Plum Pockets Identification Guide

The Fungus That Never Makes a Mushroom

Plum Pockets forces you to abandon the usual cap-and-stipe routine. Taphrina pruni produces no fruit body at all. It lives as a perennial mycelium in the bark, buds and developing fruits of plums and blackthorn, and its only visible expression is the deformed fruit it induces on the host. Identifying it therefore means learning to read a distorted plum rather than a mushroom, and the sequence below follows the order in which the characters actually become obvious on a hedgerow walk.

First Impression and Setting

You nearly always notice the effect before you notice the fungus. Walking a blackthorn hedge in late spring, or passing an old damson or bullace in an unmanaged orchard, your eye catches a scatter of pale, oversized, oddly curved fruits among the normal green ones. They look inflated, as though someone had blown into them, and they hang limply because they weigh almost nothing. On blackthorn the deformity is often concentrated on one or two branches while the rest of the bush carries perfectly ordinary fruit. That patchiness is itself a clue: the mycelium is systemic in particular shoots rather than distributed evenly through the plant.

Reading the Deformed Fruit

Pick one and work through it slowly.

• Shape and size: an infected fruit is elongated, often flattened and slightly curved, commonly two to three times the length of a healthy fruit at the same date. A sloe that should be a small round bead becomes a bean-shaped or banana-like pod 2 to 4 cm long. The outline can be irregular, with shallow ridges or a slight twist along its length.
• Colour: pale green or yellow-green at first, then dull yellow, then buff, and finally brown as the tissue dies.
• The bloom: this is the decisive character. As the fungus matures, the surface takes on a fine, even, whitish to greyish mealy bloom, like a dusting of flour. Wipe a patch with a fingertip and it comes away, leaving the shinier skin beneath. That bloom is the fungus's entire fertile layer, a continuous palisade of naked spore sacs pushed up through the host epidermis.
• Interior: cut one lengthwise. A healthy young plum has a firm stone forming at the centre. An infected one is hollow, a bladder of thin spongy tissue surrounding an empty cavity, with the stone absent or reduced to a shrivelled trace. This stoneless hollow is what gives the name pocket plum, and none of the common confusions share it.
• Fate: by midsummer the pockets shrivel, blacken, and either drop or persist on the twigs as wrinkled mummies well into winter.

Twigs, Leaves and Brooms

Follow an affected fruit back along its branch. Infected shoots are frequently thickened, slightly swollen at the nodes and paler than their neighbours, and they may proliferate into a dense brush of short twigs, the witches' broom growth form. Leaves on those shoots tend to be smaller, blistered, slightly curled and yellow-green, and they can carry the same fine whitish bloom on the underside. Finding fruit distortion, shoot deformity and leaf blistering together on a single branch is as close to a complete field diagnosis as this genus allows.

Substrate and Season

The hosts are members of Prunus: wild blackthorn in hedges and scrub, and cultivated plum, damson and bullace. The window is short. Deformed fruits become conspicuous from late spring into early summer, the bloom peaks over a few weeks, and by high summer the evidence has darkened into mummies. Cool, wet springs produce far more infections, because the spores need surface moisture to germinate on opening buds. Old unmanaged hedges and neglected orchard corners give the best chances. Between seasons the fungus persists in bud scales and bark crevices as yeast-like cells.

Sampling the Fertile Layer

There is no spore print to take here, so substitute a scrape. Draw a razor blade or scalpel flat across a bloomed surface, transfer the scrapings to a slide in water, and look at low magnification. You should see a dense palisade of cylindrical to club-shaped asci standing side by side, each seated directly on the host epidermis with a small basal foot cell, and no enclosing wall or fruit body of any kind. Around and inside the asci you will usually find masses of small ovoid budding cells, because the ascospores of Taphrina multiply by budding while still inside the sac. Naked asci in a palisade plus budding cells is the microscopic signature of the genus; the host plus the hollow fruit fixes the species.

Separating It From Look-alikes on Structure Alone

• Brown rot fungi in Monilinia also wreck plums, but the fruit keeps its normal outline and its stone, the tissue turns firm and brown, and the surface bears buff pustules arranged in concentric rings rather than an even mealy bloom.
• Other Taphrina species separate on host and organ. T. deformans blisters and reddens peach and almond leaves and leaves the fruit alone; T. betulina builds birch brooms; T. alni converts alder cone scales into long protruding tongues; T. padi makes closely comparable pockets but on bird cherry, Prunus padus.
• Insect and mite galls on Prunus are solid when cut, usually with a larval chamber inside, and never develop a fertile bloom.
• Virus damage such as plum pox gives ring-marked, blotchy, sometimes lumpy fruit, but the stone still develops and the surface stays clean.
• Frost-checked or physiologically aborted fruit is small and hard rather than inflated and hollow.

Field Checklist

Host in Prunus; fruits inflated, elongated, curved, often flattened; interior hollow with no stone; surface with a fine even whitish bloom that rubs off; shoots on the same branch thickened or broomed and leaves blistered; late spring to early summer; scraped bloom shows a palisade of naked asci containing budding spores.