Answer: Heat stress can cause the leaves of your apple tree to turn yellow and drop. Drought stress can do the same thing. And, diseases can also cause the leaves to yellow and drop but affected leaves typically show spotting or other symptoms. Without more information I can't really diagnose the problem with your tree. If it is heat stress, it will recover when the weather cools; if it's drought stress, you'll need to water deeply and regularly to help your tree recover. Good luck with your apple tree.
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