Fungus on an Acer.

What is coral spot?


Coral spot, so-called because after affected branches die they develop pinhead-sized pink fungal pustules, is a disease caused by the fungus Nectria cinnabarina. It causes die-back of branches of woody plants, but the causal fungus is a weak pathogen and its presence often indicates that the plant also has other problems.

Among broadleaf trees and shrubs, Acer, Aesculus (horse chestnut), Carpinus (hornbeam),Fagus (beech), Juglans (walnut) and Tilia (lime) are among the more susceptible to attack. It is rarely found on conifers.

The small coral-pink fungal pustules may be seen at all times of year.



Symptoms


You may see the following symptoms:

• Die-back of branches, usually those that have been broken or pruned further up
• After death, pinhead-sized pink pustules of the causal fungus appear on the bark

Control


Non-chemical control
Always prune in dry weather. When carrying out routine pruning, cut branches through the collar (ring of slight swelling found at the base of branches). Healing of wounds occurs most quickly here, compared to leaving stubs (snags) or cutting flush with the bough or trunk. If die-back occurs after pruning, remove dead material to avoid further infection.

Prune out infections promptly and cut back to healthy wood. Do not leave dead wood to moulder and generate spores in damp corners of the garden.

Chemical control
There are no specific fungicidal controls for this disease.

If it is necessary to prune in wet weather when risks of infection are high, or if the plant has previously shown a particular susceptibility to the problem, use a wound paint (Vitax Medo, Bayer Garden Arbrex Seal and Heal, Growing Success Prune and Seal) to protect the cut.

However, wound paints are not recommended for routine use as they are thought to interfere with healing and may even provide a better environment for rots.



Biology


The causal fungus is very commonly found growing as a saprophyte (a term describing a fungus that feeds on dead organic material) on dead wood, especially old hazel peasticks. Spores from the pink pustules on the bark are dispersed in rain splash and wind-blown rain.

Spores can infect trees in a number of ways:

• By colonising dead branch stubs, areas of dead bark or young tissues killed by spring frost
• Through lenticels (air pores) in living bark if the tree is stressed for other reasons

Although dead wood is colonised initially the fungus can then move on into living wood to cause die-back. After death, the fungus grows out of the dead bark to form the characteristic small pink pustules that produce the spores.