British trees and shrubs face sudden death

5 May 2009 by Tom Marshall

Britain's woodlands, historic gardens and lowland heaths are at risk of a new epidemic, and action is needed to prevent a repeat of the Dutch Elm Disease outbreak that killed most of the nation's elms in the 1970s.

Beech forest

Beech woodlands are vulnerable to the disease.

The Department for the Environment, Food & Rural Affairs has commissioned scientists at Imperial College London to assess how it has been dealing with a spreading outbreak of Sudden Oak Death disease, which is caused by two closely-related fungi, Phytophthora ramorum and Phytophthora kernoviae.

Both have been found in trees and shrubs in the New Forest, in south-west England and as far north as Staffordshire; it's believed they may have entered the country early in the decade and been spreading ever since - slowly at first, but lately at a worrying pace. The government has earmarked £25m over five years for efforts to control them.

This is potentially one of the biggest threats to British trees and shrubs at the moment,

- Dr Clive Potter, Imperial

The disease gets its popular name from the havoc it has wreaked in the oak forests of California and Oregon in the US, causing foliage to die back and ultimately killing millions of trees. Among British trees, beech is seen as most vulnerable, while rhododendrons are the key host species. This means that old woodland gardens, where shrubs like rhododendrons are often planted under a canopy of trees, are especially vulnerable.

Other common species at risk are ash, yew, magnolia, and viburnum. And worryingly, heathers and plants of the Vaccinium genus, which includes shrubs like the bilberry, are also vulnerable. These plants are cornerstones of heathland habitats, which Britain has in exceptionally rich supply.

"We're at an early stage in the outbreak, and the evidence is still being collected" says Dr Clive Potter, a geographer from Imperial's Centre for Environmental Policy who is leading a team that also includes economists, biologists and plant pathologists. "But this is potentially one of the biggest threats to British trees and shrubs at the moment; some of the scenarios our models are producing suggest that a full-scale epidemic cannot be ruled out."

The team is already doing related research into the problem, with funding from the Rural Economy & Land Use (RELU) programme. They have been investigating how the lessons of the 1970s Dutch Elm Disease epidemic could be relevant to the new threat.

Lessons of the past

"We know that some of the policies pursued in the 1970s inadvertently promoted the spread of Dutch elm disease," Potter says. "For example, felling diseased trees, but then transporting the timber without removing the bark, enabled the pathogen to survive and move around the country very rapidly."

While this specific insight is not in itself likely to help with the current outbreak, comparable lessons may help in assessing the effectiveness of current policies. It's thought that sudden oak death is spreading partly via the nursery trade and the movement of plants around the country that it involves; existing rules on plant passports and inspections for disease may have to be tightened.

It's also possible that restrictions on visitors to gardens and woodlands could be needed. The fungus is thought to be able to spread across the country through soil stuck to people's shoes. At present many country houses belonging to organisations like the National Trust have woodland gardens that are vulnerable to infection. If the situation worsens, there may need to be restrictions on entry to gardens or compulsory inspection of footwear after a visit.

"It's ironic that people enjoying the natural environment by visiting gardens could be agents of the disease's spread," says Potter.

The £24m RELU programme aims to draw together research across different disciplines to understand the social, economic, environmental and technological challenges that rural areas face.

RELU is an interdisciplinary collaboration between the Economic & Social Research Council (ESRC), the Biotechnology & Biological Sciences Research Council (BBSRC) and NERC. Additional funding comes from the Scottish Government and the Department for Environment, Food & Rural Affairs.