That's a bit galling

Okay, I've not suddenly gone all-out bonkers on looking at leaves but whilst pottering about in the garden after work I noticed some bits. And what it reminded me is that looking for leaf-mines will inevitably present a variety of galls, smuts, mildews, rusts and other on-leaf oddities that might be just as difficult to identify.

Some are obvious and easy to ID like these 'spangle calls' from the asexual generation of the wasp Neuropterus quercusbaccarum that are all over my oak sapling.

Some are obvious to see, but take a bit of tinterweb searching to ID like this fungal disease on a cultivated rose that I am sure is Rose Black Spot (Diplocarpon rosae).

And some are going to be hand's-up with no idea, like this 'thing' that looks like a nipple that I found on the garden silver birch on the underside of a leaf. It looks very regular, and very much like it should be a gall caused by either a wasp or mites although there is no sign of any disruption to the leaf. But I can't immediately find anything like it on the tinterweb so far. One suggestion on a Facebook forum was 'spider poo' ........