What is it ?

Silverleaf (Chondrostereum purpureum) is a fungal disease that attacks deciduous trees and shrubs including apples, pears, cherries, plums, peaches, apricots, nashi, berryfruit, poplars, willows, silver birches, eucalypts and roses. The fungus can easily move between the different host plants. For example, silverleaf on peach trees can also infect apple trees.

What does it look like ?

Figure1: Symptoms of Silverleaf on an apricot tree

The silverleaf fungus enters the tree through wounds, causing the xylem tissue to become darkly stained. If you cut back into old pruning cuts, you can often see this staining within the branch.

The characteristic symptom of silverleaf is a silvery sheen on the foliage. The fungal toxins, and/or enzymes in the sap stream, damage the leaf cells. Light reflects through these damaged cells giving a silvery effect. Sucking insects such as mites and thrips can also cause a similar silvering effect on trees.

Leaves over the whole tree may be silvered if the fungus enters through a large wound in the trunk. Entry through a smaller wound may produce symptoms on just one branch.

The extent of silvering also can depend on the tree species, age, vigour and the degree of infection. Some species, such as apples, show some silvering symptoms and then recover. Others, especially stonefruit, become progressively more silvered and eventually die.

What is the life cycle ?

Bracket-like silverleaf fruiting bodies form on dead wood. The fruiting bodes produce spores which are carried by wind and enter wounds of susceptible plants. If conditions are right, infection will take place. In some cases, the fungus will continue to spread and eventually kill the host.

What conditions are needed ?

Drizzly, rainy, foggy or humid days with no wind or sun - in other words days on which the household washing won't dry - are perfect for silverleaf spore release and infection.

Spore discharge starts with the onset of rain, peaks several hours later and continues as long as the relative humidity stays above 75% and the fruiting bodies remain hydrated (soft and flexible to touch).

Spore release, and hence the risk of infection, is higher at night.

Where is it found ?

Silverleaf is found in temperate climates world-wide. It can affect trees throughout New Zealand, but is more of a problem in damp, humid regions.

Why is it a problem ?

The disease can potentially affect all parts of the tree decreasing production and, at worst, causing death. Some of the consequences of silverleaf include:

• reduced leaf area
• death of leaf cells (which leads to a loss of reserves leading to starvation and death)
• reduced root growth (causing poor nutrition and an increased susceptibility to Phytophthora)
• smaller and fewer fruit
• reduced colour in apples
• an increased incidence of water core
• poor storage of fruit
• tree death.

What are the current controls ?

Paying attention to orchard hygiene is the best way of reducing the amount of spores on the orchard. Poplars and willows can act as hosts for silverleaf so shelterbelt management is important. Shelter should be trimmed during the growing season so wounds can heal rapidly ; trimmings need to be mulched or burnt.

Good pruning practices (described in the following articles), will help to reduce the likelihood of infection.

Protecting pruning wounds can prevent the entry of silverleaf. However, it is better to leave the wound untreated than to apply home-made or substandard wound dressings. If the wound dressing is of the wrong formulation, it will smother beneficial fungi that may provide some protection against silverleaf. Biological control agents, which naturally inhibit the silverleaf pathogen, have also been trialed as wound protectants.

Once the symptoms appear, injecting the tree with chemical or biological control agents may prevent the spread of disease. The most successful of these biological control agents is Trichoderma.

If the tree succumbs to the disease and dies, remove it by autumn and burn. When removing shelterbelts, cover the stumps with dirt to smother the fruiting bodies of C. purpureum.

• By trialing a number of wound paints and dressings, Dr Adrian Spiers of HortResearch, Palmerston North has developed the wound dressing Garrison™ which is now commercially available. Some of the background research which went into developing Garrison™ is described in the article :
  • Silverleaf : wound infection, wound protection. The Orchardist. February 1994.

• Recent work has shown how dramatically silverleaf can affect the yield and quality of apples and peaches, see :
  • Silverleaf - how does the disease affect tree productivity and fruit quality ?

• HortResearch scientists have also been studying the enzymes produced by the silverleaf fungus which cause cell wall breakdown, see:
  • Silverleaf fungus (C. purpureum) hydrolase activity in liquid culture [120K]

• One of the best ways of preventing silverleaf infecting fruit trees is through correct pruning techniques. Some of these are described in :
  • Pruning to prevent silverleaf

• Strains of the micro-organism Trichoderma control silverleaf disease in the laboratory and in the field. HortResearch scientist, Dr Robert Hill has shown that by injecting liquid formulations of Trichoderma into nashi and Asian pear trees even severely affected trees can recover completely. In addition, a pruning paste containing Trichoderma has greatly reduced the spread of silverleaf in infected nashi orchards in trials.

• Dr Adrian Spiers has had variable results injecting Trichoderma into silvered apples and stonefruit. Further trials with this promising biological control agent are continuing. Injection of chemicals has given effective results. This technology is being developed for grower use as soon as possible.