Powdery mildew spores normally enter your garden with the wind. However, if you’ve experienced powdery mildew outbreaks in the past, subsequent outbreaks may also start from dormant spores in surrounding dead vegetation or weeds.
Plant resistant cultivars in sunny locations whenever possible. Prune or stake plants to improve air circulation. Make sure to disinfect your pruning tools (one part bleach to 4 parts water) after each cut.