Have you tried the "home remedy" solution of 1 tablespoon of baking soda and a few drops of liquid dish soap per gallon of water applied at least weekly until the problem is gone? Also, good sanitation practices (removing dead and congested foliage, keeping dead leaves picked up, etc) may go a long way to help in the future. There are other similar suggestions online.

There are probably some great synthetic chemicals out there for this problem, but I'm not familiar enough with any of them to make a suggestion.