Tree Health – November 2015

Tree Health – November 2015

This account is based on a presentation given by Dom Collins of FERA, organised by the Woodland Trust. It was held at Ashton Court Bristol on 26th September 2015.

Much publicity has been given to Ash Wilt disease caused by Chalara fraxinea. This has now been found close to Glastonbury and Radstock, and it seems inevitable that it will eventually devastate the Ash trees around Bristol. Perhaps of even greater significance to the Ash tree is the invasion by the beetle the Emerald Ash borer. (Agrilus planipennis) ( ). This beetle leaves ‘D’ shaped exit holes in the bark of the Ash tree. Affected trees develop strong epicormic growth. Any sightings of this (or of other pests and diseases) should be reported to ‘Observatree’, a collaborative project between Forest Research, the Forestry Commission, APHA, Defra, Fera, the National Trust, Natural Resources Wales and the Woodland Trust.

Mountain Ash (also called Rowan, Sorbus aucuparia) is an important amenity tree which is susceptible to the disease Mountain Ash Ring Spot Virus. /This causes circular and linear lesions on the leaves leading to the decline and eventual death of the tree. Although the pathogenesis of the disease is still not fully understood, the genome of the virus has been fully sequenced, The disease appears to be confined to Rowan and the vectors are likely to be mites of the Eriophyid genus.

Two species of introduced longhorn beetle have been especially destructive. Notification of recent isolated cases necessitated the destruction of trees in the vicinity in order to confine the insects. The Citrus Longhorn beetle (Anoplophora chinensis) and the Asian Longhorn Beetle (Anoplophora glabripennis) originate in East Asia and have been imported on timber.

The Pine Processionary Moth is not yet established in the UK although it is widespread around the Mediterranean..It appears that only members of the genus Pinus are susceptible, but attacks can cause complete defoliation.

The Oak Processionary Moth attacks many species of oak sometimes causing complete defoliation. The caterpillars also carry irritating hairs which can cause serious health problems for humans. Many records have been made in the vicinity of London. The following health precautions for both Oak and Pine processionary moth caterpillars are given by the forestry commission.

  • Do not touch or approach nests or caterpillars
  • Do not let children touch or approach nests or caterpillars
  • Do not let animals touch or approach nests or caterpillars
  • Do not try removing nests or caterpillars yourself

Horse Chestnut Leaf Miner , first found in the UK in 2002 is now well established. It is caused by the micro-moth Camereria ohridella. Although the appearance of the trees is spoiled by the dying leaves, fortunately the trees recover well in subsequent years. However they are undoubtedly weakened and are then susceptible to other pests or diseases.

The Oriental Chestnut Gall Wasp (Dryocosmus kuriphilus) causes leaf distortion of the Sweet Chestnut (Castanea sativa). The galls are formed on the midrib or the petiole of the leaves, causing infected leaves or twigs to die.

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