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Five tree pests and diseases threatening Welsh woodlands

Help stop the spread

Arriving with clean boots and tyres and practicing good biosecurity can help reduce the spread of the disease.

If you think you’ve spotted any of these pests or diseases, report it on the Tree Alert website.

Phytophthora ramorum 

Phytophthora ramorum, or P ramorum, is a fungus that infects and kills larch trees. It also affects a number of other trees, shrubs and plants.

It has also been found to infect rhododendron and native bilberry.

Symptoms on larch

  • Wilted, withered shoot tips with blackened needles
  • Infected shoots shedding their needles
  • Prematurely bleeding cankers exuding resin on the upper trunk and branches

Causes

  • P ramorum spreads easily in moisture and water such as moist air currents, mists, fog and watercourses
  • Contaminating spores can also be spread by animals and on footwear, vehicle wheels, machinery and equipment

Outlook and risk for Wales

Chalara ash dieback

Also known as Chalara dieback of ash, it is a serious disease that is killing ash trees across northern Europe.

Symptoms

  • Dark lesions – often long, thin and diamond-shaped – appear on the trunk at the base of dead side shoots
  • The tips of shoots become black and shriveled
  • Blackened, dead leaves – may look a bit like frost damage
  • The veins and stalks of leaves, normally pale in colour, turn brown
  • Saplings have dead tops and side shoots
  • In mature trees, dieback of twigs and branches in the crown, often with bushy growth further down the branches where new shoots have been produced
  • In late summer and early autumn (July to October), small white fruiting bodies can be found on blackened leaf stalks

Causes

  • Chalara dieback of ash is caused by a fungus, Hymenoscyphus fraxineus
  • The disease is spread by spores from the fruiting bodies of the fungus produced on fallen ash leaves. These airborne spores can disperse naturally via wind over tens of kilometres

Outlook and risk for Wales

  • Trees outside of woods (in parklands, gardens, hedgerows, along roads etc) are most at risk
  • Chalara ash dieback has been found on newly planted ash trees as well as older, established ash trees in Wales
  • Chalara can spread easily from place to place by spores on clothing, footwear or vehicles
  • More information about Chalara

Emerald Ash Borer Beetles

Emerald ash borer causes many ash trees to die within 2 to 3 years of infection.

Imports of wood and wood packaging means it poses a significant risk to our woodlands in Wales.

Symptoms

  • Thinning or yellowing of foliage
  • Fissures in bark, 5-10 cm in length
  • Woodpecker activity, as the birds strip away bark to get at the beetles
  • D-shaped holes, 3mm in diameter, produced by emerging adults

Causes

  • Emerald Ash Borer beetle, Agrilus planipennis
  • Larvae burrow through the bark and feed in the tissues than transport water and nutrients, creating tunnels that eventually kill the tree

Outlook for Wales

Acute Oak Decline

Acute Oak Decline is a bacterial disease that affects mature English and sessile oak trees, usually of at least 50 years but has been spotted in younger trees with a diameter of 10-12cm.

Symptoms

  • Black weeping patches on stems and trunks
  • Longitudinal splits or fissures in the bark at the site of the bleeds
  • Lesions and necrotic tissue underlying the bleed points
  • Canopy health seems fine in early stages but grows visibly thinner as trees approach death

Causes

  • Thought to be bacterial, but the wood boring buprestid beetle may be partly to blame

Outlook for Wales

  • Acute Oak Decline has been found in Wales in at least one location
  • Trees that are most affected die within 4-5 years of developing symptoms whilst it appears others are capable of recovering from the disease
  • More information about Acute Oak Decline

‘Phony peach’ - Xylella fastidiosa 

Xylella fastidiosa, also known as ‘phony peach’ is a disease causing bacteria that affects a wide range of woody plants and broadleaved trees. 

Symptoms

  • Symptoms vary, depending on host species and susceptibility, but include marginal leaf browning known as leaf scorch, foliage wilting and withering of branches
  • With severe infections in some of the most damaging host/subspecies combinations, dieback, stunting and eventual plant death can occur, for example in olive trees or grapevines

Causes

  • Xylella fastidiosa invades the xylem vessels that transport water throughout plants and causes symptoms that range from leaf scorch to tree dieback and death
  • In the natural environment it is transmitted by xylem-fluid feeding insects such as leafhoppers

Outlook for Wales

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