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Threats to our Trees

Thursday, May 18, 2017

John Lockhart and James Broom provide an update on the pests and diseases attacking trees in the UK.

Chalara fraxinea: ash dieback

The fungus Chalara is present across much of England, and key stands at risk include pole-stage, unthinned ash (Fraxinus excelsior) plantations, coppice and natural regeneration. In such places it is important to thin by 20–30% to increase airflow, which will help prevent a build-up of spores.

In mature stands, it is advisable to remove ash trees that have more than 50% of the crown affected by dieback as these are likely to be severely stressed, have limited genetic resistance and will be actively spreading spores.

Emerald ash borer

Originating from Eastern Asia, the Emerald Ash Borer (Agrilus planipennis) is a species of beetle that has already killed extensive amounts of ash in northern America following its arrival in packaging material. It should be expected to appear in the UK and estates and landowners need to diversify their ash stands.

Phytophthora ramorum

Although many Phytophthora species are present in the environment, ramorum is of greatest concern. Trees of the genus larch (Larix) particularly Larix kaempferi (Japanese Larch)– a common timber species in the west of Britain and the uplands – are susceptible to the pathogen. Infected stands are given a statutory plant health notice by the Forestry Commission (FC) and landowners are then required to fell all trees within 100m of the infection immediately.

Infected larch timber is also subject to a movement licence issued by the FC. Phytophthora ramorum has also been found to affect Sitka spruce (Picea sitchensis) and Douglas fir (Pseudotsuga menziesii) in laboratory trials, although no infections have yet been observed in the wild. More recently, Phytophthora ramorum has been observed in Devon and Cornwall on sweet chestnut (Castanea sativa).

Dothistroma needle blight 

This fungus causes premature needle defoliation on pine species – reducing growth rate and therefore timber yield, which can lead to high mortality – and is of particular concern in the lowlands where Corsican pine (Pinus nigra subspecies maritima) is often a major component of commercial woodlands, in particular on lighter soils in the east of Britain.

Scots pine (Pinus sylvestris) is also susceptible. The preferred management method is regular thinning to increase airflow and prevent the conditions under which the fungus can thrive.

Acute oak decline

Suspected to have first arrived in the UK 30–35 years ago, acute oak decline is a condition that commonly affects mature trees.

Trees in decline will have vertical, weeping fissures that seep black fluid down the trunk, known as stem bleeds, and some die four to six years after the onset of symptoms. The larval galleries of the native buprestid beetle (Agrilus biguttatus) are usually found in relation to the lesions, with their characteristic D-shaped exit holes identifiable in the bark.

Oak processionary moth

The principal risk from this moth is to human health, as the fine hairs of the caterpillars are an irritant. This pest is currently restricted to London, Surrey and Berkshire, but is spreading relentlessly.

It is extremely important to notify the FC on discovery of the caterpillar. Current management is expensive, and focuses on manual removal of nests and caterpillars along with careful application of insecticide in Spring. The threat level is medium, in terms of its implications for timber, but is of critical concern in urban or more populous areas.

Massaria disease

This is found on plane trees (Platanus x hispanica) and causes lesions on major branches that can lead to branch drop. This is an obvious concern in urban settings, where London plane is such a critical component of urban tree resource. Regular assessments should be undertaken and appropriate remedial work carried out.

This is an edited extract. Read the full article on tree pests and management in the July-August 2016 issue of the Land Journal