Coryneum or shot hole disease 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, peach, almond, apricot trees, sometimes even on plum trees (especially ‘D’Ente’), walnut trees, blackcurrants, currants and raspberries. Plum trees are more often affected by Coryneum microstichum, which also affects Kerria, these popular shrubs forming small yellow pom-poms in spring.

How to recognise Coryneum?

This cryptogamous disease usually appears in summer, but as early as spring on cherry laurel.

In general, Coryneum fungi initially cause brightly coloured round spots on foliage, about 3 mm in size:

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

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

• Peach and almond trees respond with leaf drop even when only lightly affected.
• When young shoots are infected, characteristic round spots appear on the shoot from which gum sometimes oozes profusely, sometimes causing shoot dieback.
• Attacked buds fail to burst.
• Fruits are often affected on almond and apricot trees, which then show a more or less lumpy surface marked with small grey-brown spots; sometimes also on cherry (small brown spots that become pitted), causing fruit growth to stop followed by fruit drop.

Coryneum microstichum, more frequent on plums, causes, in addition to foliage perforations, small cankers on branches around buds that lead to branch dieback. On Kerria, elongated brown-violet spots with a dried centre appear on young branches, on which blackish punctuations (the spores) form. This results in dieback of shoot tips.

Symptoms can be mistaken for those of an attack by bacteria (bacteriosis due to Pseudomonas or Xanthomonas) or by virus, or by an overly concentrated treatment, notably copper, or by scorch caused by drops of water on foliage during the hottest hours of the day.

Factors favouring Coryneum

The survival of the fungus occurs mainly in cankers and buds, and even in fruits which release spores.

The fungus causing 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 continue to progress within the plant during winter. In spring, rain and wind contribute to the spread 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: avoid excessive pruning of healthy stone-fruit trees, except peach tree, which only flowers on one-year-old shoots!
• Remove and burn fallen leaves beneath infested trees.
• Collect infested fruit carrying spores.
• Treat with copper (copper oxychloride) cherry and plum trees just after leaf fall in autumn and again at bud burst period. For peach tree, favour products such as Cuivrol or Ferticuivre, lower in metallic copper and less toxic to the tree (formed of copper sulphate or copper oxychloride, trace elements and plant macerates), 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 peach trees. You can apply just before flowering and immediately after first petal fall.
• Disinfect pruning wounds with a copper solution and coat larger wounds with wound-sealing mastic.
• Limit nitrogen fertiliser applications and avoid overhead watering of your orchard.