How to thicken leaves of semi bald Chinese Pistache tree?

Question:One of the Chinese Pistache tree seems to have such thin (few) leaves compare to the other Chinese Pistache trees. Only 15 and 30 ft away along the same tree, 2 other Chinese Pistache trees planted which have very thick normal leaves. All have the same age of about 10 years. The tree with the thin leaves is planted in a center of my house side walk with lawn around it. The other 2 trees are planted in the same side walk but there is no lawn around them. All the 3 tree seemed to be doing well and had the same thickness of leaves up to about two to three years ago. What might be the problem with the thin leaves tree? What can I do to fix it?
Thanks,
Ron-

Answers:
Sounds like some kind of parasite is affecting the root system.Probably grubs.You can get granules that you wet down and they will kill them.Also get some root stimulater and mix as directed and add it to the ground around the tree.If its been caught in time this should help.Root stimulaters main ingredient is adole.You should be able to find it at any home and garden store.