Taphrina pruni (Pocket Plum)

English name: Pocket Plum

(other scientific names include Ascomyces pruni, Exoascus insititiae, Exoascus pruni, Taphrina insititiae)

As the species name suggests, Taphrina pruni infects Prunus domesticus (Plum) and Prunus spinosus (Blackthorn or Sloe) fruits to form pocket plums. It also infects the shoots of Blackthorn to cause stunted or swollen distortions.

T.pruni is a member of a group of fungi that don’t produce fruit bodies. Instead, the spore germinates on the plant surface before penetrating the flesh seeking refuge and nutrition. Infected fruits tend to become elongated, often more so on one side than the other, to produce the pocket-like shapes, presumably referred to by the English name.

As a member of the ascomycete fungi (including the cup and flask fungi), T.pruni produces its spores in tubes called asci (plural; singular = ascus). These asci penetrate through the surface of the fruit where the tip releases under pressure, shooting the spores out into the air.

There are hundreds of Taphrina species worldwide, In Scotland you may also find T. alni (Alder Tongue), T. padi (Bird Cherry Fungal Gall) T. betulina, (Birch Besom) and T. johansonii (Aspen Tongue). The first two species in this list have been the focus of recording effort by the Highland Biological Recording Group’s successful ’TRY’ project. All Taphrina fungi are obligate parasites i.e. don’t grow without a host.

Another species of Taphrina, T.deformans, is well known to fruit growers as the cause of Peach leaf curl disease. Infection requires humid conditions and it is this disease that restricts commercial growth of peaches and its relatives to dry climates or glasshouses with very carefully controlled humidity.

There are 204 records currently in the FRDBI, mainly recorded from May to July, although the withered fruits have apparently been recorded throughout most of the year. Here is the current distribution of T.pruni on the NBN. There aren’t many sloe on the Blackthorn around my home this year. Don’t let this put you off going out to look for this species though because well over half of the few fruits on the first hedge I checked were infected. The species must be far more widely distributed in Scotland than the NBN map suggests so please send your records in here or pass on to your local fungus group.

David Genney

2012

Sources:

Ellis, M.B. and Ellis, J.P. (1997) Microfungi on land plants: An identification handbook. Richmond Publishing. 868pp.

Ingram, D and Robertson, N (1999) Plant disease: A natural history. The New Naturalist. HarperCollins. 287pp.

Sloe infected with Taphrina pruni (left) and uninfected (right)

Sloe/Blackthorn (Prunus spinosus) infected with Taphrina pruni

Habitat: Blackthorn (Sloe) - Prunus spinosus