Now is the time of year to be carrying out pruning work on Prunus spp. including cherries and plums. With a lot of orchard pruning taking place over winter while the trees are dormant, Prunus spp. often get lumped in with other orchard trees, but it is often better to wait until spring and early summer to minimise the risk of silver leaf disease.
Silver leaf (Chondrostereum purpureum) is a fungal disease which attacks a wide range of deciduous trees and shrubs, however Prunus species are particularly susceptible to it. The fruiting bodies of the fungus produce spores which are carried by wind and infect trees and shrubs through wounds, mainly caused by pruning.
The fungus produces most of its infectious spores in autumn and winter where drizzly, rainy, foggy or humid days with no wind or sun provide the perfect conditions for spore release and infection. By pruning susceptible species at this time, you will open up wounds which will be slower to heal as the tree is in its dormant stage and will provide an entry point for infection.
Once infected, the characteristic symptom of silver leaf disease is a silvery sheen on the foliage. This is caused by the fungal toxins in the sap stream being transported to the leaves and causing the leaf tissue to separate, giving the leaf a silvery appearance. Symptoms can develop throughout the whole tree or on just a few branches depending on the point of infection, however, once branches die, fruiting bodies grow on the dead wood and the cycle starts again. When branches are cut across, an irregular, dark stain may also be observed in the centre.
As with many infections, prevention is better than cure. Good orchard hygiene can reduce the amount of spores on the orchard, minimising the risk of infection. Poplars and Willows can act as hosts for silver leaf so careful management of the surrounding trees is just as important as managing those trees within the orchard boundary. Potential host species surrounding the Orchard should be pruned during the growing season to enable wounds to heal rapidly, with trimmings being burnt or mulched. Removal and burning of major deadwood can also help minimise the amount of spores by reducing the available growing medium for the fungus. Dead or dying trees should be removed by autumn and burned before fruiting bodies can develop and release spores over Autumn and Winter. If you are removing trees completely, try to grind out the stumps or cover them over with soil to smother the fungal fruiting bodies.
It is also important to utilise good pruning practices which can be found in detail in BS3998:2010 Tree Work Recommendations. If you are using a Tree Surgeon/Arborist, check that they have a copy of this British Standard and that they are working to it. If you are undertaking small scale pruning yourself, speak to your Arborist for advice on pruning. A good Arborist will care about the health of trees and will be happy to provide a reasonable amount of advice for free to help you manage your trees effectively. It is most important that all pruning cuts result in a minimal exposure of unprotected wood and cuts are cleanly made with a sharp pruning saw/secateurs so that they can heal quickly and cleanly.
Which brings us back to where we started! If you are thinking of carrying out work on Prunus spp. now is the time to be undertaking this work to the above guidelines. In spring and summer, there are not only less Chondrostereum purpureum spores about to infect open wounds, but the trees are actively growing rather than dormant which means they can heal more quickly, covering over and compartmentalising the wound to prevent infection.
Similar silvering of foliage can be caused by cold, drought or other forms of stress. This is known as false silver leaf and can be identified by the absence of stain in the wood.
Binding, wrapping or painting pruning cuts is no longer recommended. The best thing to do is allow the tree to heal itself naturally while giving it the best chance of success by following correct pruning techniques and observing the best time of year for work to be undertaken.