Mistletoe is a pretty parasite and useful for holiday decoration - but it's a parasite. And it's a growth that is a dead give away that your tree is in stress.
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 trees results from several factors. Some varieties like hackberries have built in problems and 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 control plan. Prune all the mistletoe out of the tree. It can be put in the compost or chopped up and sprinkled on fire ant mounds. 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. Next apply the Sick Tree Treatment, with the first step (and most important one) being the exposure of the trunk flare. All soil and mulch should be removed from the flare. It has been reported to us that the mere application of dry molasses will cause the mistletoe to fade out but the entire procedure is the more sure way to go, plus it will help with issues other than parasites.