There are no organic or chemical sprays that work to control mistletoe. On the other hand, it can be controlled. Mistletoe infests trees that are weak. Weakness in the immune system of trees results from several factors. Some varieties have built in problems. Hackberries for example almost always have a lot of mistletoe. Other trees such as elms and oaks develop mistletoe infestation and other parasites because of stress caused by improper planting, too much or too little water, too much or the wrong kind of fertilizer, soil compaction, soil contamination and/or the use of toxic chemical pesticides.
Here's the plan. Prune all the mistletoe out of the tree. Remove infested limbs completely if it can be done without ruining the shape of the tree. Notch into large limbs that can't be removed. I used to say that this is the only place I recommend the use of pruning paint. That was a dumb recommendation. Pruning paint and toxic wound dressings should never be used. At the most slather a little Tree Trunk Goop on the wound.
The well-known Mistletoe is an evergreen parasitic plant, growing on the branches of trees, where it forms pendent bushes, 2 to 5 feet in diameter. It will grow and has been found on almost any deciduous tree, preferring those with soft bark, and being, perhaps, commonest on old Apple trees, though it is frequently found on the Ash, Hawthorn, Lime and other trees. On the Oak, it grows very seldom. It has been found on the Cedar of Lebanon and on the Larch, but very rarely on the Pear tree.