Ash pests and diseases

Ash bud moth

Wilted leaves caused by Ash bud mothThe Ash bud moth, Prays fraxinella, is a native species of micromoth. The moth caterpillars feed on Ash leaves and make tunnels in Ash buds or in the bark at the base of new shoots.

If holes are mined in the base of the bud, the leaf shoots may either fail to flush (open and grow) or may flush but then wilt and blacken. If a fungus enters the wound caused by the caterpillars, it can kill the whole shoot.

Attacks are cyclical, and severe damage can exacerbate Ash decline (see below).

Look out for:

  • wilted and blackened leaf shoots (from May onwards)
  • small entrance/exit holes in buds and at the base of wilted shoots
  • silken webs and fragments of bark (frass) may also be visible
  • symptoms are most obvious on younger trees.

Could be confused with:

Ash key gall (or cauliflower gall)

Ash key galls are caused by the mite Aceria fraxinivorus and are very common. They actually indicate a healthy tree, as the mites prefer healthy trees.

The mites are very small and less than 0.5 mm long so may be easily overlooked, although they might be visible with a magnifying glass.

The galls make Ash keys heavier, so wind dispersal is hindered and the seeds are not carried as far.

Look out for:

  • a woody encrustation on the stalk of the Ash keys
  • the crusty galls are green at first but later become brown
  • older galls can remain attached for over a year, so they stay visible all-year round.

Nectria canker

Nectria canker is caused by the fungus Neonectria galligena. It is a native disease which may be linked to existing wounds, causing dieback of twigs and sometimes leading to broken branches.

The canker (patch of dead or infected wood) is centred on small branch stubs or their remains.

They are smooth or ridged, rather than gnarled or knobbly, and will only appear on one side of the trunk.

Look out for:

  • the canker looks as if someone has pushed their thumb into Plasticine
  • older cankers look like a target with concentric rings of dead wood
  • in the autumn you may see the fungal fruiting bodies – small (1-2 mm) orange-red spheres or dots at the edge of the canker.

Could be confused with:

  • bacterial cankers, which unlike Nectria cankers will spread around the circumference of the trunk and look rough in appearance.

Ash decline

Tree showing signs of Ash declineAsh decline is caused by a combination of factors that affect the roots and cause a gradual decline in the tree. It is common in hedgerow trees beside ploughed fields, especially on drier sites.

Dieback is progressive over a number of years, mainly starting in the upper crown. It allows the entry of decay fungi causing loss of timber value and creating a safety hazard for trees in public places.

Because it is progressive, trees will need to be monitored over a number of years. However, recovery is possible.

Look out for:

  • death of a number of twigs and branches
  • poor growth in the crown

Could be confused with:

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