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Powdery Mildew Prevalent on Foothill Oaks this Year
By Scott Oneto, Farm Advisor, University of California Cooperative Extension

Powdery mildew developing on a young black oak leaf. Copyright © 2011 The Regents of  of the University of California. All Rights Reserved.
Powdery mildew developing on a young black oak leaf. Copyright © 2011 The Regents of of the University of California. All Rights Reserved.
Many blue oak trees in California foothills might be more accurately described as “silver oaks” this year. From a distance, they shimmer with a silvery halo. On closer inspection the outermost leaves are coated with a white to gray powdery fuzz

The cause is powdery mildew. Powdery mildew, a group of fungi that causes a white, flour-like growth on the surface of leaves, is common on roses, begonias, grapes and many other ornamental plants.

Powdery mildew rarely kills the majestic trees. Even small seedlings that have all of their leaves severely infected usually survive and recover.

Powdery mildew makes it more difficult for the affected leaves to photosynthesize and produce food, and if it’s severe enough, it can also result in the leaves distorting, curling up, dying and falling to the ground. But most affected trees will simply grow a new crop of leaves later in the summer or the following spring. And if weather conditions return to a more normal pattern next year, with little or no rainfall after March, it is unlikely that powdery mildew would continue to be severe or widespread.”

Some people may be inclined to treat affected trees with fungicides. However, these treatments are most effective before the symptoms first appear, which occurred weeks or months ago. It is also generally not recommended to treat trees in wildland settings. There are too many trees to treat and the potential environmental risks of applying fungicides across a large landscape can outweigh the benefits. Above all, don’t panic and cut down the trees, even if all their leaves fall off.

The trees are still very much alive, losing their foliage is just the oak’s way of dealing with an unwanted pest. By this time next year they should again be leafed out without that silver covering currently observable.

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