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.