Pear tree bug

Does anyone know what this bug is (was)? The leaf undersides are well populated with lumps like this, about 10mm across, which look like egg groups, or nests. Where whatever-it-is has come out, it's left a bag-like structure behind. The strands are all coming from the rim of a hole.
Top side of the leaf has a thumbprint sized yellow /brown stain.
It doesn't look right for any of the pear tree ailments I can find.

Thanks

 

Yes pear tree rust.
Best to remove infected leaves and burn or get rid of them off site.
There's not much in the way of chemical control available to amateur gardeners (the good stuff is now banned) but Westland Plant Rescue Fungus Control applied at the right intervals starting next Spring might do the job.

 

Thanks - yes seems to be a "good" year for it!
It doesn't seem to be a huge problem until it defoliates the tree, which isn't happening to this one.
Will look for a local juniper! here seems to be a lot of variety in information on the web about what the juniper's gall looks like, and the story's mixed in with hawthorn rust.