Mildew is a perenial problem on these two favorite perennials. Cut down the stems now and destroy them along with any leaves that have dropped. Remove any organic mulch round the base and get rid of that as well, as the spores of the fungus overwinter happily there and will splash up in the spring to infect new growth. Try to improve air circulation round the plants next year. As the new stems grow, edit each clump down to about 1/3 to 1/2 the stems that appear. If the bed seems crowded with other plants remove some to get better air movement. The plants should be grown in full sun and not be stressed for lack of water. If you see infection starting - small white or grayish spots on the stems and leaves, apply a spray of 1T canola oil: 1T baking soda:3 drops of dish soap: 1qt water. Mix well and spray to cover the stems and leaves. The best news is that there are several new varieties of both perennials that have been bred for mildew resistance. These include 'David' phlox and 'Marshall's Delight' and 'Raspberry Wine' beebalm (Monarda).