Hello, it looks as though you have powdery mildew on the leaves, and the fact that the leaves are curling also indicates this, can be a problem through long dry spells, possibly treat with a fungicide spray, julien.

One tablespoon of organic washing up liquid, or yucca extract, plus two teaspoons of baking soda, per gallon of water. Apply about once a week, until the nights get warmer.