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What is causing the browning on the branches of my Palo Verde tree? 
On air question from August 14th, 2010.
Segment 2. Transcription starting at 1:18 into the segment.
Rosie: ...let's get to Arizona homeowner's landscape and gardening questions. We're going to start with Sue in Glendale. Good Morning Sue and welcome to Rosie on the House.
Sue: Hi Rosie, thanks for taking my call; I love your show. I have a Palo Verde and I've tried to research research it on google for about a month now and I don't know what kind I have. Mine is a smooth trunk one; its the one where branches are coming out everywhere. And so about six month ago it got a brown spot on it and my tree is slowly dieing. So I cut off the branch, took it down to my local nursery. They couldn't identify if it was a bug or if it was a fungus and so he recommended I cut every dead part out of my tree and still where the parts I cut, he told me it was just going to die. It's like a 20 foot tree; is there anything that you think that I can do?
Jay: Without seeing it, I would just be totally guessing as to what you might try. Now is this Palo Verde; was it there originally, is it native, or was it planted?
Sue: No, we plated it. We live in the hidden manor area so our dirt is with all river rock like everybody else. We planted it about 12 years ago after another tree had died in that same spot. It's been health until about 6 - 8 months ago and then where the branches were splitting, like where a new branch was coming, there was a brown spot. I began to watch it and it grew and grew and grew. That arm didn't die, but sticks and parts of the tree started to die and then huge branches just started to die.
Jay: It would be really helpful if you could email some pictures to Rosie on the House...because I'd just be throwing things against the wall right now.
Sue: Well, when we pulled the outter layer off, I guess its bark on a Palo Verde, green part, underneath it's just like sawdust. There's not really a hard part of the limb.
Jay: Have you seen, are there any entrance holes in the limb like bores?
Sue: No we haven't. We've looked and there is not.
Rosie: Here's what we're going to do Sue...email some pictures and you will be next week's featured gardening show & tell...hopefully we can save your 12 year old Palo Verde tree because watching a tree you've nurtured, it's 20 foot umbrella coverage shade; it almost brings me to tears just thinking about it.
Jay: Yeah, you don't just plant another one that's that size already.
Rosie: Sue, thanks for the call!
Expert Advice
Jay Harper I would guess "Mites" they're not uncommon on palo verdes and most of the damage appears out on the ends where mites would be most likely to occur! she could spray with spinosad, or use systemic granules or both, also pruning out the damage and fertilizing to stimulate some replacement growth, i think the tree will be fine.
Terry Mikel I checked the photos out and with the comment about the 'saw dust' under the bark leads me to think that the plant has had some stress (Examples: water, too much too little; wind knocked it around and loosened the roots and/or root girdling) With the stress then the Powder Post Beetles laid their eggs and the tiny little grubs are working on the wood. No effective treatment. By removing the dead branches and then determining the stress(es) would help a lot to get the tree back up and running. If the stress is girdling roots then it is harder to get it back on it's feet .
Michael Bialowitz It’s likely to be the Palo Verde Borer. The larva could have been laid as much as five years ago and they are just now coming out with the extra rains from early this spring. No real good treatment, just properly water and fertilize the tree so it can naturally combat the borers.