Powdery mildew which forms concentric rings on Populus tremuloides. Undersides of leaves appear mostly unaffected, at least at this stage.
Witches' broom Gall (Cystotheca lanestris)
Edgewood Park and Natural Preserve,
Redwood City, CA
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Reference: Field Guide to PLANT GALLS of California and Other Western States by Ron Russo- ISBN 978-0-520-24886-1
Gall induced by a fungus (Cystotheca lanestris)
Edgewood Park and Natural Preserve,
Redwood City, CA
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Reference: Field Guide to PLANT GALLS of California and Other Western States by Ron Russo- ISBN 978-0-520-24886-1
Ex American trumpet vine (Campsis radicans)
These leaves of [1] appear to be hosting fungus of which I think as "powdery mildew", though I don't know enough to say that's correct.
Host this species: https://www.inaturalist.org/observations/96842847