As I look at it, it looks like regular powdery mildew, which has been growing for a while there. The high humidity is the probable cause of this issue, but in order to save these plants immediate action needs to be taken. The powdery mildew slowly eats through the leafs, but the growth can be stopped in various ways. This article - www.growingagreenerwor...ry-mildew/
describes various things you can try. I've tested most of the natural remedies, and they actually work. And if there's no results in a couple of days, you should probably consider hiring a company to help you. Most professionals use various specialised mixtures and methods to remove the mildew without endangering the plants. One company I've encountered are these guys - www.gardeners-melbourn...intenance/
. Their garden maintenance service is pretty great and covers everything, including pests and plant diseases.