That is really nice! I have some English Oaks and 2 (left) black Oaks planted in the ground. Harry Harrington has a care guide for oaks which you should read, powdery mildew is a big problem when the foliage gets too dense, thin them out and put them in a spot that has very good air circulation (Windy). Also a good fungicide that is absorbed into the leaf and will stay effective for a couple of months would help a lot. I have found if you use sulphur or copper they are covered with the stuff for half the season, at least here, it's very humid for half the summer.