Disease status

• 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.

Other hosts

• 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.

Varietal susceptibility

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

Distribution

• 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.

Symptoms and recognition

• 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.

Disease cycle and epidemiology

• 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.

Disease monitoring and forecasting

• Examine orchards for signs of silver leaf during the growing season.
• Mark affected trees so that they can be watched.

Chemical control

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.

Cultural control

• 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.

Biological control

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.