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Oak Disease Pathogen Found in Southern Calif. Nursery

The fungus (Phytophthora ramorum) responsible for sudden oak death thrives in cooler, wetter climates.

The California Oak Mortality Task Force, during a March 17, 2004 meeting at Sonoma State University, announced that Phytophthora ramorum, a fungus that causes sudden oak death (SOD) was found on camellias at Monrovia Nursery, in Azusa, just east of Los Angeles. It's reportedly the first time the disease has been identified in a Southern California nursery.

Said to be the largest horticultural nursery in California, Montrovia Nursery has been in business for 76 years and produces more than 2,200 varieties of perennials, conifers, woody ornamentals, shrubs, trees, citrus, camellias, rhododendrons, vines, ferns, grasses and topiaries. Other than the home nursery in Azusa, it also operates nurseries in Dayton, Oregon, Visalia, California, Springfield, Ohio, La Grange, N.C. and Cairo, Georgia. These wholesale nurseries, covering some 4,724 acres, ship plants to more than 5,000 garden centers nationwide.

The California Department of Food and Agriculture has the job of trying to identify any potentially infected material that might been shipped unknowingly in the last year. The disease has reportedly been found in more than 40 plant species worldwide.

All host plants at the Monrovia Nursery are to be inspected and plantings within 10 meters of infected plants will be on hold for 90 days and inspected. The pathogen was reportedly traced from shipments from a Washington state to Southern California.

The California Oak Mortality Task Force reports that coast live oak (Quercus agrifolia), tanoak (Lithocarpus densiflorus), and California black oak (Quercus kelloggii) are dying in large numbers in central coastal California due to SOD. The fungus affects many other tree and shrub species, including rhododendron species, California bay laurel (Umbellularia californica), huckleberry (Vaccinium ovatum), and California buckeye (Aesculus californica). There are 38 species susceptible to the fungus, 22 of which are identified as hosts by the USDA. (The full list is available at two plants most likely to be spread the fungal infection are the California bay laurel/Oregon myrtle and Rhododendron spp.

The task force had confirmed back in October 2002 the presence of the pathogen in at least 12 California counties. The infection may occur on the trunk and branches (cankers) or on leaves (leaf spot). Infection does not necessarily result in death of the plant, the task force points out; mortality occurs when the cankers expand in the truck.

The fungus thrives in cooler, wetter climates. In California, it is found only in the coastal counties.

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October 20, 2019, 6:05 pm PDT

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