spider mites cause white spots when the damage the leaf, but it's not consistent with the powder.
It's possible your water has large amounts of lime and calcium in it. When you run it through your humidifier it could be leaving deposits on your plants.
Mine do fine at 30% humidity. Clean off all your leaves, shut off the humidifier, and see if it comes back.
Make sure you fix this before you flower while you still can.
You may need to take evasive action.http://www.rollitup....lls-leaves.html
"Identification, Symptoms and damage:
Early signs of powdery mildew include white powder/fuzzy patches on leaves (usually low in the canopy) and a fuzzy white coating on lower stems. Note: powdery mildew can be wiped off the leaves for a quick visual check"
Edited by toilofday, 14 May 2010 - 07:44 PM.