Trees can suffer damage by a number of causes, including fungi, bacteria, viruses, weather, insects, animals and humans. Sometimes they succumb to the attack, sometimes they can fight back to protect themselves. You can’t always see how trees react to damage, but two of their responses are particularly visible: burrs and bacterial tree ooze.


What are they?

You may have noticed trees with a strange knobbly growth or two on their trunk. They're easy to spot once you start looking!

This abnormal growth is called a burr. In American English it’s known as a 'burl', so you may find it referred to as both.

This growth is a mass of shoot and bud tissue that grows manically and forms a distinctive growth on the trunk. At its most basic, it's not unlike a benign tumour in an animal.

Why do they form?

Burrs are thought to be a response to stress, but the stressor can be difficult to identify. The growths usually form over a wound which heals over during the period of irregular growth. The burr protects the tree from any further damage at that site.

Are they harmful to trees?

To some people burrs can look unsightly, but they don’t cause any harm to the tree. They are simply a unique feature of the tree in question. In fact they can have economic value, as the erratic growth of the plant tissue results in unusually-patterned wood that can be beautiful when worked carefully.

Bacterial tree ooze

What is it?

Bacterial ooze on trees doesn’t appear common in the UK, but you’d recognise it if you saw it – it looks like slime.

There are different types of bacterial ooze, and they’re not very well studied. At its most basic, it forms when a tree gets damaged and subsequently infected with bacteria. In some circumstances, if the bacteria can feed on the tree sap and nothing prevents it from multiplying, it will eventually form this slime.

Why does it form?

Like all plants, trees have an immune system which should protect them from severe infections like this. Bacterial ooze happens when the tree is unable to heal a wound and prevent the bacteria from feeding on the sap.

Is it harmful to trees?

Bacterial oozes are often fatal. It will rot the tree as the bacteria 'eats' it, ultimately leading to the tree’s death.

Without knowing what bacteria is causing the problem, it’s difficult to know how contagious it is. The bacteria involved are often present in a woodland anyway without causing any problems. As the ooze usually forms when specific conditions occur on a tree, it shouldn’t spread in woodland.

Bacterial oozes may be accompanied by other pathogens that further harm the tree. For example, slime flux is a type of bacterial ooze that is a mix of bacteria and yeast. It has quite a distinctive orangey-yellow appearance. The yeast and bacteria ferment the tree sap, leading to an unpleasant smell and attracting insects to the ooze.

How you can help

If you spot a tree with bacterial ooze on it, we recommend you let the owner know. The rot can weaken the tree, so they may need to check it is safe. Please also fill out a Tree Alert form so that we can track the prevalence and spread of bacterial ooze around the UK.

More on tree growths, burrs and galls