Minimizing the Risk of Sudden Oak Death

In some plants it's deadly, in many it may be nothing more than a few dots on a leaf. As we continue to learn how to diagnose and understand its impacts on our gardens, study up so you spot the disease before it spreads.

Sudden Oak Death is a new disease caused by Phytophthora ramorum, a fungus-like pathogen that has killed tens of thousands of oak trees in California and is known to affect and sometimes kill other nursery plants. It was identified in the mid-1990s and scientists and nurseries are doing all they can to learn more about the disease and how to fight it.

P. ramorum is extremely difficult to identify as its symptoms are common to many plant disorders, including other diseases and non-living factors. For example, sunburn is one disorder that can be confused with P. ramorum. DNA testing is required for disease verification. P. ramorum is spread by water splash and its spores may remain dormant in the soil for several years, making it difficult to find and eradicate.

Approximately 52 plant genera have been identified as possible hosts of this plant disease. In some plants it is deadly, in many it may be nothing more than a few dots on a leaf. As science catches up, we will learn better ways to diagnose the disease and determine its effect on the ecosystem and our gardens.

The most susceptible plants appear to be broadleaved evergreens such as camellias, viburnum, rhododendrons, pieris and kalmia. Syringa (lilac) is also a concern because it encourages the disease to produce spores.

Camellia recall

In spring 2004, shipments of camellias from one California nursery – some of which were infected with P. ramorum – were sent to garden centres across North America. British Columbia was the only region to launch a public recall in a bid to prevent the disease from spreading into the urban or natural environment.

Trained inspectors were sent throughout the Lower Mainland and Vancouver Island to check the plants of homeowners who purchased camellias. Suspect plants were sampled, then destroyed. Less than one per cent tested positive for P. ramorum. Locations with positives are tested regularly until the Canadian Food Inspection Agency (CFIA) is convinced the disease has been controlled.

The camellia recall was a success because of team work. The CFIA ensured the program met its standards, the BC Ministry of Agriculture and Lands provided technical support, media ensured the public was aware of the recall and the BC Landscape & Nursery Association managed the process.

Status of P. ramorum in Canada and U.S.

The Canadian Food Inspection Agency surveys Canadian nurseries annually, looking for new and other ‘quarantineable’ diseases, including P. ramorum. In 2003, P. ramorum was identified in B.C. at one wholesale nursery; in 2004, new finds were identified at three nurseries and several garden retailers. In 2005, positives were found at one wholesale nursery and a few garden retailers, all in the Lower Mainland or on Vancouver Island. The CFIA takes aggressive action to eradicate the disease when it is found, including site quarantine, destroying infected and adjacent plants, and sanitizing and monitoring the site for several years.

CFIA traces potentially infected plants from place of origin to final location, and as a result, have found P. ramorum-infected plants at a few landscape projects. Their normal stringent measures are carried out.

In 2005, the United States recorded 55 positive sites in seven states. Most were in the three regulated states of California, Oregon and Washington, but a few were found in Georgia, Louisiana, Tennessee and South Carolina.

B.C. minimizes risk of disease

The BCLNA has developed a P. ramorum nursery certification program to minimize the risk of Canadian or U.S. nurseries receiving or spreading P. ramorum. The program includes plant sampling and testing; mandatory training; implementation of best management practices and an annual audit to ensure procedures are being followed. Over 300 B.C. growers are voluntarily participating in the program, which is under review as a model for other jurisdictions in North America.

Assistance in developing the P. ramorum program was provided by the CFIA, Agriculture and Agri-Food Canada, the BC Ministry of Agriculture and Lands, Canadian Nursery Landscape Association, Investment Agriculture Foundation, and the BCLNA’s Industry Development Council.

For more information on P. ramorum:

(Select “Nursery Programs and scroll through the two P. ramorum and Canadian Nursery Certification Institute sections in the sidebar.)