What are those small grey bumps on your plums? Why does your fruit not have a stone inside? What’s causing your leaves to curl up? This article covers the most likely culprits.

While plum trees are generally more resilient to disease than your apples and pears, there are several microorganisms and pests that will target them. Some varieties, such as Victoria, are more at risk than others.

The main two categories of plum tree diseases are those caused by fungi and those caused by pests.

Fungal plum tree diseases

If your plums are disfigured and lack stones inside, you are looking at the work of Taphrina pruni or ‘pocket plum’.

Brown rot is fairly easy to spot due to the small grey bumps that form on the fruit, underneath which the flesh will be discoloured. Left to develop, the fruit will shrivel on the branch. Brown rot can spread quickly so we recommend removing and burning all affected fruit as soon as possible. Pick healthy fruit as soon as it’s ripe and remove and use any plums that have fallen on the ground.

Other fungal threats to plum trees include shot hole (Coryneum blight), silver leaf disease and powdery mildew.

Plum tree pests

Plum trees can be targeted by aphids, especially the plum leaf-curling aphid (April to June) and the mealy plum aphid (July to September). As the name suggests, the former cause the leaves of plum trees to curl. This is often mistakenly blamed on a virus. The good news is that aphids won’t damage the fruit and a mild infestation can usually be dealt with using soapy water and a bit of elbow grease. One of the biggest challenges is noticing the aphids as they are very small.

Larger insects that target plum trees include scale flies, moths, flies and wasps. 

The female plum moth (Grapholita funebrana) lays its eggs on the underside of fruit in May. Once hatched, the caterpillars will burrow into the plums, often leaving no external sign. Inside, the pink caterpillar and its brown waste will give you a nasty surprise.

The plum sawfly (Hoplocampa flava) is another pest that leaves few signs but renders the fruit inedible. These flies emerge from the soil in the spring and lay their eggs in the blossom. The maggots will then enter the fruit, leaving just a small brown mark, and develop within.

While some insecticides will work on these pests, you may prefer to use a pheromone trap rather than spray your fruit.

If wasps are regularly feeding on your fruit, you probably have a nest nearby. These insects strategically build their nests near sources of food so the only course of action is to have the nest destroyed or moved.

Then you have pigeons. For some unknown reason, these birds like to strip the leaves from plum trees and damage the stems in the process. While small trees can usually be protected by a net (or, even better, a cage), you may need to experiment with bird scarers on larger trees.

Other causes of plum tree damage

Bacterial canker can affect any tree, infecting cracks or wounds in branches. The characteristic dark, sunken wounds should be pruned out and the whole branch removed at source where possible.

While plum trees are good at surviving frost, the plums themselves are not so hardy. If your tree is in a frost pocket, and it is not too big, consider wrapping it with fleece. If you have yet to plant your tree and are worried about frost, go for a variety such as Czar, Pershore or Blue Tit which are more resilient.

Are you still unsure about what’s attacking your plum tree or how to deal with it? Contact Arborcure for a thorough survey of your fruit trees. Our experienced specialists will get to the bottom of your problem and prevent it getting worse.