Plum trees may produce generous harvests of some of the most mouth-wateringly juicy fruits, but they’re not without their problems. They often tend towards what’s known as biennial or alternate bearing, where trees produce a bumper crop one year, only to take the following year off. Cropping so prolifically can also cause branches to become so weighty that they actually snap under the strain.
There are also two serious diseases to be aware of. Silver leaf and bacterial canker can weaken or even kill plum trees.
These issues are heartbreaking when they occur, but the good news is they can usually be avoided by carefully performing one simple annual maintenance task: summer pruning.
Silver Leaf Disease in Plum Trees
Silver leaf disease can affect other fruits such as apples and cherries, as well as some other trees and shrubs, but it is most usually associated with plums. In autumn and winter, spores of Chondrostereum purpureum may infect trees through wounds, so it’s important to avoid pruning at this time.
In summer the leaves of infected plums take on a silvery sheen (although the leaves themselves are not infectious), then once the wood dies the bracket-shaped fungi appear, ready to produce more spores. They are a whitish colour, with a woolly surface and purplish-brown underside.
Environmental stress can cause a similar silvering, but it’s likely to affect the whole tree, not just selected branches as with silver leaf. Cut through a branch for a certain diagnosis. Branches infected with silver leaf will have a tell-tale dark stain at the centre.
Bacterial Canker in Plum Trees
Bacterial canker is another serious disease that affects plums and other trees of the same plant family (Prunus). It’s harder for the trees to resist the disease in winter, so again summer pruning is the safer option.
Sunken, dead and often oozing areas of bark will become apparent from spring and early summer, and shoots may die back. Branches can quickly die of this disease. In summer small holes appear in the leaves. They look like they’ve been used for shotgun target practice, giving the disease its alternative name of ‘shothole’ (careful how you say it!).
Oozing bark, though alarming, isn’t necessarily a sign of the disease. It’s not uncommon on plums and related plants, and in the absence of the distinctive dead, sunken patch it’s likely due to other less serious issues, such as stress or a wound.
Summer Pruning Plum Trees
Thinning fruits can be regarded as the first stage of pruning, as it reduces strain on the branches. Do this in early summer while fruits are still small and can be pinched out with your thumb and forefinger.
July and August is the best time to prune plums in most Northern Hemisphere locations. Make sure your tools are sharp and sturdy enough for the job; secateurs can be used for very small twigs, but anything larger calls for loppers or a saw. It’s better to use a more powerful tool and make a clean cut than to struggle to force the blade of a lightweight tool through a thick branch.
Before any pruning task, stand back and take a look at your tree. The first thing to do is identify and remove the 3Ds – dead, diseased and dying wood. If you see signs of silver leaf, cut well beyond the area where the dark stain shows in the centre of the wood to make sure that no trace of the fungus remains.
Once that’s done, step back again and look over your tree. Do any branches point inwards, cross over each other or look like they’ll cross in the future? Branches that rub together will cause wounds, so they should be removed. Cutting them out sooner rather than later means you can make smaller cuts that are less likely to become infected. Soft shoots can be pinched out with your thumb and forefinger.
But remember, plums fruit on young wood. Avoid removing branches that are less than three years old unless you have to.
Choose a point just above a bud. Make your cut so that it slopes away from the bud to help rain to drip off. If you need to cut back to the main trunk, cut close to the raised ‘collar’ but not into it. Don’t leave a stub of branch sticking out – it will only die back and could introduce disease.
Some plum varieties, like ‘Victoria’, tend to grow branches that spread outwards horizontally. These branches begin to droop when they’re heavy with fruit, and are liable to break. Make sure to choose upward-pointing shoots to counteract this.
Don’t use a wound paint or wrap to try and seal the wound. This used to be the recommendation, but best practice nowadays is to leave pruning wounds open to ‘bleed’ and heal naturally. This is healthier for the tree, unless there is a recurring issue with silver leaf or other diseases.