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Large gall on chestnut leaf, red on the upper side, green & reddish on the underside. The infected leaf is stunted.
I spotted this gall in the woods behind our house; many of of the trees here are chestnut trees (Castanea sativa).
If the ID is right, this is bad news, because it means that the worst chestnut pest has now reached the vicinity of Paris, and is thus spreading North.