Powdery Mildew is a airborn spore....spores are not systemic are they
1 spore is all it takes for a outbreak of PM.....filtering your air intake is a nessary step to prevention
For a total eradication of the spore..a 100% cleaning of your grow area is nessary (1/4 cup bleach to 1 gallon of water) floors walls ceiling equipment
also a 100% total coverage of your plant with a fungicide is nessary at the same time you clean your Grow room
I used neem oil with greencure mixed together...applied once a week for 2-3 weeks to make sure all spores on the plant have been killed