Bacterial tree ooze: how to identify it

Every now and then we get sent photos of weird things spotted in woodland (which we love – keep them coming!). One of our supporters got in touch recently to show us some slime she had found growing on a tree. The faint of heart may wish to look away now!

Bacterial tree ooze can easily go unnoticed. (Photo: F. Nicholas)
Bacterial tree ooze can easily go unnoticed. (Photo: F. Nicholas)

Slime like this is known as bacterial ooze. There are different types of bacterial ooze, and they’re not very well studied. Bacterial ooze doesn’t appear to be that common in the UK, and it can easily go unnoticed. At its most basic they form when a tree gets damaged and subsequently infected with bacteria. In certain circumstances if the bacteria is able to feed on the tree sap and nothing prevents it from multiplying it will eventually form this slime.

The ooze is starting to spread away from the wound and across the bark. (Photo: F. Nicholas)
The ooze is starting to spread away from the wound and across the bark. (Photo: F. Nicholas)

Trees, like all plants, 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. Bacterial oozes are often fatal; the ooze that forms 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 an ooze might be, but in most cases the ooze itself only forms when specific conditions occur on a tree so shouldn’t spread in a woodland. The bacteria involved are often present in a woodland anyway without causing any problems – the ooze forms when something goes wrong and the bacteria breeds out of control.

Slime flux is a type of bacterial ooze that often has a foamy appearance and an unpleasant odour.  (Photo shared on our social media platform)
Slime flux is a type of bacterial ooze that often has a foamy appearance and an unpleasant odour. (Photo shared on our social media platform)

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 orange/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 that you let the owner know. They may need to check the tree to ensure it is safe (the rot may weaken the tree). Please also fill out a Tree Alert form (https://www.forestry.gov.uk/treealert) so that we can track the prevalence and spread of bacterial oozes around the UK. And if you get good photos, email them over to us as we’d love to see them!

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