Powdery Mildew at 50% RH?

I just picked up 5 different clones and I believe one of them had powdery mildew on it before I brought it in, now my largest plant in the room is infected pretty hardcore (~10 patches), but the RH of my grow tent is never above 50%. I've had PM in the past REALLY badly, but the tent was sterilized with hydrogen peroxide, could this have originated in my room or did I definitely bring the PM in with this clone? Also, how is it spreading so fast with such low humidity? Thanks for your help!

go get some actinovate ,,,,its a bacteria that colonizes on ur plant and will stop and keep it from happening helps with mold and rott also,,,,a good thing to have trust me its the one thing i cant grow without since i have used it no problems at all