This appears to be a type of leaf gall that we have encountered regularly (but infrequently) over the years. (You can view a sample image of the gall in the linked page.) The organism responsible is not yet known, though the majority of plant galls are caused by either insects, mites, or fungi.  It does not cause serious harm to the plant and can be ignored, though if you prefer, you can snip off the individual leaves with symptoms and dispose of them.

Miri

Thank you Miri!