Plants affected
Plums, especially the cultivar 'Victoria', other fruit trees
(cherry, peach, apricot, pear and apple), many ornamentals,
e.g. laburnum, poplar, Portugal laurel, rhododendron, flowering
cherry and rose.
Symptoms
Foliage of woody plants assumes a silver sheen. Other signs are a brown irregular stain in the wood, which can be seen if an affected branch is cut or sawn across.
Only one or two branches may show discoloured foliage. A branch with discoloured foliage usually begins to die back.
Silvering is not always seen in the foliage of some plants such as rhododendron, but brown staining of the wood and dieback of the branch may occur.
Biology
Spores of the silver leaf fungus Chondrostereum purpureum (syn. Stereum purpureum) enter through recently injured surfaces such as pruning cuts, broken branches and frost cracks. They produce fungal threads, which grow through the living wood, killing the tissues. The fungus does not spread into the leaves and there is no danger of infection from the silvered leaves.
The fungus produces a toxin which spreads upwards in the sap and causes the cells of the upper leaf surfaces to separate, so that air accumulates between the cell layers, altering the light-reflecting qualities of the leaf and giving it a silvery appearance.
Fruiting
bodies develop on dead wood, forming tiers of small, purple
or brown bracket-like structures with a whitish woolly upper
surface. They also develop on recently exposed stumps and
felled logs.
The spores produced cause new infections, occurring in fruit trees from September to May.
Control
Cut out and dispose of dead wood, before the fruiting bodies have developed, at a point 10-15cm (4-6in) beyond the area where the stain in the tissues ceases. Remove broken branches. Prune plums and other susceptible plants in the summer months (between June and August) when there is little risk of infection.
Apply a proprietary wound treatment immediately: phenolic formulations (Arbrex Seal and Heal), tar acids (Vitax Medo) or octane acid (Growing Success Prune and Seal).
When pruning several trees, disinfect pruning tools between trees.
Boost the vigour of an affected plant by good cultivation, feeding both with a complete fertiliser and a foliar feed if the tree is small enough to be sprayed.
Plum cultivars showing slight resistance include: 'Black Prince', 'Blaisdon Red', 'Jefferson', 'Marjorie's Seedling', 'Merton Gem', 'Monarch', 'Pershore', 'Purple Pershore'.
Note: Silvering of the leaves can also be due to other factors. 'False silver leaf' may have a variety of causes such as severe insect infestation, cold weather conditions, extreme drought or a form of malnutrition. Where false silver leaf occurs there should be no staining of the wood. If in doubt get expert advice.