How to recognise it
Initially leaves develop a faint, silvery sheen, which is more noticeable on plum trees. This is generally confined to a single branch, but will then spread. After silvering, the leaves will split and may brown around the margins and the mid-rib.
Stems which are affected will turn dark brown just under the surface (as the disease stains the xylem vessels) and eventually die back. On apples the bark may also become papery and peel off (though this can also occur due to other problems such as apple canker and poor soil texture). On rhododendrons long strips of dead bark may radiate from the point of infection. After the branch has died, from late summer onwards, flat or bracket-shaped fungi, with a lower surface of purple coloured spores, develop on the bark. The top surface of bracket-shaped fungi, or under the top surface of flat fungi, is white/brown and hairy with spore-forming bodies which are soft and slippery when wet, brittle and shrivelled when dry.
Why it’s a problem
The disease may spread from the original branch to the rest of the tree. Although this doesn’t always happen, if it does the tree will need to be destroyed.
Even if limited to one branch, that branch will eventually die and, in the meantime, will look unsightly if the leaves are affected. Fruiting will also be reduced while the tree is fighting the infection.
Where you are likely to find it
Mainly affects trees and shrubs in the Rosaceae family, including plums (which tend to be the most seriously affected by the disease), almonds, apples, apricots, cherries, hawthorns and roses. Where other plants are infected, they may not show the leaf symptoms. Other plants which can be susceptible to silver leaf include currants, gooseberries, laburnums, poplars, rhododendrons, willows, silver birches, eucalypts and pears.
Infections are usually initiated between September and May when the fungal spores are released from the bracket-shaped or flat fungi.
How to prevent it
Where silver leaf has been present in the past, pruning of susceptible plants should be done in the summer, or in late spring for plums and other Prunus species; this avoids the main period of infection.
When pruning take care to make the cuts cleanly, so you don’t provide potential entry points for the infection. The cuts on particularly susceptible plants may be painted with protective wound paint to protect them while the pruning wound heals.
Damsons and greengages are generally pretty resistant to silver leaf, as are plants grown on the rootstock ‘Pixie’, so these would be good options if the disease is prevalent in your area. The plum cultivar ‘Victoria’ is particularly vulnerable to silver leaf and should be avoided if you have any concerns.
How to get rid of it
Many plants will recover naturally from an attack of silver leaf, so it’s best to wait some time after you’ve noticed the silvering before you take action. If branches start to die back as a result of the disease they should be pruned back beyond the spread of the brown colouration, to the next adjoining stem.
Where the entire plant is infected, or silvering starts to appear on suckers growing from the roots/rootstock, then it is infected throughout and should be removed (roots and all) and destroyed (burned). This should be done before September to prevent the spores developing and spreading to other plants. Don’t leave the wood lying around as this may become a source of infection for other plants.
Is it good for anything?!
Other useful information
The spores infect plants through wounds in the wood and then emit toxic substances through the plant’s transportation system, these enter the leaves and when they make contact with air they create the silvering effect. There is no direct fungal infection of the leaves.
A nutritional disorder called ‘false silver leaf’ bears similar symptoms, although the stems of affected trees are not stained brown. False silver leaf can be fixed by mulching and feeding the affected plant.
In the UK the 1923 Silver Leaf Order specified the destruction of all silver leaf infected wood before July 15th each year. This order has now been revoked (although its contents are still good common sense for gardeners to follow).
In the USA, Canada and other countries silver leaf is used as a biological control for trees which are considered weeds in managed forests (eg red alder, aspen and Sitka alder).