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Silver leaf – additional information

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 recommended as sprays for control of silver leaf.

  • Pruning wounds, particularly large wounds on scaffold branches where major tree restructuring is being done, should be painted immediately with a suitable wound protectant paint.
  • This will protect against infection by silver leaf fungus while the tree is developing its own natural callus tissue.
  • Currently Bezel (tebuconazole) is recommended as a paint for application to pruning wounds for control of Nectria canker.
  • This product should also give some control of silver leaf.
  • Latest time of application is not specified but it should be applied to the wound as soon after pruning as possible.
  • There are no other suitable wound protectant paints.

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.