Tracy explains that powdery mildew on phlox and monarda is caused by high humidity and lack of air circulation. Also temperature fluctuations like warm dry days and cool nights work in favor of powdery mildew. She writes that cutting back the foliage after blooming may provide new disease free foliage. You must if you are going to cut the foliage back, give the plant some extra moisture. Also remember to disinfect your pruners in a 10% bleach solution (1 part bleach/9 parts water) or at the very least wash them with soap and water to restrict the chance of spreading the disease further. Tracy says that thinning may help to provide more air circulation, but also may not. She found with phlox that thinning the plant to provide air circulation did not always work well. She feels that choosing resistant varieties of both plants is the way to go.
The monarda didyma varieties that are resistant to or only slightly affect by powdery mildew include 'Jacob Kline', 'Blue Stocking', 'Colrain Red', 'Gardenview Scarlet', 'Marshall's Delight', 'Sunset' and 'Violet Queen'. I'm going to leave it up to you to find and google a variety you are interested in planting. Hope this clears up powdery mildew for us all! Rainy (Yay!) day outside. Hope you are having a great day. Talk to you later.