4 p.m. and April 23 at 9 a.m. For more information, email wolfgang.schweigkofler@dominican.edu