Just came across this post and I have to add my comments. It looks like powdery mildew to me. This is fungus family and comes when air is moist and warm. If not caught early, it will suck life out of the leaves. If you wait too long to get rid of it, it leaves spores on everything.
I lost all of my mature African violets this past Marcch/April. I have two 5-shelf units and one unit had nine trays of my mature violets. I didn't notice it at first. Then I spotted the mildew on one whole tray of about nine to ten plants. I sprayed them with diluted Lyslol couldn't save a one. I sprayed the violets with milk, a fungicide spray, and neem oil. I had waited too long to catch them in time. I lost them all. I had leaves potted in another room, so I still had violets.
The message here is to catch it quickly. Another forum member suggested putting little saucers of sulfur in each tray, on every shelf. That is what I'm doing now. My air-conditioning in on until probably October. Come March or April when we shut it down I will have to be eagle-eyed and watch those violets like a hawk. I sure don't want to lose any more.
