This is Silverleaf Fungus (Chondrostereum purpureum) taking hold on a Saw-toothed Azara (Azara errata) at the South London Botanical Institute. Actually I wouldn't have known which fungus it is but I was there yesterday and several of the SLBI's expert gardeners and botanists all concur it's Silver Leaf. Silver because it makes leaves turn silver and purpureum because it develops a pronounced purple tinge.

 Infection is by airborne spores landing on sapwood that has been recently exposed e.g. by pruning or damage. This patch is by a branch cut sometime in the past though that's not necessarily the point of entry. It's a fungus of old dead wood but affects living trees too. Prunus species are said to be particularly vulnerable which makes sense. Cherry trees for example often have dead and dying branches among the living ones. I remember we had a plum tree in the back garden which ended up being more dead than alive. It kept on going with bits dropping off until eventually only a stump remained.

 Unfortunately Silverleaf may prove fatal to the Azara in the long run and we discussed whether an amputation might help, decision pending. Azara serrata is generally described as a large shrub but the one at the SLBI has become a small tree. It's multi-stemmed from a couple of feet above ground level and would remain shapely if the trunk affected is to be removed. In which case I offered to do the job of taking that limb out which may or may not save the rest. 

 It would be a shame if the Azara is a goner but it is a botanical garden and that includes fungi; I find the form and texture of funguses absolutely fascinating!