Pistachio trees are noticiably thin, discolored, heavy seed and early leaf drop, what is happening?
I have seen this in several cases, most much more severe. We believe it was due to environmental influences such as ; coming out of wet winter and a period of some 5 weeks without rain. Most folks had turned their irrigation off because of this and were unaware of the growing situation. As some indicator plants like flowers and/or evergreens began drooping and browning some began watering, others later when plant damage showed up.
Dehydration in trees is one of the most severe disorders a tree can face(life changer), when it occurs, and especially in the spring when moisture is necessary for leaf/twig expansion development, significant harm comes to the tree and opens the tree up for attack of invasive insects such as borers.
It is important to understand that considerable attention now must be given so that this situation does not occur again. What that means, is remove unnecessary barriers that will inhibit progress such as buried or covered root flares and inconsistent water management.
Irrigation gives you that consistency but most systems are designed for turf, trees of course are a high plant with much more complex root systems thus requiring additional fluid. In our field, trees require 10-15 gallons per diameter inch per watering, that is a base line, variables like species tolerances, exposure, slope, drainage, soil type and volume are all taken into consideration.
In most cases it would require due diligence through water management, recovery of health estimation from this type of event, is generally 3-4 seasons, in your case I believe you caught it earlier.
Bottom line don’t miss a watering.