I came across this article from
@agseedco a while back that explains it far better than I can.
How to cure Powdery Mildew By: Todd McCormick What is Powdery Mildew? Unfortunately, powdery mildew really is a mystery to science and many growers, some believe PM a systemic pathogen that permeates via the plant’s vascular system, others believe is it superficial pest that spreads across the...
agseedco.com
"I would make the comparison to trying to grow a cannabis plant in too dry of an environment, as the same as trying to drive a car across a desert with just water and no antifreeze/coolant in the radiator, you may make it, but it's way more prone to overheat, and while you plant may make it through a dry environment, it's more than likely to have problems associated with lack of humidity.
Powdery mildew mostly attacks plants in dry environmental conditions, even if the plant is in a greenhouse or outside, depending on the environmental conditions leading up to the infestation of powdery mildew, I would now blame it on mother nature giving you add dryer condition than you expected and your plants suffering. I'll be the first person to admit that for a long time I really didn't take minimum humidity requirements into consideration, and I inadvertently dried the wax off of the leafs inadvertently."
This was a huge part of my issue with PM last winter, I think. I had a dehumidifier that kept the room under 55% rh, and it still attacked the plants like a savage until I started spraying micronized sulfur. There haven't been any signs of PM since getting that humidifier, that room stays between 75-80 degrees all the time, with 60-70% humidity, and I haven't seen PM in months.