Ramorum Disease found in Japanese Larch trees in Scotland for first time

Tree felling is under way to tackle an extremely damaging plant disease that has been found in trees in Scotland for the first time.

Ramorum disease, caused by the pathogen Phytophthora ramorum (P. ramorum), has been confirmed in Japanese larch trees in a forestry plantation on the Craignish Peninsula in Argyll. About 1.25 hectares (3.1 acres) of the trees are being felled as part of measures designed to prevent further spread of the disease.

P. ramorum was first detected in Britain in 2002, and although it had been detected in Scotland in a number of plant and shrub species, including rhododendron, it had never previously been found in trees here. Even in England and Wales it had initially affected only a small number of trees, and usually only trees standing close to heavily infected Rhododendron ponticum plants that are found in many woodland areas.

However, in 2009 it was unexpectedly found killing extensive areas of Japanese larch trees in forestry plantations in south-west England, and this year in Wales, Northern Ireland and the Republic of Ireland. This was the first time that it had been found in a commercially important conifer species anywhere in the world.

Dr Bob McIntosh, Director of Forestry Commission Scotland, said, “Following aerial surveys earlier this year it looked as if Scotland had remained free of ramorum disease on Japanese larch. It was therefore a real blow when infection was confirmed at this site.

“Although the area of infected trees appears to be relatively small, it is still of real concern, so all the trees must be felled to try to minimise the risk of the disease spreading.”

P. ramorum sporulates (produces reproductive spores) very heavily on Japanese larch, and the best scientific advice to prevent the disease spreading is to fell the trees to kill the living tissue on which the organism depends.

“I therefore urge all owners of larch trees to inspect their trees regularly for signs of decline or poor health, particularly next spring when they produce new needles, and to contact us if they suspect they might have ramorum disease.”

The most vulnerable area is western Scotland, because the disease thrives best in wetter climates. It can be spread in mists, watercourses, air currents and water splash, on boots, vehicle tyres and equipment, and by the movement of infected plants.

The symptoms on Japanese larch trees include wilting shoot tips  and needles turning black and falling prematurely, and cankers that bleed resin can appear on the branches and upper trunk. However, ramorum disease of larch is more difficult to identify in winter because larch trees shed their needles in the autumn, although tell-tale resin bleeds can still be visible on the trunks.

Anyone who suspects their larch trees in Scotland might have ramorum disease is required to contact the Tree Health Diagnostic & Advisory Service (DDAS) of the Forestry Commission’s Forest Research arm at:
Northern Research Station, Forest Research, Roslin, Midlothian, EH25 9SY; tel: 0131 445 2176; email: ddas.nrs@forestry.gsi.gov.uk.

Ramorum disease is not harmful to humans or animals.

Further information about this disease is available on the Forestry Commission’s website at www.forestry.gov.uk/pramorum.

• The Craignish site illustrates the difficulty of diagnosing P. ramorum infection. Only about 30 per cent of laboratory tests of suspect tree tissue collected around the country have produced conclusive positive or negative results. Although the Craignish trees displayed all the signs of P. ramorum infection to an expert eye, and P. ramorum had been isolated from ground litter there, the first laboratory tests of suspect foliage collected from the trees returned negative results. However, because the Forestry Commission was unable to attribute the damage to other causes, it carried out another round of visual inspections and scientific tests using equipment known as a lateral flow device, and has concluded that the cause can only be P. ramorum.

• Aerial surveys of western Scotland in the summer identified 12 sites that were rated as ‘suspicious’, but after closer inspection the symptoms at the other 11 are now thought to have been caused by other factors, such as deer or squirrel damage.

• P. ramorum is a ‘quarantine’ organism under European Union law, and its suspected presence must be notified to a relevant authority (Forestry Commission, Scottish Government, Fera or the Welsh Assembly Government). It was first found in Britain on a viburnum plant in a nursery in 2002. It can kill many of the plants and trees that it infects, but symptoms vary according to the species. It also infects bilberry (Vaccinium myrtillus, known as blaeberry in Scotland), an important ground-cover plant of woodland and heathland environments. One site on Arran has been identified with infected blaeberry, and action has been taken to eradicate the pathogen there.

• P. ramorum has not been found infecting any European larch (Larix decidua) or hybrid larch (Larix x eurolepis), which are also grown in Britain, but these species are being closely monitored. Complete figures are not available for Japanese larch alone, but all three larch species together cover about 65,000ha (160,000 acres), or 5.1 per cent of Scotland’s woodland area. An estimated 2600 hectares of Japanese larch have been felled or are due to be felled in south west England and Wales.

• Larch is a durable, versatile timber that tolerates changes between wet and dry conditions, resists rotting when used in the ground, and is easily stained, worked and finished. It is therefore in demand for outdoor uses such as fence posts, fence panels, exterior wall cladding, boat-building, sheds and furniture, and indoor uses such as flooring and chipboard.

• P. ramorum does not harm larch timber, which can still be used provided a number of biosecurity measures, including brushing down (or, if necessary, pressure washing) timber trucks between every journey, are taken to prevent accidental spread of the disease during timber movement. The Forestry Commission is permitting the logs to be taken to specially licensed mills that meet the biosecurity requirements, including destroying bark, which might be contaminated.

• P. ramorum has caused the deaths of millions of North American native oak and tanoak trees in California and Oregon. However, in laboratory tests, Britain’s two native species of oak, sessile and pedunculate oak, have been demonstrated to be more resistant than their American cousins. Only a tiny number have been infected, and all of them were standing next to heavily infected rhododendron bushes and were therefore exposed to heavy ‘inoculum pressure’.