gall on deciduous tree

gall on deciduous tree

Observation - gall on deciduous tree - UK and Ireland. Description: very large growth on a deciduous tree, don't know which type due to lack of leaves, measuring 45cm across. Is this parasitism by an insect or infection by fungus, bacteria, etc?

very large growth on a deciduous tree, don't know which type due to lack of leaves, measuring 45cm across.
Is this parasitism by an insect or infection by fungus, bacteria, etc?