Plum Pockets

Plum Pockets is caused by Taphrina pruni (with the closely related Taphrina communis responsible for the same effect on American plums). Like all Taphrina species it belongs to the Taphrinomycotina, an ancient lineage of leaf- and fruit-parasitic ascomycetes that alternate between a parasitic filamentous stage and a free-living yeast stage.

The fungus infects the ovary of the flower or the very young fruit. Instead of developing normally, the fruit elongates and inflates, the flesh becomes spongy and the stone fails to form, leaving a hollow cavity — hence the names bladder plum and pocket plum.

The transformation is so complete that the affected fruits look like a different structure altogether: pale, flattened, curved and often two or three times the length of a normal plum on the same twig.

Affected fruit

Distorted fruits are elongated, banana- or bladder-shaped, 2-6 cm long, flattened or slightly curved, and pale green to dull yellow rather than the normal blue-black of a ripening sloe or plum. They feel light and spongy, and cutting one open shows a hollow centre with no stone.

Fungal layer

By early summer the surface develops a fine, powdery grey-white bloom made up of a continuous layer of asci standing on the skin. Older pockets shrivel, turn brown or blackish, and may cling to the twig as dried, wrinkled mummies.

Shoots and leaves

Infected shoots are sometimes swollen and distorted, and leaves on affected twigs may be slightly thickened or curled, though the fruit distortion is far more conspicuous.

Spore print

Not applicable — the spore sacs are microscopic and borne directly on the fruit surface.

The fungus occurs on wild and cultivated Prunus — blackthorn (sloe), damson, bullace and domestic plum in Europe and Asia, with Taphrina communis filling the same role on American wild plums. It attacks the flower ovary and very young fruit, and occasionally shoots.

Hedgerows, scrubby woodland margins, gardens and orchards are typical places to find it. Galls appear in late spring and early summer as the fruits size up, and the swollen pockets remain visible on the twigs for weeks. Cool, wet weather at flowering time favours infection; between years the fungus overwinters as budding cells on bark and in bud scales.

Taphrina pruni is a parasite of living meristematic tissue. Yeast-like cells overwintering on the twig surface are washed into opening buds and flowers, where they germinate and the mycelium invades the developing ovary wall. Fungal growth regulators cause the fruit tissue to enlarge and stretch while suppressing normal stone formation, producing the hollow bladder.

Once the pocket reaches full size the fungus produces its surface layer of asci, discharges ascospores, and those spores bud into the yeast phase that survives on bark until the next flowering season. As with other Taphrina species there is a single infection period per year and no secondary spread between fruits.

Insect galls on blackthorn twigs and fruits are firmer and usually contain a larval chamber, whereas Plum Pockets are hollow throughout and lined with spongy fungal-invaded flesh. Brown rot fungi shrivel plums into hard, wrinkled mummies covered with grey concentric pustules, but the fruit keeps its stone and roughly its shape rather than elongating into a bladder.

Other Taphrina distortions are separated by host and organ: Taphrina deformans puckers peach leaves, Taphrina caerulescens blisters oak leaves, Taphrina betulina makes birch witch's brooms, and Taphrina alni produces tongue-like outgrowths from alder cones.