A concentric leaf mine on a Holm Oak leaf (Quercus ilex). (But actually, not a leaf mine. I edited the title to reflect reality after receiving a helpful comment!)
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I came across a number of these features on Holm Oak leaves at just one site earler this month. They aren't mines, but seem to be the result of a fungal infection. You can follow up and perhaps identify the fungus from here - http://www.bioinfo.org.uk/html/Quercus_ilex.htm
The reasons fro saying that this is not a mine - no frass in the blotch; no sign of a larva and no sign of an exit hole.
I started to wonder about that after making the entry! I agree it can't really be a mine. Thanks.
Lat/Lng: 51.3512, 0.0779
OS grid ref: TQ447633