Answer: Sudden wilting is usually associated with drought, too much water (soggy soils drive out oxygen and plant roots need oxygen as well as moisture) or disease such as verticillium wilt. Start by checking the soil moisture and drainage to make sure everything looks right. The symptoms of verticillium wilt include a black streak inside the branches. Cut one or two of the branches off and inspect for this symptom. If you find none, look for crown or root rot (mushy parts). If you still can't pinpoint the problem, the tree may have just become so stressed that it wasn't able to tolerate the growing conditions. I'd replace it. But just in case there is a disease present, don't plant a new kouse in the ground where the old one died. Try a different type of tree or, if you really want a kousa, plant it in another spot.
Best wishes with your landscape!
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