Powdery Mildew

How can I treat a powdery white substance on my plants?  (Powdery Mildew)

Symptom(s):

Powdery white substance appears on leaves of plants. 

Cause(s):

High humidity and/or poor air flow causes the naturally occurring fungi to grow.

Control(s):

Reduce humidity, prune to improve air flow and avoid wet, shady areas.  Apply dormant oil spray or fungicide during active growth. 

Reference(s):