Silver leaf, caused by Chondrostereum purpureum, is a fungal disease that affects a variety of plants of the Rose family, including mountain ash, pear, apple and cotoneaster.
Affected wood will have a brown stained appearance when cut. Grey shelf-like fruiting bodies, running parallel with the branch, will be seen at infection sites as the disease progresses. Leaves will have a silvery cast when compared to those on unaffected branches.
Prune out infected wood immediately when silvering is noticed. Make cuts starting 30 cm (12") back from any visibly affected wood. Sterilize pruning tools between cuts with gas line antifreeze for a minimum of one minute. Burn or bury infected wood immediately after pruning.
Silver leaf can only infect plants through wounds, so ensuring your plants are wounded as little as possible will help prevent the disease from occurring.