Fungus on Holm Oak

A concentric fungus infection on Holm Oak (Quercus ilex).  High Elms, 29 December 2012.  Reflected light.

A concentric fungus infection on Holm Oak (Quercus ilex).  High Elms, 29 December 2012.  Transmitted light.

I don't know what fungus caused this, but it was too interesting to ignore.

Holm Oaks are our only evergreen oaks.  They are not native to the UK, but are naturalised and well established.  Only the saplings have spiky leaves like this, which protect them from grazing.  Leaves on mature trees have a single point at the end.