I probably should have added that the stuff I use is 5%
The mixture depends on the concentration of the H2O2 solution. Most stuff in the shops is between 3 and 8 percent. You can get higher concentrations, but don't go crazy. Unless the final mixture is over 10-15% H2O2, it'll not only kill the spores, but any other critters as well. Anything higher risks greater damage to the plant. The reaction that destroys the bad guys releases water and oxygen, so it's pretty safe to work with as long as you keep if off your skin. Won't hurt the plant much either, but the potential is there for slight damage to the outer cell walls. Not enough to be terribly concerned, really.