Question
Our 4 year old Chinese Pistache Tree suddenly appears to be dying.  The leaves turned brown, and are falling to ground, clearly dead leaves.  We do not believe there is any root rot.  This tree is part of our landscape, there is a wood fence about 3-4 feet behind, annual and perenial flowers are around and below, we have a drip system that waters these. We see some blackish brown flying insects on the tree.

Answer
Hi Cindy:
I doubt if the insects are directly related to your issue.  Look the trunk over for evidences of loose or damaged bark. Difficult to say what the problem may be w/o seeing the tree in person. Check the obvious- water. Too much or not enough?  Has it been hot/dry? Lack of water can lead to leaf drop.  Using your fingernail, scratch a small spot on several of the smaller twigs. Look for green beneath the bark. Again, do this on several random twigs.  If they are all green beneath, I suspect the tree may have been stressed and this may be temporary. This is like being a detective. Another few questions; Has anything been sprayed on the tree or poured on the soil under the tree? Any digging near the tree?

You can contact your local county (US) Extension office and often, they can arrange to make a house call to see the tree. Also, visit with your local nursery for their comments.

Regards
Steve