Sick plum tree

Question

Suddenly the leaves of the tree have holes and they are drying up.

Answer 1

There are several possible culprits from the ranks of bacteria and bugs. The first line of defense in good cultivation practices, so the tree is healthy and able to repel invaders. This article has a number of ideas: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-dropping-leaves.htm
The only way to know for sure is to take some leaf and twig samples to the Extension Service for analysis and recommendation on treatment. This link will help you locate the nearest one; https://www.gardeningknowhow.com/extension-search/