What is silver leaf?
Silver leaf is a fungal disease particularly affecting plums, damsons and greengages, though it can also infect apple, pear, apricot, cherry, and non-fruit trees such as birch, hawthorn, laburnum, poplar and others.
Spores of the Chondrostereum purpureum fungus enter the tree through fresh wounds from September to May. Fungal threads are produced that spread through the wood, gradually killing it. They also produce a toxin that is carried upwards by the sap to the leaves, where it causes a silvery sheen, formed because of the separation of the upper leaf surface from the tissue below. This changes the way the leaf surface reflects light – the silver leaves don’t have the fungus in them.
On dead parts of the branch or trunk, so-called “bracket fungi” develop, a cluster of circular, concave forms that begin as purplish but then become brown or light grey. The bracket fungi release spores from September to May, which are carried by the wind.
How to deal with silver leaf
Ensure that the infection is due to this fungus. “False silver leaf” can appear in some types of tree just as a result of stress, principally caused by drought or malnutrition, and in this case, the leaves become silvery even though the fungus is not present. If you look after your trees, ensuring that there is enough moisture, giving them sufficient nutrients and using mulch on the ground below the canopy, false silver leaf is unlikely to develop. The first symptom of true silver leaf is branch dieback. You should cut out the entire branch, and if the branch is at least 2.5 cm in diameter, you will see brown staining on the cut wood, more clearly visible when you wet the wound. Cut progressively downwards, in other words towards the trunk, until no staining is present. All of the branch above the staining will be infected. Remove the infected wood from the garden or orchard and burn it or dispose of it safely. Paint wounds with heal and seal. If you don’t have a heal and seal paint, you can use ordinary household emulsion paint to create a barrier. Disinfect the pruning tools that you have used.
If the disease spreads to the trunk, and you are certain that it is caused by true silver leaf, it is best to cut the tree down and remove as much of the stump as possible. Be careful with firewood logs and old tree stumps in and around your garden, because the fungus often grows on this dead wood as well, becoming a source of infection.
Silver leaf is the main reason why plums, greengages and damsons should not be pruned from September to May. Pruning wounds offer an easy place for spore entrance. If you prune between June and August, the tree naturally produces a gummy exudate that prevents the entrance of silver leaf spores. Read an article about how to prune plum trees.