Silver leaf – additional information | Apple Best Practice Guide

Silver leaf – additional information

Silver leaf is a wood rotting disease that frequently attacks apple trees and may cause death of trees or tree parts.
It is usually most common on older trees particularly those that have been top worked or with large pruning cuts.

Silver leaf has a wide host range.
The disease is more serious on plums and cherries, but also attacks pears and other members of Rosaceae including peach, apricot, nectarine, almond, hawthorn, blackthorn, rose and Portuguese laurel.
Many other woody hosts are also attached such as laburnum, horse-chestnut, currant and gooseberry, poplar and other forest and hedgerow trees.

Apple varieties most frequently attacked are Newton Wonder and Early Victoria, but all apple varieties are susceptible.

The disease occurs in most parts of the world where apples are grown including USA, Canada, Australia, New Zealand, Japan and most parts of Europe.

Leaves on affected trees have a characteristic silver appearance which appears soon after petal fall.
Whole trees or individual branches may be affected.
The silver leaf symptom is due to a separation of the upper epidermis from the palisade layer in the leaf.
Necrotic areas may appear on severely silvered leaves.
Affected branches when cut often show a purple discolouration in the wood.
The fungus causing the wood decay produces a toxic substance that when translocated to the leaves, induces the silver leaf symptoms.
Affected trees often have evidence of large wounds or pruning cuts.
Affected trees may decline over several years before dying.
Once the tree or tree parts die, they may become covered in the bracket-like fructifications of the fungus.
These are variable in size and shape, but usually 1.5‑3 cm across, 1-2 cm wide and 0.2-0.5 cm thick.
Their lower surface is smooth and purplish and the upper hairy and pale brown in colour.
Fruiting bodies are never found on live wood.

Other problems that may be confused with silver leaf

The silver foliage and fungal fruiting bodies are very characteristic.
Silvering of foliage may also be caused by frost.
The bracket fruiting bodies of silver leaf may be confused with those of the many-zoned polypore (Coriolus versicolor).
The latter are usually larger (4-10 cm across, 3-5 cm wide) and distinctly coloured with concentric zones of black-green, grey-blue, grey-brown or ochraceous-rust with a white or cream margin and very common on the remains of dead tree trunks in orchards.
C. versicolor is a saprophytic fungus colonising dead or dying deciduous trees.

The fruiting bodies are produced on dead wood in autumn and spores (basidiospores) are released from these during wet weather and when temperatures are above freezing.
Spore release occurs from autumn to the following June.
The spores are spread by wind and rain and infect wood through pruning wounds, bark splits, pruning stubs, grafts, etc.
Infection of wounds more than one month old is uncommon.
Apple trees are most susceptible to infection from September-May.
In June, July and August, trees are much less susceptible to infection as they are actively growing and capable of forming barriers in the wood against fungal invasion.

Silver leaf does not spread from tree to tree through the soil and silvered leaves are not a source of inoculum.
Spread of the disease on pruning implements is also unlikely.

There are no fungicides currently recommended as sprays for control of silver leaf.

Bezel (tebuconazole) was previously recommended as a paint for application to pruning wounds for control of Nectria canker and this product gave some control of silver leaf.

Some trees may recover from silver leaf if vigour is increased by fertilisation or other cultural practices.
Affected trees should be monitored and grubbed and burnt as soon as they die, before silver leaf fruiting bodies appear.
Avoid pruning, especially major tree restructuring, during wet weather, when wounds are more likely to be infected by silver leaf.
Wood from affected trees should not be stacked at the orchard edge as the fungus will fruit on the dead wood which will provide a large source of inoculum.
Neighbouring trees and hedges should be checked for dead branches and signs of the fruiting bodies.
Affected branches should be removed and burned.

The fungus Trichoderma viride which is antagonistic to silver leaf was developed as a biocontrol agent for control of silver leaf.

It was used either as a spray to pruning cuts or as pellets introduced into the trunk of the healthy tree.
The treatment proved effective in small scale trials, but the results were inconsistent in multisite, larger scale trials particularly on apple trees.
The fungus was available commercially as ‘Binab T’, but currently is no longer registered in the UK.