
Appearance
Forms large, globose, thick-walled, hard, mottled, brownish or reddish galls on the side of the acorn cup, easily detached and dropping to the ground in the fall, always on red oaks. Pink when fresh, becoming yellow and then blood-red, diameter 1/2 to 1 inch, on black, red, scarlet, Spanish and scrub oaks. August and September, shrivels, and becomes very hard.References:
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https://bugguide.net/node/view/507502