What is your %RH and temperature hi/lo, both day and night?
You must have an exhaust. That could very well be PM and the damage from it. It is a fungus that grows in high humidity and likes cool temperatures with a swing down at night that is drastic enough to cause condensation on the leaves (dewfall). Even if it is not PM, it could very well be another type of fungus.
Go to wlamart and get a bottle of Serenade. Use as directed. It's safe and organic.