Coryneum or shot hole of stone fruit trees

Which species are susceptible to Coryneum?

Coryneum beijerinckii, also known as Stigmina carpophila, is responsible for shot hole disease on cherry trees, peach trees, almond trees, apricot trees and sometimes on plum trees (especially d’Ente), walnut trees, blackcurrants, redcurrants and raspberries. Plum trees are more often affected by Coryneum microstichum, which also affects Kerria, these popular bushes forming small yellow pom-poms in spring.

How to recognise Coryneum?

This fungal disease usually develops in summer, but as early as spring on cherry laurel.

Generally, Coryneum fungi initially cause brightly coloured round spots on foliage, about 3 mm:

• reddish with dark border on cherry, peach and cherry laurel,
• brown-purple on apricot,
• orange, red or purple on plum (3 to 5 mm in diameter),

then the centre greys and becomes perforated, hence the term “shot-hole”.

• Peach and almond react with leaf drop even when only slightly affected.
• When young shoots are infected, characteristic round spots are seen on the twig from which gum sometimes oozes abundantly, sometimes causing dieback of the twig.
• Attacked buds fail to open.
• Fruits are often affected on almond and apricot, which then show more or less lumpy surfaces marked with small grey-brown spots, sometimes also on cherry (small brown spots that become pitted), leading to halted fruit growth followed by fruit drop.

Coryneum microstichum, more frequent on plums, causes, in addition to leaf perforations, small cankers on twigs around buds which lead to dieback of the twigs. On Kerria, elongated brown-violet spots with a dried centre appear on young twigs on which blackish pits (the spores) develop. This results in desiccation of the terminal part of the twigs.

Symptoms can be confused with those of an attack by bacteria (bacterioses caused by Pseudomonas or Xanthomonas) or viruses, or with damage from an overly concentrated treatment, especially copper, or with scorching caused by drops of water on foliage during the hottest hours of the day.

Factors favouring Coryneum

The fungus mainly survives in cankers and buds, and even in fruits which diffuse the spores.

The fungus of shot-hole disease of stone fruit trees causes economically significant damage, especially during wet winters and springs. With rain and temperatures above 2°C, the fungus can indeed continue to progress within the plant during winter. In spring, rain and wind contribute to dissemination of spores onto flowers and young leaves.

During years with fairly dry winters, the fungus often remains inconspicuous.

Control and treatment of Coryneum

• Prune infested wood as soon as possible and burn it. Remember to avoid excessive pruning of healthy stone fruit trees, except peach tree which only flowers on one-year-old shoots!
• Remove and burn dead leaves under infested trees.
• Collect infested fruits carrying spores.
• Treat with copper (copper oxychloride), cherry and plum trees, just after leaf fall in autumn and again at bud burst period. On the peach tree, favour products such as Cuivrol or Ferticuivre, lower in metallic copper and less toxic to the tree (made from copper sulfate or copper oxychloride, trace elements and plant macerations), which can be applied two or three times during the season at low doses (3 to 5 g/l). Treatments against peach leaf curl generally prevent shot-hole disease in the peach. You can intervene just before flowering and immediately after first petal fall.
• Disinfect pruning wounds with a copper solution and coat larger wounds with healing mastic.
• Limit nitrogen fertiliser applications and avoid sprinkler irrigation of your orchard.