|What can I use to treat trees with rotting beginning where branches have been cut off?
|The decay in the cut branches is due to fungi which decompose the dead inner wood. The types of fungi we treat with sprays are those that attack living plant tissues. As a result, there is little to do for pruning wounds other than proper pruning techniques, and practices that promote tree health and therefore wound healing.
Branches should be pruned just outside the ridge of bark that encircles the branch where it attaches to the trunk or to a larger branch. If you cut it too close, you will create a larger wound and remove the raised ridge which promotes rapid healing. If you leave a stub, it will die and begin to decay while physically preventing the proper closure of the wound. A proper pruning cut on a healthy, vigorous tree will heal quite rapidly.
Pruning paints are only recommended when there is a particular insect or disease in your region of the country that poses an immediate threat to the fresh wound tissues. I'm not aware of any that would apply to your area.
If you feel that the decay has created a cavity that will not allow the callous tissue to have a base to support it as it tries to close back together, you can fill a small area with one of the aerosol foam products (available from hardware and building supply retailers) used to fill spaces around a door or window during construction. These products expand into the space and dry to a hard surface. They can be used to reconstruct an artificial "inner wood" surface to support the callous as it closes the wound.
Good luck with your trees. They are truly an asset to your property!