What You Should Know
Chondrostereum purpureum is a fungal disease of trees caused by the fungus plant pathogen. It attacks most species of the rose family Rosaceae, particularly the genus Prunus. The common name is taken from the progressive silvering of leaves on affected branches. It is spread by airborne spores landing on freshly exposed sapwood. For this reason, cherries and plums are pruned in summer, when spores are least likely to be present and when the disease is visible. Silver Leaf can also happen on fruits like apples and pears. Plums are especially vulnerable.
It is a pathogen of various deciduous trees including species of Acer, Aesculus, Alnus, Betula, Crataegus, Fagus, Larix, Malus, Ostrya, Picea, Populus, Prunus, Salix, and Sorbus.
After starting as just a crust on the wood, the fruiting structure develops undulating intergrowing brackets up to about 3 cm broad, which have a tough rubbery texture. The edges and fertile lower surfaces show a fairly vivid violet color while the fungus is growing, and the upper surfaces have a grey aspect (sometimes with zonation) and are covered with whitish hairs. After a week or two, the fructification dries out, becomes brittle, and turns a drab brown or beige. Infected wood can be recognized because it is stained with a darker tint.
The spores are rounded cylinders approximately 5-8 µm x 3-4 µm in size. The hyphal structure is monomitic with clamp connections.
Geographically it is roughly speaking just as widespread as its hosts - it is common in woods, orchards, and tree plantations in temperate climates.
Chondrostereum purpureum is not considered edible. However, it does have an effective economic use in inhibiting resprout and regrowth of cut tree stumps. This application of the fungus can particularly be used by the electrical industry for stumps near power lines. The species is also undergoing testing as a possible control for competing for vegetation in conifer plantations by the British Columbia Ministry of Forests and Range.
Other names: Silver Leaf.
Chondrostereum purpureum Mycoherbicide
This mushroom is commercially available as a method of combatting forest weed trees (sic) such as aspens, beech, birches, maples, pin cherry and poplars, and other species. The fungus is applied directly to the weed trees in a nutrient paste which can be stored and handled conveniently.
The first regulatory approval was granted in 2001 to Myco-Forestis Corporation and targeted species "including birch, pin-cherry, poplar/aspen, red maple, sugar maple, and speckled alder in the Boreal and Mixed forest regions of Canada, east of the Rocky Mountains". It had not been reported as of 2001 to cause many diseases in coniferous tree species.
According to a 2007 regulatory decision of the Canadian Pest Management Regulatory Agency, the use of this control method in paste form on Sitka spruce and red alder will only have a limited impact on non-target trees since the fungal spores are ubiquitous anyway and healthy trees are resistant to attack.
Chondrostereum purpureum Treatment
There are no fungicides currently recommended as sprays for control of silver leaf. Bezel (tebuconazole) was previously recommended as paint for application to pruning wounds for the control of Neonectria canker but this approval lapsed in 2015. Other coatings, e.g. BlocCade, provide a physical coating for protecting plant wounds against infection caused by fungi such as a silver leaf. To be effective, wounds must be treated immediately after pruning.
Effective control is mainly dependent on orchard hygiene and cultural measures.
Remove and burn dead trees before silver leaf fruiting bodies are formed.
Do not stack wood from felled apple trees at the orchard edge as silver leaf fruiting bodies may form and provide a large source of inoculum.
Check surrounding hedges and woodland for silvered trees and silver leaf fruiting bodies and remove and burn.
Avoid pruning in wet weather when the risk of the silver leaf fungus infecting wounds is much greater.
Where silver leaf has been present in the past, pruning of susceptible plants should be done in the summer, or 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.
Many plants will recover naturally from an attack of silver leaf, so it’s best to wait sometime 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 coloration, 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 from developing and spreading to other plants. Don’t leave the wood lying around as this may become a source of infection for other plants.
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Photo 2 - Author: Christopher Stephens (Creative Commons Attribution-Share Alike 4.0 International)
Photo 3 - Author: Michel Langeveld (Creative Commons Attribution-Share Alike 4.0 International)
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