It took very little on-line research to come up with the culprit, the gall-causing mite Cecidophyes nudus. Galls are quite often easy to identify - as a gall - especially as they're usually on a specific host. There is a very useful spreadsheet download here which lists gall-causers by the host plants. But once you delve into the gall itself, being sure that you have actually found an example of the gall-causer is more difficult - and if I'm completely honest it seems wholly unnecessary. The causer is usually some microscopic thing that you will not see without optics and high mag; it's almost superfluous as you inevitably identify the causer from the gall rather than the other way around. Still, I usually give it a go though the results are barely worth taking up blog space. But today I have bugger all else to post so here we go.
Whilst the upperside of the affected leaf shows bumpy raised bulges, a general shot of the underside of one bulges shows a dense matting of hairs. The underside of the unaffected parts of the leaf has similar hairs, sparsely distributed, so I am making a great assumption that the activity of the gall-mite causes the leaf to grow more hairs??
When I first looked at this, at relatively low x20 mag, there were a number of obvious pale green mites dashing around on the surface of the hairs ....
It would be easy to shout 'bingo' and leave it at that, but I was sure these mites were incidental (well, at least in so far as I didn't think they were the gall-causer). I searched a bit more using x200 mag on the USB crapstick, and found something else - perhaps a small dipteran larva?
As you can see, it wasn't doing much but at least it was within the hairs rather than scampering around on top of them. Doesn't look very mite-like though does it. I then searched a bit deeper and found what I was looking for. A much smaller and more elongated mite actively milling about on the leaf surface. Which was almost impossible to track of and keep in focus.
I feel sure that this is indeed the gall-causing mite Cecidophyes nudus. It does not feature very highly on my list of things what I have seen that I would really like to see again. And quite frankly, there is nothing mighty about this mite.