I don't really know what the right strength is for your situation, but this website advises very dilute strengths, like 0.2% H2O2 for sick and fungusy plants.
http://www.using-hydrogen-pero...
Since drugstore peroxide is around 3%, you need to dilute that around 16 times:
2 ounces peroxide per quart of spray
or
1 cup per gallon.
Some people use it stronger.
Remember that you can also buy "food service hydrogen peroxide", but it is more than ten times stronger than drugstore peroxide.
