Sometimes I get powdery mildew on my rosemary, sage too. Usually its in the winter when its been cool, rainy and foggy. The first time I grew silver sage, its first winter was just gruesome...almost every single sage
plant was infected. I've also had it on new cultivars of rosemary which I was trialing for the first time too. And then it seemed some cultivars were more suseptible than others...Things to try: Increase sunlight if you can, Keep foliage as dry as possible, Plenty of free air movement, Baking soda, I've heard works well. The theory is that it alters the pH of the leaf's surface so the powdery mildew can't get a foothold. It's used a lot on
roses to prevent blackspot. I want to say its 1 tsp. of baking soda to 1 qt. of water with a tiny drop of liquid soap, like 'Safers' or 'Impede' used as a spreader sticker (helps to adhere to leaf surface). I'm not 100% sure of the proportions, though, but 1tsp-1qt. sounds safe to me without harming the
plant I'm trying to save. 'Sunspray', a superfine horticultural oil, works on the same principle as the baking soda and you can follow label directions. As far as eating or cooking with your rosemary, depending on its size of course, I'd wait if the
plant was small since it will need all its energy to recover. If its large enough, go ahead and snip off a few of the cleaner stems. Hopefully this is not 'too much information'. Sometimes I just don't know when to stop.
