A client called, alarmed about his beautiful, mature blue oak. He had been away for a two-week trip. When he returned, he saw the oak, which had looked perfectly healthy before they left, now looking half dead.

On close inspection I noted most of the leaves that had emerged first, after the tree came out of dormancy, were dead or nearly so, with a brown, spotty appearance on the upper surface. The undersides of the affected leaves showed well defined but irregular flat, grayish patches.

Leaves nearer the new shoot tips were generally normal in appearance.

A closer look with a magnifier revealed no structure typical of any insects.

The symptoms are consistent with a powdery mildew infection that started when early spring weather — warm days, cool nights, and humidity lingering from heavy rainfall — was conducive to infection and subsided as the days and nights warmed up and the soil dried later in spring.

My problem was the fact that the infections were on the undersides of the leaves, while most powdery mildew infections appear on the upper surface of the leaves, as I have recently noted on dozens of valley oaks and on “London plane” sycamores.

So, it was time to dig into the reference books.

And here is where it got tricky: powdery mildew generally infects the upper surface of leaves while downy mildews infect the undersides. And the two different diseases require very different conditions.

Powdery mildew thrives in warm days, cool nights and high humidity and is actually suppressed by rainfall. In contrast, downy mildew requires low temperatures, high humidity or free moisture on the leaves.

See? Wet leaves suppress powdery mildew while wet leaves favor downy mildew. In a way, downy mildew is like black spot on roses, a closely related “Ascomycete”. It typically builds up on roses when they have rainy spring weather and have not had thorough dormant cleanup pruning.

Digging further, I found a few references to downy mildews on oaks, but nothing matched the symptoms on this blue oak.

After a brief detour into downy mildews, I found a reference (“Oaks in Urban Landscapes,” UC Publication 3518) and a photo of a powdery mildew that infects the undersides of leaves on some species of oaks: Cystotheca lanestris.

It turns out that this the same fungus causes “witches broom” in coast live oaks, tight clusters of gray to white shoots. It is often seen in coast live oaks that are growing very vigorously and experiencing cool nights and high humidity. Soft, immature leaves are very susceptible.

With the generous rainfall this spring, trees received welcome relief from drought but there is a flip side to it. Humidity, warm days and cool nights favor powdery mildew infections in soft new leaves.

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Think back to the blue oak. The leaves that appeared a little later were not infected, when the rain had quit, weather had warmed and humidity probably fell as everything generally dried out. And the garden area around it is not irrigated, i.e low humidity.

As far as treatment is concerned, avoid the notion to fertilize the tree. Soft new growth is susceptible to infection, so fertilizing, and excess irrigation can make powdery mildew infections worse.

Referring to “Pests of Landscape Trees and Shrubs” (UC Publication 3359), there are various spray materials that may suppress powdery mildew infections, depending on the plant or crop.

Sticking to shade trees, I gather from the UC publication that prevention is the best approach, and the most promising spray materials include certain oils (stylet oil, jojoba oil, neem oil), and specially formulated soaps (M-Pede and Safer).

And then there are synthetic fungicides, which licensed pest control companies can apply.

On the blue oak mentioned above, it looked like the stem tips and younger leaves were all clean, and the buds were already forming for growth next spring. In a case like that, it might be best to just wait and see what the weather does next spring and watch closely for symptoms of infection before pulling the trigger on the sprayer.

Bill Pramuk is a registered consulting arborist. Visit his website, www.billpramuk.com. Email questions to info@billpramuk.com or call him at 707-226-2884.

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