Answer: Based on your description I am not certain what is happening to your plant. Since one is fine, your care must be good. It might be a problem with poor rooting, such as a large rock underground or encircling roots, it might be a problem with a disease such as verticillium wilt, or it might be something else such as accidental herbicide damage or a blockage in the watering system (if you use one.) Since the problem began just as the heat of summer picked up I would suspect a root problem, but it is impossible to diagnose this kind of symptom long distance. I would suggest you work with your county extension and/or professional certified nurseryman to obtain a specific diagnosis of the problem and then work from there. I hope it can be saved, losing so much foliage is a sign of a serious problem.
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