Q: Several weeks ago I purchased four hydrangeas. I sprayed three of them with Sevin to keep insects away. The fourth one was not sprayed and seems healthy.
A: Carbaryl (Sevin) doesn’t usually cause chemical burns on leaves but that is exactly what you have. It could have been that high temperatures and high humidity caused the chemical to sit on the leaves too long and that caused the damage.
In any case, I don’t think this will turn into a bigger problem. It isn’t a disease, so it won’t spread. Just leave the plants alone, water appropriately and they will grow out of the damage.