Sick Series: Acute Oak decline

19th October 2018

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Dieback is a common condition in Britain’s oak tree population, but in recent years the new menace of acute oak dieback (AOD) has appeared. The combination of a new strain of bacteria accompanied by an infestation of a particular species of beetle is thought to cause and/or make worse the situation, and it is a problem that is rapidly spreading – with AOD currently identified from Somerset to East Anglia and as far north as the Midlands.

The symptoms of AOD are easy to spot:

  1. Stem bleeds – a dark fluid that seeps from cracks or lesions in the bark and runs down the trunk. This fluid may flow at certain times of the year, starting in the springtime, and stopping at others, allowing the fluid to dry and turn a lighter brown or grey colour.
  2. D-shaped holes in the bark – these are the exit holes of the buprestid beetle, a pest which gets in under the bark of the tree, lays its eggs and after two years of feeding on the tissues of the tree, matures and leaves to find another home.
  3. Rapid decline – trees with AOD are likely to fall into decline and dieback within just a few years of the symptoms first appearing. Though some trees will recover, the death rate is quite high.

However, none of these on their own will point to AOD as these symptoms also exist in perfectly healthy trees or those with other conditions.

It is believed that the infection of the bacteria causes lesions to appear in the softer tissues of the tree which grow, causing the bark to crack and split and producing the stem bleed fluids. The buprestid beetle is then believed to sense the opportunity of getting in under the tough bark of the tree to lay its eggs. The beetle’s larvae eat their way around the inside of the tree’s bark, which can make things worse for the tree by cutting off water supplies to stems and branches and make dieback even worse.

If you have any species of oak tree in your garden and you see one or more of the above symptoms on your trees, don’t panic. If they are on one tree and the dieback is not too pronounced – dieback being where sections of or the entire tree canopy wither and do not grow healthy leaves – then monitor the situation, minimise the potential for spreading by not touching any stem bleeds, and destroy all plant material that falls from the tree (i.e. do not compost). However, if several trees in your garden start to show symptoms or one tree shows a marked decline then it’s always best to call in an expert.

A tree surgeon will be able to review the combination of factors, environment and symptoms present to assess whether you should go for the management option, where your trees are monitored over a period of time to see if they recover, or if felling is the only choice, as it sometimes is, in an effort to protect the other trees in your garden and those of your neighbours.

Scientists are looking into the disease with some urgency, given its spread. Defra has funded a £1.1m research project to better understand the disease and how it might be tackled in future, but for now it’s a case of minimising the spread of this disease, and for that we need vigilant homeowners to report signs of AOD.

If you have spotted any of the symptoms of acute oak decline on your trees, call us on 0208 292 8992 and we’ll happily pop round to give you our professional opinion for your peace of mind.

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