First, leaves appeared burned on top edges and became a very pale yellow/green. Now the highest leaves have just disappeared, all but the center that was through the middle of the leaf, forming a kind of skeletal appearance. The wisteria is three years old and up until now has been great. I fed it with Baby Bio yesterday evening and sprayed it with a bug killer this evening.
If it's just started, I wouldn't get too nervous. Fall is in 5-7 weeks, and it's almost time for leaves to start looking past their prime. As tough as wisteria is, it should be fine next spring.