Answer: Premature leaf drop can be heat/drought stress related or could be a sign of a fungal disease. With redbuds, if a fungus attacks it can affect the foliage but you won't notice the problem until the tree defoliates, usually in late summer. I'd inspect the leaves for signs of fungals spores. If the leaves look normal and without black spots or specks, then the leaf drop is weather related. Hope this helps solve the mystery!
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