What is this?
I have a pussy willow with these white things on the underside of the leaves. What are they?
Deschutes County Oregon galls
Thank you for your question. The structures on your plant are called galls, and are created by insects, but in your case, probably by an eriophyid mite, and you can read about the process here: http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/willowgallsaw.html
Since they don't hurt anything, you can just enjoy one of nature's more interesting processes!