Pocket Plum is a minor disease that affects damsons in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
Developing fruit turns elongated, flattened and hollow, with no stone inside, ranging from light green through grey to a light orange as it ages. A white bloom of fungal spores appears on the surface, then the fruit shrivels, dries out and falls, usually well before it would normally ripen. Damsons are themselves a type of plum, which is why this is the same disease and the same fungus wherever you see it named as either plum pocket or pocket plum.
What causes it
Cool, wet weather right when buds break in spring is when infection happens, and infection is rare in warm, dry weather.
Is this disease active now?
Infection happens at bud break in spring, but the distorted, gall-like fruit doesn't become obvious until midsummer, with galls reaching full size by July and August.
Symptoms
Only the fruit that was infected is affected: it's inedible and drops, but the rest of the tree's crop is unaffected. Deformed twigs can also thicken and grow abnormally, and dense clusters of dead and live twigs, known as witches' brooms, sometimes develop where the fungus has overwinteredKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → in bark or bud scales.
Life cycle
The fungus overwinters on old infected fruit still clinging to the tree or fallen on the ground, and in bark and bud scales. It invades new tissue as buds break the following spring, going on to affect twigs, leaves and developing fruit through the season.
Why this matters in Scotland
A cool, wet spring right as buds break is an ordinary Scottish spring more often than not, and that's precisely the weather window the infection needs.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
Remove infected branches, witches' brooms and affected fruit thoroughly, before the white fungal bloom releases spores
Prune out infected twigs, limbs and brooms. Don't add the removed material to a compost heap, since that keeps the spores in the garden rather than removing them. Since spores are airborne, this reduces rather than eliminates the risk of new infections where other Prunus species, including wild blackthorn, are growing nearby.
Myths about pocket plum
5 common claims about pocket plum, checked: 2 busted0 disputed1 untested2 hold up
“You can still eat plums or damsons affected by pocket plum if you cut away the deformed part”
Pocket plum doesn't leave a good portion to salvage: the whole fruit fails to swell or ripen properly, stays hollow with no stone ever forming, and turns papery as it ages. There's no unaffected core to trim down to, because the deformity runs through the entire fruit rather than sitting in one spot.
What to do: Pick and bin (don't compost) any fruit showing the flattened, hollow, stoneless shape as soon as you spot it, rather than trying to salvage part of it.
1 source
- RHS: Pocket plumInfected fruits do not swell or ripen and are worthless to eat.
“Spraying a copper fungicide like Bordeaux mixture at bud burst prevents pocket plum”
There are no fungicides available to amateur gardeners for controlling pocket plum in the UK. Copper-based Bordeaux mixture, once recommended for this, has been withdrawn from amateur sale.
What to do: Rely on cultural control — remove and destroy infected pockets/pouches and prunings promptly, since there's no home spray available.
1 source
- RHS: Pocket plumThere are no fungicides available to amateur gardeners for the control of pocket plum
“Some plum or damson varieties are resistant to pocket plum, so you can plant your way out of the problem”
Neither RHS nor the Pacific Northwest Extension handbook names any plum, damson or ornamental Prunus variety as resistant or more susceptible to pocket plum. Both cover identification and cultural control, but neither has a cultivarA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition →-resistance section for this disease.
Our read This isn't a case of sources disagreeing, it's that the claim doesn't appear to have been tested or published by either institutional source we checked, so there's nothing to confirm or bust.
What to do: Don't choose a variety on resistance claims for this disease specifically; rely on removing infected material and accept that any Prunus tree can be affected in the right weather.
2 sources
- RHS: Pocket plumThe page covers identification, biology and control but names no resistant varieties.
- Pacific Northwest Plant Disease Management Handbook: Plum pocketsThe page covers cultural control and life cycle but names no resistant varieties.
“Removing infected fruit and branches promptly reduces how much pocket plum comes back next year”
The fungus survives winter on old diseased fruit still clinging to the tree or lying on the ground, and in bark and bud scales, ready to infect new growth the moment buds break the following spring. Taking that infected material away before it can shed spores cuts down the amount of fungus left to overwinter on the tree, though it won't stop reinfection completely if other Prunus trees are growing nearby.
Our read Neither source spells out a measured before-and-after reduction figure; the case rests on the overwintering mechanism both describe rather than a trial result.
What to do: Prune out infected twigs, witches' brooms and affected fruit before the white fungal bloom appears and releases spores, and put the removed material in the bin rather than the compost heap.
2 sources
- RHS: Pocket plumRemoving infected material will not completely eliminate the risk of new infections if other Prunus species are nearby, but it will reduce the threat.
- Pacific Northwest Plant Disease Management Handbook: Plum pocketsTaphrina pruni overwinters on old diseased fruits clinging to the tree or on the ground, and cultural control involves pruning out infected twigs, limbs and brooms.
“Pocket plum only takes hold after a cool, wet spring right when the buds break”
Infection of leaves, shoots and fruit happens specifically in wet, cool weather at bud break; a dry spring at that window gives the fungus far less opportunity to establish.
Our read RHS's page is silent on weather conditions, so this rests on the Pacific Northwest Extension source alone rather than being cross-confirmed by both.
What to do: Watch for the disease especially after a wet, cold spring, and check developing fruit through midsummer, since the deformed pockets don't become obvious until they reach full size in July and August.
In Scotland: A cool, wet spring right as buds break describes an ordinary Scottish spring more often than not, which is the exact window this disease needs.
1 source
- Pacific Northwest Plant Disease Management Handbook: Plum pocketsInfection of leaves, shoots and fruit occurs in wet, cool weather when buds break in the spring.
Frequently asked questions
What should I do first if I find pocket plum in my garden?
Start with remove infected branches, witches' brooms and affected fruit thoroughly, before the white fungal bloom releases spores.
Is there a spray that works?
No. There are no amateur chemical treatments available for pocket plum. The controls that work are cultural: the measures listed above.
When are pocket plum most active in Scotland?
Check the seasonal activity section above. Scottish timings can run a few weeks behind the UK-generic figures.
Sources
2 sources, recorded with what each was used for
- RHS1: definition, causal organism, affected crops, identification, symptoms, active-by-month timing, the one non-chemical control, and the no-fungicide-for-amateurs stance
- Pacific Northwest Extension18: independent confirmation this is Taphrina pruni (not the related American species Taphrina communis), the cool-wet-weather-at-bud-break infection trigger RHS's own page doesn't cover, overwintering mechanism, low economic/garden severity, and independent agreement on pruning as the cultural control