Unfortunately alot of info is missing for me to know for certain what is going on with your tree, but I'll give it a go.* You say you transplanted a 5 year old tree.* I'm thinking it has been in the ground and you recently dug and moved it.* It is not a container or b&b tree that you just purchased.* If so, I'm wondering if you root pruned it first and, if so, when.
You don't say where you live, but if you are in the US it is summer now and is the hottest time of the year for this.* The best time to transplant a plum tree is in early spring before they leaf out.* It would also be helpful to see pictures to see if the tree is planted too deeply or too heavily mulched.* You also don't say if you watered before you dug and how you've been watering since transplanting.
With all that said, leaves are expendable to a tree and they will shed them in times of stress.* If you see new growth, it's best not to cut it off by pruning.* Best not to prune a transplanted tree for at least 6 months.* If you prune now the tree will have to use it's energy to heal the cuts as well grow new roots.
Here's how to transplant, look for the rootflare, mulch and water so you can see if you did it right.
Lots of helpful info here on roots, depth of planting and regeneration of cut roots.
This site shows which trees should be transplanted in spring and also has some good prep and transplant info.
If you have more questions, don't hesitate to ask.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.