For that advice you should consult the label. Try finding out specifically what kind of mildew you have first (assuming that is what the problem is). The most common types are powdery and downy mildew. I'm guessing that you have powdery because of the white stuff, but you can look at some pictures of the two on-line to verify this for yourself.
My memory says that they use sulfer for powdery mildew on mangoes. The trouble is that because of high levels of fungicide use, some mildews have built up resistance to certain fungicides. If you only get partial control, switch to a different fungicide rather than spraying the same one again.
You may also notice that the affected leaves turn brown after applying a fungicide. Typically, this happens because the mildew had already killed the leaf cells, but they had not had time to dry up yet.
That is pretty much what I know, I hope it helps. My miracle berry never suffers from fungus problems, so I can't give you more specific advice.