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Thank you to Richard Reinert & his daughter Vivian Miller who sent us this great question and leaf samples! Earlier this summer, the leaves on Richard's Chinese pistache got black spots, turned yellow, and fell off. Black spots normally indicate a disease of some sort, so that probably is what's going on here. Most diseases need a wet environment for their spores to germinate and cause any damage.
Our summer was hot and dry, but we did have fairly wet spring. Chinese pistache trees do like more dry environments. They're very popular in my former home of El Paso, where they do very well in such a dry environment. In addition to humid air in our spring, with moist air around the leaves, our soil usually has a lot of clay, so that keeps the roots of this tree far too wet, which will also cause leaf drop.
How can you tell if black spots are fungus or damage from sucking insects?
Damage from sucking insects begin as a yellow spot, which grows and then forms a black or brown spot in the middle of the yellow. Unlike the spots on this leaf, the pattern of insect damage spots will be a little less sporadic and they'll usually be about the same diameter all across the leaf.
As with many leaf issues on trees, leaf drop from stress of any kind is pretty normal. It's a response to stress and the tree will grow new leaves this fall or next year. The damage is not long term.