This is called Powdery Mildew or "Blight" as some of the old school gardeners will refer to it as... The best thing I have found for it is organic seaweed extract used as a foilar spray daily. Growth Plus is what I use.. a small bottle is about 15 bucks and will go a long ways.. 10ml to a liter of water, and mist your leaves top and bottom every day. This will not only feed your plants.. but seaweed has natural anti-fungal properties and will kill/prevent powdery mildew and bud mold.. stop spraying about 10-14 days before harvest for best taste results Good luck!