Moved from ID Request. Thanks for the follow-up--please see the guide page for information. The "later naked" part of the gall description makes me want to check Bassett's original description to see if it really makes sense, but Andricus ignotus definitely seems like the best option. The other somewhat similar Andricus galls are found on white, post, and live oaks.

I am almost never able to find a satisfactory ID for these woolly oak galls using Felt (1). They clearly belong in category #16 of leaf galls, "woolly galls attached to the leaves and covered with woolly fibers." This section is divided into five parts:

a. Galls composed of small cells, each with a separate crown of wool.
b. Galls composed of seed-like cells in groups and covered with wool.
c. Galls with large cells, not seed-like, but clustered under wool as before.
d. Galls with small cells and bonded together by a hard substance.
e. Galls many-celled and under wool.

The distinctions among these categories are not clear to me, although I'd venture that the cells in this particular gall are not "large," so not c. Part e includes only two species, which both make rusty brown galls, so we can rule that out. The species in part d are all on canyon live oak or Quercus arizonica, so that can be ruled out. This leaves a fairly short list of options, but I don't know how to choose among them because bur oak isn't listed as a host for any of them (a through e). Maybe someday my request for (2) through interlibrary loan will come through and I'll have something more to say.