Once an oak has the oak root fungus Armillaria (often due to excessive watering), most treatments will be ineffective. Be sure the base of the tree is free of mulch and other organic debris and slightly expose the root system at the root collar. While fertilizing may not help, try using one bark application of Agri-Fos + PentraBark. This application provides minimum nutrients, but strengthens the defense mechanisms of the tree. Of course, stop watering unless absolutely necessary. Water should never be administered from July-September.
Are there any treatments available for an oak with Armillaria (oak root fungus)?