I have a tree year old Wisteria. Each spring as the flowers are developing on their pendulous racemes something must be sneaking up onto the central stalk of the raceme, scarring the juicy outer layer and sucking the sap, or so it looks like. It leaves a scar 2,3 mm up to 1 cm long. As the wound dries up, it turns brown and the entire cluster dies and eventually falls of. Beside ants, I never see any insect on any part of the plant. I see the bite being fresh in the morning, therefore suspecting some possible nocturnal invader. I have repeatedly sprayed it with Bug B Gone. I used a dormant oil kit on it and managed to burn the leaf buds in the process. I painted the trunks with a Vaseline, hoping something would stick to it, but to no avail. Does anyone have any idea what should I do? Or am I defeated?