A few weeks ago I noticed some swollen brown bumps on its leaves; the back of the leaf was covered with a fine white dust. Therefore, I wrote to the Plant Doctor at the Botanical Garden here in St. Louis. Like this:
Dear Plant Doctor,
My pin oak needs your help. The symptoms are: many round brown bumps on the top of the leaves that correspond to brown stains on the back of the leaf. There also seems to be some whitish dust on the back of the leaves. The unfortunate thing is that I believe the whole tree is affected by this.
I was wondering if you can tell me what the disease may be and if there is any treatment for it. I am attaching a few photographs.
And the good doctor wrote back with unbelievable news:
Your good pictures tell us that your pin oak has been attacked by two different organisms, a small wasp and a fungus. Neither is life-threatening, but each can make the leaves unsightly. They also make good clean-up of affected leaves and their disposal very important.
The wasp causes those bumps, called leaf galls. The female wasps inject the leaf tissue with a growth regulator during the process of egg laying. There are many types of leaf galls. For more information, see our web page on the subject at:
The fungus causes the whitish-gray material, called powdery mildew, on the undersides of your tree's leaves. For more information, see our web page at:
We don't recommend spraying for either leaf galls or powdery mildew.
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So, I have a pin oak inhabited by wasp larvae. There must be thousand of them, because every leaf of this immense tree is affected. When they mature, the wasps fall down, "released" by the leaf, get buried in the soil, from where they emerge to fly around a bit before resuming the cycle.
The blind cycle of life.