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The hunt for the tree killers

Phytophthora: (fi-tof-thora), from the Greek for plant destroyer. A genus of more than 70 species of plant pathogens. Best known is probably P. infestans, a close relative of P. ramorum. P. infestans was responsible for potato blight.

It all started with a diseased Viburnum growing in a nursery in West Sussex, in April 2002. Scientists testing samples from the Viburnum confirmed the disease Phytophthora ramorum, otherwise known as ‘sudden oak death’ a so-far incurable infection that has destroyed 80 per cent of one oak species in some parts of California and Oregon.

The disease attacks around 50 different trees and shrubs each year in America costing tens of billions of dollars in damage, including the cost of destroying infected nursery stock.

This was the first discovery in Britain and, during the next 18 months or so, around 370 similar infections, mostly in nurseries, would be found. So was the great British Oak tree about to become extinct? Thankfully, it seemed unlikely. P. ramorum

However, in October 2003 it was reported that P. ramorum had struck once more in Britain: this time it was a mature, 100 year old exotic southern red oak tree, native to North America. Within days two mature northern red oaks were confirmed as infected in the Netherlands.

By December 2003, another 10 trees had been diagnosed with P. ramorum at a further two sites in Cornwall, and this time it was even more worrying as several trees of a native British species, beech, were among those affected. So too were a sweet chestnut, horse chestnut, Turkey oaks and Holm oaks. There were, however, still no cases discovered in our native white oak species. These new hosts matched those predicted from tests carried out by FR.

The Forestry Commission immediately began what was to be the largest, most rapid survey of woodland ever undertaken in response to a specific threat.

Specialist teams checked more than a thousand woodland sites across the length and breadth of England, Scotland and Wales in all sorts of weather conditions. The aim was to establish how many trees were infected and remove any infected shrubs, as well as informing policy development.

Norman Day, of FR’s technical services unit, said: "This was a mammoth task given the area it covered and the amount of surveys required. However, after an intensive briefing, we were able to concentrate solely on completing the surveys and we managed to finish the job within the three month window.

"Between December 2003 and March 2004, inspections were carried out at 1217 high-risk sites across Britain: 395 in England, 512 in Scotland and 310 in Wales."

As Joan Webber explained, " ‘High risk’ sites are those characterised by factors such as the climatic conditions (the pathogen favours a mild yet moist environment) and whether rhododendrons are present. In contrast, ‘low risk’ sites are those with a climate match of less than 60 per cent. The climate match is assessed by comparing long term climate data in Britain with parts of Oregon where P. ramorum is found. The west of Britain appears a generally higher risk."

A further 131 low risk sites were surveyed in England and Scotland, giving a total of 1348 survey sites checked. Of the samples collected and analysed, all were confirmed as negative.

Roddie Burgess, head of plant health, welcomed the findings, and praised the survey teams from the FR’s technical services units, who carried out the massive operation. Although no cases of P. ramorum were found, Roddie stressed that the results did not prove categorically that Britain’s woods were free from the disease. However, if it is present, the level of infection would be very low, and Roddie confirmed that the policy of eradication wherever the disease was found was the right one to follow.

P. ramorum is now known to have been present in at least two European countries since 1993 without causing widespread devastation. Studies carried out in 2000 by Clive Brasier, Joan Webber and colleagues from Forest Research established that our native common or white oaks were relatively resistant to the fungus, compared to those in America. In Europe, where it has now been confirmed in at least nine other countries, its main hosts, or carriers, were confirmed as Rhododendron, Viburnum and Camellia. 
Last updated: 11th May 2017

What's of interest

This article (second half on page "Future plans") was first published in the Forestry Commission staff newsletter, Slasher in December 2004. Permission to reproduce, in full or in part, may be obtained from the Forestry Commission Press Office, 0131 314 6550.