Q.plum tree
Dear Sir/ Madam,
I have plum tree with some kind of disease, I dont know what it’s called or how I should treat it , I have attached photos so please could you help me and advise me what to do.
Thank you.
Maysa

This is not a disease, but Lichen growing on the bark of the tree.
Lichen is not generally harmful to the host plant, though it can be an indication of growing conditions for the tree.
https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm