My fern-leaf Cosmos - varieties Pinkie and Purity - have been brilliant all summer. But in the last couple of weeks the leaves of some plants have become mottled with pale blotches and then rapidly wilt and turn brown and now the blooms are starting to wilt as well. It seems to spreading to all thsee varieties, but not to the broader leaf varieties nearby e.g. Brightness. It doesn't rub off the surface like downy mildew. See picture. Any ideas as to the cause and possible control?