From looking at it,I'd say that its Powdery mildew, is it just powderery or is it weblike?..,
what is your grow area like? are you foliar feeding?,how are the temps?/humidity?
I am saying this because most causes for powdery mildew are usually caused by damp/humid an poorly ventilated conditions.
You could spray a fine mist of alkaline water and bi carb on the effected plants to stop it, but be careful not to soak the budsites as you may get budrot!