Pocket Plum (Taphrina pruni) fungal pathogen of Blackthorn – June 2023
I was inspecting this years possible crop of sloes, hoping to make some sloe gin in Autumn, and I came across a couple of trees with malformed fruit. I have never seen these before.
But with reading up I found out some interesting info.
Aphrina pruni is a fungal plant pathogen of blackthorn (Prunus spinosa) that causes a chemically induced distortion of the fruit (sloes), that make the fruit swollen on one side, making it into a banana shape. The fruits are totally inedible.
Strangely in the above image you can seen one sloe fruit has not been affected by the pathogen, not sure why ?
Can also be found on most other Prunus species like bird cherry (Prunus padus), almond (Prunus amygdalus), peach and nectarine (Prunus persica) occuring as Peach Leaf Curl. However Mirabelle and Greengage varieties of Prunus domestica seem to be more resistant. Similar Taphrina pathogens occur on Birch, Hawthorn and Alder.
This pathogen is generally under recorded in the United Kingdom, but it is found throughout the temperate Northern Hemisphere.
In some of the photos you can see a white bloom on the fruit. This bloom is where the airborne spores are released into the air. These settling onto other trees and getting into the crevices of the bark. The spores mature and invade the tissue of the tree, gradually invading the flowers from within the tree, and into the ovaries causing a pseudo-pollination and an enhanced cell division of the fruit. Thus starting the whole cylce again.
In the UK the gall forms in May/June, reaching full size in July / August, persisting into September, some overwintering, but most falling to the ground.
As a fungus, cool and wet weather conditions promote the germination of spores, whilst warm and dry weather results in infection rarely taking place. There appears to be no chemical treatment, apart from maybe trying a copper based fungicide. So the only options are to cut out the affected branch and burn, and the collect any fruit before it falls and burn. Generally chemical control of Peach Leaf Curl (Taphrina deformans) is not effective, also cutting branches is not effective either.
Further info
https://www.naturespot.org.uk/species/pocket-plum and https://www.woodlands.co.uk/blog/flora-and-fauna/junes-fungi-focus-pocket-plum-taphrina-pruni/