Preventing Powdery Mildew on Garden Plants

My garden plants were covered with powdery mildew this summer. Can I spray something on them this fall to prevent problems next year?

No, spraying now will not help. The disease is dormant and fungicides are not effective at this stage. The best thing you can do is a thorough fall cleanup. Remove insect and disease infected plant material to eliminate at lease one source of future pest problems. Discard, do not compost, pest infested materials as most of us do not compost at high enough temperatures to kill disease pathogens. Check with your local municipality as most allow you to discard small amounts of pest infected yard waste.