The Walks has a wonderful collection of trees, many of them known to have been there since the 19th century. Older trees often have prominent fungal brackets, which themselves can be very old. New research is establishing that fungi can be present in a tree throughout its entire life – sometimes even present in the original seed! Seeing fungi on a tree does not necessarily mean that the tree is dying or unsafe. Trees can live for many years with various different fungi present, often replacing decayed wood with new wood to maintain structural strength. However, if this equilibrium is upset (e.g. by physical damage to the tree from drought, flooding, insect infestation, or pruning of branches and roots), the tree may decline and decay fungi may then make the tree structurally unsound.
The species in the picture is probably Ganoderma australe which is a perennial fungus, growing a little larger every summer/autumn. The rusty coloured dust are the spores (reproductive cells) that are released annually and allow the fungus to colonise new sites with the aid of the wind and rain.