Aceria pseudoplatani

Aceria pseudoplatani causes the sycamore felt gall that is found on the leaves of sycamores ("Acer pseudoplatanus") or Norway maple ("Acer platanoides"), and is caused by an acarine gall-mite.
Appearance.
The gall forms a bulge on the upper epidermis where the pigment colour is usually light yellow to start with and brown later on in the year. The appearance on the lower epidermis is a concavity with cream or white coloured felt-like mat which later turns brown. The bulge is due to the erinae or hairs being wider at their top. In purple-leaved varieties of the sycamore the patches are pink. The size of the felt-like patches is variable and they may appear from late spring onwards.
Distribution.
The Sycamore Felt Gall is quite common and is widespread through the United Kingdom and is also recorded from Poland, Belgium and Germany.
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