Threat to ornamental gardens

It was by compete chance that a microscopic fungal-like disease called Phytophthora lateralis was found to be lurking amidst the broad acres of North Yorkshire.

Only one outbreak had previously been reported in England, not in some large woodland or beautiful public garden but in a boundary hedge of a Plymouth industrial estate.

The team of Forestry Commission researchers who found it in Yorkshire were, at the time, on the lookout for something different. They had flown from an airport on the Lancashire coast to conduct an aerial survey for a similar disease they feared might be infecting larch trees on the North York Moors.

But after their survey proved negative, the light aircraft was returning to Lancashire when its flight path took it over the Washburn Valley, to the west of Harrogate. One of the team just happened to notice that below the plane was a block of dead and dying trees.

Several days later, when Forestry Commission scientists returned on the ground to investigate, they discovered the trees were not larches but a popular ornamental species called the Lawson’s cypress. The location of the diseased trees was a shelter belt of woodland straddling the banks of the River Washburn at Blubberhouses, between Yorkshire Water’s reservoirs at Thruscross and Fewston.

The trees not only covered one of the most popular walking areas on the north side of the Leeds-Bradford conurbation, but also spread into the grounds of an activity centre owned by the British Canoe Union.

The disease has devastated woodland and gardens on the west coast of North America, where the tree is known as the Port Orford cedar. When introduced to Europe for use as an ornamental tree in the mid-19th century it was renamed after the Edinburgh nursery that imported it, Lawson & Son. The fungal infection attacks the roots and then spreads through the inner bark and cambial layer between the bark and wood around the base of the tree. The infection girdles the trunk and stops the flow of nutrients.

Diseased Lawson’s cypresses have been found in France and Holland, and in the UK besides being identified in Plymouth and the Washburn Valley it has also now been confirmed on Loch Lomondside in Scotland and in Northern Ireland.

It has forced Yorkshire Water, owner of some of the land affected, to start taking precautions reminiscent of the 2001 foot and mouth disease epidemic.

Anyone visiting on Yorkshire Water business has to clean their boots with industrial methylated spirits in order to stop the infection spreading. Walkers are asked to brush mud off their boots and then wash and dry them at home before walking elsewhere.

Geoff Lomas, catchment and recreation manager, says, “We get a notice – in this case from the Forestry Commission – informing us that we have had a confirmed outbreak. This means that we can’t move the trees unless we have a licence, and they can only be taken to a disposal site licensed to accept them.”

The Forestry Commission said: “The disease could represent a serious threat to the ornamental plant industry if it became established here.”

The Lawson’s cypress trees at Blubberhouses are thought to have been planted in the 1970s. Both infected and apparently healthy trees will all have to be felled. The disease can remain in the soil for up to ten years.

Geoff Lomas added, “There are many important public gardens in Yorkshire with these trees. Also, my neighbour has them, and if they die in two years’ time there’s probably only one source of the disease he’ll be looking at.”