My mum has an old plum tree in her garden, probably 20 years or more old. It has always produced loads of fruit until a so-called tree surgeon hacked it back early this year. It's been pretty grim looking since then. Today she noticed the trunk and branches are covered in mushrooms. ( Pic is below. ) It has been several days since she was near the tree so she's not sure how long they have been there.

I've been googling but the only similar thing I found was mushrooms growing round the base of the trunk, not all over. Will this tree have to be cut down ? Or is there something to treat it with ? Mums also worried it will spread to her other fruit trees.

I would greatly appreciate some help on this !