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Plum Trees: Pests and Diseases

ellie.white
Published on September 7th 2018
14
A bird sitting on a branch

Fruit and Veg

Plum trees are resilient fruit trees. When properly looked after, it's unlikely you'll come across pests or disease that will cause too much damage. However, as we all know things can go wrong that cause our plants to become unwell. This is mainly due to plants getting old or frost damaging them, making them susceptible to pests or fungus.
Here we've listed some of the problems you might encounter with your plum trees, and the solutions that will save the day.

Plum Moth

Grapholita funebrana
Plum Moth, Grapholita funebrana
Plum Moths are horrible pests that affect plums, damsons and gages. The caterpillars feed on the insides of ripening plums making them inedible and causing the fruit to turn brown from the inside out.
How to Identify the problem...
  • Fruits ripen early or are misshapen.
  • When cut open, you can find caterpillars inside the fruit. The caterpillars are 12mm long, pale pink with a brown head.
  • Excrement is left in the fruit (pictured below).
Grapholita funebrana excrement left behind in the fruit
Solutions: Non-Chemical Control - Pheromone Traps
You can buy pheromone traps from garden shops or order them online. These are used to lure in the insects so that they cannot escape and breed. Sex pheromone and aggregating pheromones are the most common types that you can buy. These tend to be most successful when the plum moth is in adult form and there are only a few of them.
Pheromone Trap
Solutions: Chemical Control
  • If your trees are small enough to be sprayed, you can use a contact spray such as Vegetable Bug Killer or Provado Ultra Fruit. This is effective on newly-hatched caterpillars.
  • Neonicotinoid acetamiprid can be used.
  • Spray in about the third week of June and apply the second application around 3 weeks later.
  • Make sure to always read the labels on dosage and harvesting. (Plants in flower should not be sprayed.)
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Plum Sawfly

Hoplocampa flava
Plum Sawfly is a less frequent problem than moths but have a similar effect on your fruiting trees. When in larval form, they will tunnel into the young fruit before leaving to hide in the soil while they pupate. This causes damaged fruit to fall at an early stage in June. 'Victoria' Plum is more susceptible to sawfly than other trees.
Solutions: Plum Sawflies are controlled the same way as Plum Moths.

Brown Rot

Monilinia laxa and Monilinia fructigena
A plum fruit with brown rot
Brown rot is a fungal disease that affects apple, pear, plum and other fruit trees. It causes the fruits to turn a brown colour and go rotten. This is usually noticeable around mid-summer onwards.
How to Identify the problem...
  • Fruits will have a brown rot spreading from wounds.
  • Rotten fruit will remain on the trees rather than dropping off.
Solutions: Non-Chemical Control
  • Remove and dispose of any of the rotten fruit on and around the trees, as soon as you see the infection.
  • Remove any damaged fruit to prevent the fungal infection spreading any further.
  • Cut out any spurs that are infected and dispose of them in general waste rather than your own compost.
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Pocket Plum

Taphrina pruni
Fruit infested with Plum Pocket
Pocket Plumis a fungal infection which causes fruits to grow without stones, so the fruit becomes distorted and worthless. The distorted fruits can be spotted around mid-summer.
How to Identify the problem...
  • Fruits will appear to be strangely shaped and hollow where stones haven't developed.
  • White fungus will develop on the fruits, which will cause them to shrivel and fall.
  • Sometimes stems can also thicken and become deformed.
  • The fungal infection can cause areas of twigs to die off or form dense clusters called witches' broom.
Solutions: Non-Chemical Control
  • Remove any infected fruit and cut back infected branches before spurs are produced. Dispose of branches in general waste rather than your compost.
  • There are no chemicals available for controlling pocket plum.
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Shot Hole / Coryneum Blight

Wilsonomyces carpophilus
A leaf affected by shot hole aka coryneum blight Wilsonomyces carpophilus.
Shot hole disease is a fungal infection that overwinters in infected buds and small twig cankers, leaving leaves looking as if they've been 'shot'. The infection can occur any time between spring and autumn.
How to Identify the problem...
  • Infected leaves will appear to have small red spots that turn purple with tan-white centres. The infected spots will then drop out of the leaf and leave it looking like it's been shot.
  • Fruit will have a similar appearance to the leaves and will be left with rough, scab-like spots on the skin. This causes the fruit to crack and ooze.
  • Maturer fruit that has been left infected will appear sunken and grey.
Solutions: Non-Chemical Control
  • In spring, prune out any twigs that have dead buds encircled by sunken and darkened areas.
  • When the plant begins to bloom, check that the buds do not have small red spots on them. If they do, remove them.
  • Remove any fruit with purple-red spots.
Solutions: Chemical Control
  • Soluble sulphur or Bordeaux mixture can be used. Both are best applied before the leaves start to open.
Other common problems...
  • Birds - netting can help stop birds pecking at your fruit, but you can leave any inedible fallen fruit for the wildlife to enjoy.
If you have any follow up questions please share a photo and ask us - we'd love to help.

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