Posted: 01/04/2013 at 18:40
Hi tropidog, those are 'bracket fungi', the fruiting body of a fungus which grows in the heartwood of the tree and will likely eventually kill it. There is, unfortunately, no way to control it. You could try cutting off any branches which are showing the brackets, but the fungus could well be in the main trunk. If it is a large tree, it could become dangerous as the fungi will severely weaken the wood itself and branches can fall off at any time, so I would strongly recommend asking a local tree surgeon to take a look.