A fungal disease, Powdery Mildew, shown here on the leaves of a Lilac shrub is a common problem along the coast of California and inland.
Many times it is not a serious disease appearing on leaves just as the plants are about to go dormant or die off for the winter. New growth in the spring can also be affected and the leaves usually drop off or can be removed without treatment.
The fungus is most common in the foggy, humid days of spring and fall clearing up with the onset of warmer weather or the chill of winter.
Some plants are very prone to powdery mildew and it requires spray to help control the disease on those plants- Squash, Zinnias, Hollyhocks, the older Rose hybrids and Dahlias are some plants that spring to mind.
Plants that are in areas of poor air circulation, the wrong light for the plant or overly humid areas can become so diseased that plants will never thrive, so choosing the right location for plants and not overcrowding or overwatering helps also.
With all fungal diseases it is important to remove any diseased leaves from the area below the plants as they fall or hand pinching out diseased leaves to stop the spread of the spores.
Sunset Western Garden book sites this fungus as one that likes dry surfaces so they suggest spraying plants with a jet of water early in the day to remove spores. They also site sprays of baking soda or garlic diluted as helpful non-toxic methods for treatment.
An anti-desiccant of Wilt-pruf or Cloud Cover can also help to block the action of the fungus. There are commercially available sprays available for this, if the problem persists consistently. It can be unsightly and sometimes damage new growth or newly emerging seedlings or top growth.
Examples of some commercially available sprays for Powdery Mildew and Frost Protection. Photos taken at Summerwinds Nursery in Campbell, CA
As I said this is a common, mildly serious disease which mostly clears up on its own so I usually just remove diseased leaves (if there are only a few) and make sure I rake up under the diseased plants.
