Are you saying that your acacia is now completely leafless?

It sounds like it's either dead or, at least, in serious trouble. More info would have to be known to determine (or even make a good guess at) the cause. Not all arborists are created equal, and some (especially many of the non-certified ones) just don't seem to know much about trees at all. Of course without being there, I probably shouldn't assume the situation is so similar to what I see frequently.

Any suggestions?

Post pix. And give more info: age, watering, etc.

Dan

Hello,

Thanks for the feedbacks.
My camera broke recently so for now I cannot post pictures.
Anyway, they would show an almost leafless tree with the remaining leaves that are drying. The tree is 8 years old and is watered for 2 hours every 4 days in the Phoenix summer heat (about 110F). I tried helping it by soaking the base with the hose for 1/2 hour every week, but I stopped doing it end of June. What is puzzling me (I am a novice in gardening) is: if a tree is dead, why some of the branches don't crack when bent and some do? Shouldn't all crack?

How easily a branch breaks is dependent upon a number of factors. I wouldn't use that to determine whether or not a limb was dead. You can scape a small area of bark off and see if there is still green underneath, but even that has its limitations.

Why don't you contact your local extension agency. They may be able to help you determine the cause and should be equipped to check for things like cotton root rot.

Here is a link that might be useful: Maricopa County Extension Contact Info

I'd say - based on provided info - root rot from overwatering. Co Extension may be able to confirm or deny, if they are still staffed.

Dan