Its called powdery mildew.

Seriously affected leaves must be cut off and thrown away carefully so that the dust does not go flying around.

If you live in an area where it rains a lot and the soil can tolerate sodium, then you can mix some sodium bicarb out of your kitchen in water then mist the affected area. Better solution is potassium carbonate, (potash).

You can also use sulphur powder on the leaves. Don't buy the sulphur pellets for acidifying the soil. Powdered sulphur will be referred as organic fungacide. Any half way self-respecting gardener would know what it is.

Another thing you need to do is feed the soil with high potassium fertilizer. You could also feed some potash.

Last item, when watering the plants, try avoid getting the leaves wet or watering late in the evening.

hope this helps.