Knopper galls look a little like a lump of popcorn.  When they first form in August, they're a reddish colour, and sticky to the touch.  Later they turn brown and woody.  They're caused by the tiny gall wasp Andricus quercuscalicis, which lays its eggs in the acorn.

Knopper galls have rapidly become widespread in Britain since they first appeared about 1960.  Some years they have a bad effect on the acorn crop.  The trees most affected are generally in close proximity to Turkey Oak trees.

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