Dear Steve,

Thank you for contacting Ask Extension about powdery mildew on your shrubs. The best treatment for this and all fungal diseases is, of course, prevention. 

Conditions that favor powdery mildew include moderate temperatures (60° to 80°F) and shady conditions. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus.

The best controls for powdery mildew include good cultural practices.

• Avoid the most susceptible cultivars. Buy resistant varieties.
  
• Place plants in full sun.
• Space plants and prune to provide good air circulation.
  
• Avoid excess fertilizing or use a slow-release fertilizer.
• Rake up all plant debris at all times of the year. Dispose of it in a sealed bag and place it in your trash. Do not compost.
• Use 3 to 4 inches of organic mulch below affected plants. The mulch should cover the soil out to the drip line to prevent rain splash from reinfecting the tree or shrub. Keep the mulch a few inches away from the bark.
• Fungicide sprays are not generally needed except on highly susceptible varieties of rose, crape myrtle, and rhododendron.
  

I hope this information is helpful. Please write again whenever you have questions.