A: Rosemary is a plant that likes full sun and good airflow to keep its leaves dry. Next to your door on the patio in winter is part of the issue. You’re not getting good light where they are.
Haul those plants to the front of the porch if you can. They’d like as much sun as possible! Additionally, sheared rosemary is very dense, so if they’re watered through the top of the leaves, their leaves stay damp for longer than they should. Water rosemary in containers from the side, and ideally, just water the soil in the pot without wetting the top of the plant.
Sometimes, we’ll have wet weather for several days in a row, and powdery mildew will hit your rosemary plants. Treat with Fung-onil fungicide as needed to prevent problems. But if you keep your plants in the conditions they prefer, you’ll rarely need to.
(And take off any decorative foil “hats” that the rosemary may have around their containers when you place them outside. Those covers often don’t allow the rosemary to drain properly.)