problem plum tree leaves

Question

I have a plum tree, which I planted a few months ago. It has been very healthy and growing nicely till today. This morning I noticed some of the leaves had fallen of and a lot of the leaves have brown marks on the top side of the leaves.

Answer 1

Are these marks squiggly? If so, it is leaf miners. This article will explain more about them if this is the problem:
https://www.gardeningknowhow.com/plant-problems/pests/insects/leaf-miner-control.htm

If this is not the problem, let me know what shape these marks are. That will help with identifying the problem.