Question
I have mature pistachio trees - about 18 - 20 years old, they appear healthy, good leaves, new limbs sprouted out this year as usual, but recently sap began coming from the limbs, not connected with any type of injury.  It has been very hot, (100 - 105 lately) and very dry this year.  What could be the issue?  They have never done this before.

Answer
Hi Charles,

The problem that you are describing is unusual.  This type of symptom is not associated with any major pathogens or pests in the US that I am aware of.  Here are some links to some common problems on pistachio in your area, but your symptoms don't seem to match.  It is possible that the problem is related to the dry weather and heat.
http://aces.nmsu.edu/ces/plantclinic/septoria_leaf_spot_and_a.html
http://aces.nmsu.edu/pubs/_h/h-647.pdf    

The weather conditions in NM are different than many places in the US and world, so the diseases that may be a problem in wetter climates wouldn't be found in your location.

I would recommend that you contact the county extension office in your area.  If this problem has been occurring on other trees in the area, they should be able to tell you.  Depending on your location, they may be able to visit the trees or ask you to bring a sample in to their office for examination.  This will be the best way to solve your problem.  This link will show you a map of NM and you can select your county for the address and phone number of the closest office:
http://aces.nmsu.edu/county/

So sorry that I cannot give you the exact cause of your problem.  However, I suspect that if you contact the extension office that you will be much closer to solving your problem.  Sometimes, a problem can't be solved from a written description and someone must physically examine the damage.

Good luck!