Phytophthora ramorum and Maryland


In the mid 1990s, hikers noticed tanoaks (Lithocarpus densiflorus) dying suddenly in three counties along the central coast of California - Marin, Santa Cruz and the Big Sur area of Monterey County. The canopies of these trees appeared to turn brown in a matter of weeks. A few years later a similar phenomenon was seen occurring in coast live oaks (Quercus agrifolia) and California black oaks (Quercus kellogii). As the infested area expanded and more trees died, scientists began to investigate the mysterious tree deaths. As the search for a cause continued, the name “Sudden Oak Death” was coined. The infested area continued to grow and there was concern of an epidemic in California’s oaks. Over 100,000 trees have been killed as the result of this disease.

In the summer of 2000, plant pathologists at the University of California isolated the organism causing the deaths of the tanoaks and oaks. It was an unrecognized species of Phytophthora. They soon learned that this species had been previously observed when it was isolated from diseased rhododendrons and viburnums in European nurseries in 1993. The pathogen was eventually named Phytophthora ramorum by European researchers in 2001. Species of Phytophthora are water-loving fungi that are most active during humid or wet conditions. They produce spores that can swim through water, and some species can spread spores by wind if conditions are not too hot and dry.

Knowing that the pathogen was found on the leaves of rhododendrons in Europe, researchers went back into the affected California forests. There, P. ramorum was soon isolated from more plant species, including California bay laurel (Umbellularia californica). The symptoms on bay laurel and other “foliar hosts” were limited to leaf spots, foliar blights, and shoot dieback. These symptoms can be less severe and may not kill the plants. The disease is called “Ramorum Blight” on hosts other than oak.

The disease is currently found in 14 California counties in natural forest settings, scattered along the coast from Monterey County north to Oregon, and a portion of one county in Oregon. These counties are under federal USDA, APHIS quarantines. The United States Department of Agriculture, Animal Plant Health Inspection Service (USDA, APHIS) website can be found at

The plant pathogen is not considered native to North America. When it was introduced to California it was able to establish itself in native forests and kill trees. The concern is that large areas of the United States could provide the necessary host plants and suitable climate for the pathogen to become established and cause disease. This has happened with two other tree diseases, Dutch elm disease and chestnut blight.

While scientist were learning more about the biology and ecology of the pathogen in west coast forests, nurseries and gardens throughout Europe were reporting P. ramorum on rhododendron, camellia, and other nursery stock. The pathogen was isolated from nurseries in the UK, Netherlands, Spain, and many other European countries, and despite quarantine efforts, nursery infestations in the UK became quite widespread. The affected nursery stock and garden plants included Rhododendron, Viburnum, Camellia, Pieris, Kalmia latifolia (mountain laurel), Arbutus (strawberry tree), Syringa (lilac), Taxus baccata (yew), and Hamamelis (witch hazel).

Until 2004, many considered P. ramorum to be a California forest problem, and in nurseries to be an European issue. However, there were detections of P. ramorum in west coast nurseries. In 2001, a rhododendron in a California nursery was confirmed as infected with P. ramorum, but it was surrounded by a heavy forest infestation. Then, in 2003, 17 nurseries on the west coast of the US and Canada were found positive for P. ramorum (8 CA, 6 OR, 2 WA and 1 BC). In February of 2004, a large nursery in southern California was found to contain plants infected with P. ramorum. All together, about 1.6 million potentially infected plants were shipped from west coast nurseries to nurseries throughout the United States.

Suspect plants were investigated by the states and USDA, APHIS to check for infections. Trace-forwards (where nursery stock was shipped TO) and trace-backs (where nursery stock originated FROM) were conducted to determine the source and fate of the infected plants. When found, diseased plants were destroyed in all cases in an attempt to eradicate the pathogen. However, some plants were sold before inspection. There is a risk that the pathogen may move from infected nursery stock planted in the landscape to nearby native forest vegetation. Tests have revealed that many of the oak species and under story plants in east coast forests are susceptible to P. ramorum.

In addition to the trace-forward investigations, USDA, APHIS and the departments of agriculture in each state initiated a national survey of nurseries for P. ramorum. The USDA-Forest Service is cooperating with APHIS and the States on a national P. ramorum wildlands surveys as well. To contain P. ramorum on the west coast, a new USDA, APHIS Emergency Federal Order went into effect on January 10, 2005, restricting the movement of nursery stock from California, Oregon, and Washington nurseries.

In Maryland, through the national survey and trace-forwards, three nurseries were found to have plants infected with P. ramorum. USDA protocols were followed in all instances, and P. ramorum was eradicated at those sites. Phytophthora ramorum has not been detected in the environment in Maryland.

Page last updated: October 26, 2009

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