Oak pests and diseases

Oak mildew

Oak mildewOak mildew is caused by the fungus Erysiphe alphitoides. It was first found in England in 1908 and is now common throughout Europe.

It infects younger leaves and soft shoots, especially the second growth of leaves in summer. Its severity varies according to the weather conditions in a particular year, and it is worst during warm, wet summers when humidity is high.

Oak mildew does not kill a tree but can cause it to weaken and contribute to Oak decline (see below).

Look out for:

  • a white or greyish-white powdery coating on leaves and shoots
  • if you look closely (perhaps with a hand lens), you will see that the white patches consist of tiny powdery threads
  • some leaves may shrivel and blacken.

Could be confused with:

  • other powdery mildews on other plants. These are different species of the disease, as Oak mildew only affects Oak trees.

Knopper gall

Knopper galls are caused by a tiny gall wasp called Andricus quercuscalicis. If you cut open the gall, you should see the white wasp grub (larva).

The wasps arrived in southern England in the 1950s and have now invaded most of the UK.

The wasp does not kill the tree, but affected acorns cannot germinate. It can be very common in some years, reducing the level of regeneration.

Look out for:

  • ridged and knobbly protrusions on acorns from July onwards
  • the galls make it look as if the acorns have 'melted' and they will also look greasy
  • this symptom is the second stage of infection – at first these wasps reside on Turkey Oak creating current galls, then move to English Oak to create the Knopper gall.

Tortrix roller moth

The roller moth, Tortrix viridana, is a native species of micromoth. The moth caterpillars feed on tender new leaves.

Heavily infested trees can be completely defoliated (lose their leaves), affecting the tree’s ability to photosynthesise. Although Oak trees can refoliate up to three times in a season, this uses a lot of energy. Repeated severe defoliation can weaken the tree and contribute to Oak decline (see below).

When the caterpillars pupate (become an adult moth), they roll the leaf edges around themselves, hence the name ‘roller’ moth.

Look out for:

  • edges of leaves curled up into tubes.
  • loss of leaves in May and June, which can be so severe that trees will become as bare as they would be in winter.
  • the caterpillar moves backwards when touched.

Could be confused with:

  • other common oak-defoliating moths such as Operophtera brumata and Erannis defoliaria, which generally do not cause such severe damage.

Oak decline

Oak decline is a complex disorder caused by a combination of pests, diseases and other agents (such as drought) which act together to cause a serious decline in tree health. It may be terminal, but trees can also recover.

In Chronic Oak decline, there is a slow effect over about 10-50 years, focusing on the roots.

In Acute Oak decline, symptoms appear much faster, over about 3-5 years, focusing on the parts of the tree above ground.

Look out for:

  • early signs are yellowing or fewer leaves
  • later, dead branches can be seen
  • in severe cases, dark weeping patches form on the trunk which dry to a black crust
  • D-shaped holes in the bark (about 2-3mm in diameter) which can be a sign of native Agrilus Oak beetles attracted to the weakened tree.

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Images all © Forestry Commission except Tortrix roller moth by gailhampshire on Flickr, CC BY 2.0

 

 

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