Pocket plum is a fungal infection of the young fruits of plums, damsons and some ornamental species, causing them to develop abnormally, without stones.
What is pocket plum?
Pocket plum is the name given to abnormally developed fruit of plums, damsons and some ornamental Prunus species. It is caused by an infection by the fungus Taphrina pruni, which prevents development of the stone and renders the fruit worthless. Distorted fruits appear from midsummer.
You may see the following symptoms:
- On fruit: Developing fruit elongate and are hollow, without stones. Later, a white bloom of fungus appears, then the fruit shrivels and falls. The fruits do not swell or ripen and are worthless to eat
- On stems: Twigs bearing deformed fruit may thicken and grow deformed
The fungus may also cause witches’ brooms (dense clusters of live and dead twigs). Some sources suggest these are caused by the very similar T. insititia, others that this is only a form of T. pruni.
The disease can be controlled by thoroughly removing infected branches, witches’ brooms and fruit before spores are produced. Since spores are airborne, this will not completely eliminate the risk of new infections if there are other Prunus species nearby, but it will reduce the threat.
Chemical control is not usually warranted. Difenoconazole (Westland Plant Rescue Fungus Control) is labelled to control pocket plum on several Prunus spp. Applications of copper-containing fungicides (Vitax Bordeaux Mixture or Bayer Garden Fruit and Vegetable Control) for control of bacterial canker will probably give some incidental control.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The fungus causing pocket plum is related to the pathogen causing peach leaf curl and it is assumed there is a similar life cycle, but it has been little studied.
It is thought that airborne spores released from the fungal bloom on the fruit lodge in bark and bud scales, where they grow without causing infection until the following spring. The fungus then invades the plant tissues, causing the swollen and deformed shoots and remaining in these as mycelium (the microscopic thread like body of the fungus). In subsequent years it then invades the flowers and developing fruit.