In general, I don't think its all that great of a thing to spray anything directly on the buds. I've never tried spraying my buds with anything so I don't know 100% if apple cider vinegar would harm them. I tried to dig up some information on using apple cider vinegar to kill PWM and found quite a few articles, but none specified if spraying your buds was okay. They all just said to spray down all affected leaves. In my search for info, I did a few other remedies(Baking soda, Milk, Hydrogen peroxide) that did actually mention that you should spray the buds. One person in the comments of one of the sites I was reading stated milk did wonders for their PWM. I will quote the comment here: "the way to use it is as follows dilute 400ml milk to 600ml water allow solution to stand in an open container in the sun for about an hour then spray entire plant, the science behind this is that milk, when exposed to direct sun and air, builds up natural antifungal and antibacterial enzymes that kill the powdery mildew on contact." I hope this helps and good luck in your battle!
