I started using trichoderma viride two years ago, ever since the mildew in my entire back yard has been minimal. It doesn't happen until late fall in September or October. I spray my trees once or twice a month with a spore solution from leaf emergence until august. I spray with commercially available micro nutrients (Fe, Mn, Mb, Cu, Zn, all metals basically) as a general health booster. But these metals are known to kill some microbes, so that's why I keep using trich viride.
Before that time, I used copper sulphate when the leaves were hardened off, a needle pinhead pinch (10-25mg or something?) on 3 liters of water has always been sufficient but it required three or four treatments throughout the summer to really be preventative. 10 bucks worth of copper sulphate will last you a decade or so. It's pretty cheap.
I followed some citizen scientist that got 30 people to try hydrogen peroxide on mildew at 1, 1.5 and 3% and it seems that the 1.5-3% range does seem to kill most mildew in one or two treatments within a week. But once it's dead, it has already chewed down the cuticle so the foliage is unprotected from sunburn. The tree will end up with some minor sunburns that could eventually turn dull and/or brown. On top of that, peroxide requires continuous spraying to be effective; hit every spot you see as soon as you see it, otherwise the mildew will just continue spreading. It has little lasting protective effects.
There's some bordeaux mix recipes out there that combine lime sulphur and copper sulphate, and some commercial materials that contain sodium silicate (which will friggin burn your foliage right off! But the label is correct: no more mildew! Figures..). I have read about people using potassium silicate (used in painting) as an effective mildew inhibitor but I have personally never tried it.
There are some sulphur gas mixtures, but you don't really want those fumes anywhere near you.
