California blue oak may wear silver top due to powdery mildew

By: Trish Grenfell
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Yes, we had another wet spring here in the Sierra Foothills, not as heavy as 2017, but wet enough to allow blue oaks to be vulnerable again to fungal diseases.

My blue oak leafed out in April, forming what looked like a silver crown from afar. Then the high winds in May helped my blue oak to self-prune its grey/white leaves, leaving my backyard looking like a winter wonderland (if you squinted your eyes).

My tree didn’t do this last year but it was a common occurrence last spring in the foothills.

After the record-setting rainfall in spring 2017, the fungi erysiphe alphitoides visited many oak trees, inducing the silvery cast we now know is powdery mildew.

Many residents contacted the master gardener hotline then, wondering why their trees were shedding grayish leaves in spring.

Dr. Douglas McCreary of the UC Sierra Foothill Research and Extension Center was a cooperative extension natural resources specialist whose mission in the last decades of his life was to regenerate oak trees (Project Acorn). The ANR publication “Regenerating Rangeland Oaks” was the bible for oak restoration, written and updated by McCreary in 2009.

Dr. McCreary advised in the of Spring 2017, “Powdery mildew doesn’t need rainfall, but it is favored by warm conditions, high humidity and low light and it loves young, succulent foliage.

Because California was blessed with above average rainfall this past spring, there has been —and continues to be — considerably more moisture in the soil.

Under these conditions, oak trees will grow a “second flush” of leaves, usually in May or early June, that is very susceptible to powdery mildew.

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.”

But Dr. McCreary, that was not the case in late spring 2018.

A spring of powdery mildew should not harm an otherwise healthy tree. But I am wondering about two years of powdery mildew? (I believe I only had one year.) Some people may want to manage the powdery mildew by spraying fungicides but these are effective mainly when the mildew symptoms first appear — usually before you are aware of those symptoms.

And how much was my tree damaged by the 2012-2015 severe droughts? I am worried; I love my blue oak. However, as I look up at the tree today, I see normal greenish blue leaves. Just the same, I think I will consult a certified arborist.

Dr. Douglas McCreary passed away in February of this year. Powdery mildew occurs on all groups of oaks. Trish Grenfell is a Placer County master gardener. Contact Placer County gardeners at 530-889-7388 or

Trish Grenfell is a Placer County Master Gardener.