This looks like a bacterial crown gall - caused by Agrobacterium tumefaciens or
a relative.
Margaret

Quoting Janet Boyd <janetboyd_entomologist@...>:

>
> Thank you Scotty for the list which I have passed on to Edward.
>
> He has sent me some photos for id. Can anyone help with this one on
> Hornbeam?
> Janet> Date: Mon, 17 Dec 2007 11:07:29 +0000> To:
> janetboyd_entomologist@...> From: acacia@...> Subject: RE:
> Galls on Native British trees> > Dear Janet,> > Gall4: Eruptive ?gall on
> hornbeam trunk.> > > Thanks.> > Edward
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