LONDON, United Kingdom, May 7, 2002 (ENS) - The UK government has placed a ban on imports of plants from parts of the United States and added controls on wood to protect native trees and shrubs from an American plant disease that has not become established in the British Isles.

Phytophthora ramorum, a fungus which causes a bark canker disease known as sudden oak death, is killing native oak trees and other plants in California and Oregon. Once diseased, plants can die within a few months.

A statement from No. 10 Downing St. today said plant health and seeds inspectors have found evidence of the disease in viburnum plants at a small number of nurseries. The fungus' identity was determined at the Central Science Laboratory and confirmed by Forestry Commission experts.

Tanoak bleeding with sudden oak death disease (Photo courtesy UCCE)

A ban is now in effect on the import of rhododendron, viburnum, vaccinium and oak planting material from areas of the United States where the disease is known to occur.

In the United States, P. ramorum has been found in an area running along the Pacific Coast from Big Sur, California, to Brookings, Oregon. Presently, sudden oak death occurs in areas dominated by a maritime climate zone, close to the coast with a strong summer fog influence and moderate summer temperatures.

Sudden oak death was first verified in 1995 on tanoaks near Mill Valley, California. Since then, it has been found in 10 counties in the San Francisco Bay area. It affects coastal and interior live oak, black oak, arbutus, bay laurel, rhododendrons, and viburnam.

Bay laurel infected with Phytophthora ramorum (Photo by Joe O'Brien, U.S. Forest Service)

To keep the fungus from attacking British oaks, the UK Department for Environment, Food & Rural Affairs (DEFRA) has imposed an extension of current controls on imported oak wood to include wood derived from all host trees originating in the affected areas of the United States.

Imports of oak wood from North America are already subject to measures such as removal of bark to reduce other risks.

Scientists at the University of California's Marin County Extension explain that there are two categories of hosts for Phytophthora ramorum - bark canker hosts and foliar hosts. The bark canker hosts are tanoaks and oaks that become infected on the trunks.

Foliar hosts are bays, rhododendrons, bigleaf maple, arbutus, bay laurel, rhododendrons, honeysuckle, huckleberry, and viburnam that become infected on the leaves and small branches. While the bark cankers often lead to mortality in tanoaks and oaks, foliar hosts only occaasionally die from the P. ramorum infection.

UK government officials have urged the European Commission to require Member States to carry out surveys of potential host material this summer and to report their findings later this year.

The fungus has been found infecting rhododendrons and viburnums in the Netherlands and Germany, DEFRA said today. As of April, it has been found in a number of new locations in both countries.

A young oak grown in 1964 from cuttings of the Newland Oak, largest Common oak (Quercus robur) ever recorded. Historian Cyril Hart stands beside it on the same spot in Coleford in Gloucestershire where the old oak stood. August 2000. (Photo courtesy The Tree Register)

In the UK, and elsewhere across Europe, there is already widespread oak mortality and dieback of complex cause known as oak decline. In some cases, the Forestry Commission says, the decline is associated with infection by other Phytophthoras, but these are mainly root infecting species whereas P. ramorum causes stem cankers.

Oak decline also involves recurrent episodes of drought, other root infecting fungi, repeated insect defoliation and scale insect attack.

This combination of decline factors complicates surveying to detect sudden oak death on trees in the UK, scientists say. UK Forest Research carried out investigations over summer 2001 and "no evidence of its presence was found," after surveys of susceptible areas close to major rhododendron nurseries, sites with understorey rhododendron, and sites where oak decline is prevalent. Government researchers will continue with surveys in 2002.

There is now a requirement to notify DEFRA of commercial deliveries of host plants, of any origin, into and within England and Wales.

Anyone who suspects the presence of this disease on their premises should contact DEFRA Plant Health and Seeds Inspectorate, for rhododendrons and viburnums, at Tel: 01904 455174 or the Forestry Commission Plant Health Service, for trees, at 0131 314 6414.

View the UK Forestry Commission's Sudden Oak Death Exotic Plant Alert.