Whenever I look up a pest or disease to answer a question, I invariably find something in the prevention or control section that talks about cleaning the garden. Pests that overwinter in your garden will usually overwinter in the leftovers of your garden. So, as things begin to die off, make sure you remove the dead plant material and put it in your compost heap for next year. Remove weeds from your garden to keep them from harboring both overwintering pests and current pests. Those slugs just LOVE to hide under the leaves of weeds.
Cleanliness is also important when dealing with a diseased plant.
It's really easy to pull off the dying leaves on your rose that have been struck by black spot or powdery mildew. It's even easier to drop them on the ground after you've pulled them. But, both black spot and powdery mildew are a form of fungus which will keep on growing. And, like most fungi, the spores can easily be spread by wind, rain, or even the spray of water you give to your garden. So, remove the diseased material, bag it, and throw it in the trash.