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European Canker is caused by the fungus Nectria Gallegina. It is largely a wound infecting pathogen which enters the host tissue through wounds or immature scar tissue.
The fungus which belongs to the ascomycete group of fungi produces two distinct spore types. Over the spring, summer and early autumn, conidia may be produced. These are found in creamy white conidial fructifications which develop during periods of rain. Later during the dormant period, ascospores are produced. Their fruiting bodies are spherical red perithecia which are arranged in concentric rings on mature cankers one year or older. The perithecia are very similar in appearance to red mite eggs.
The main sites of infection are:
- Leaf scar tissue following leaf fall.
- Pruning wounds.
- Fruit calyx area following petal fall.
Other sites of infection include growth expansion cracks and crevices in leaf scar tissue over the spring growth flush period, and possibly wounds created by other pests and disease.
Generally, leaf scar tissue infection is considered the main infection site, particularly during the establishment phase of the infection in the orchard. Once infection becomes well established in an orchard or locality, winter pruning wounds become a major entry point.
The level of infection is very dependant on the amount of wet weather experienced over the autumn leaf fall period and winter pruning season.
The fungus is generally very slow growing on the host, and in the case of leaf scar infection may take several months for the lesions to appear. With leaf scar infection, the first symptom to appear will be a slight blistering of the bark around the leaf scar. This "blister" is initially light brownish in colour due to the surface cells lifting away from the deeper tissue. As the lesion grows, the bark colour turns purplish brown in colour. Leaf scar infection symptoms usually occur in late winter ahead of bud break. Often these infections occur on spurs in which case the fungus works its way down the spur into the main branch, where it eventually forms the typical elliptical canker we associate with the disease.
As the infection develops, the center of the canker shrinks away from the surrounding healthy tissue to create an elliptical crevice. With time, the fungus grows steadily into the surrounding healthy tissue and as it does, the diseased tissue continues to shrink away from the surrounding healthy tissue to form the concentric ring pattern we associate with European Canker lesions.
With time, the cankers will continue to expand and will eventually girdle the branches they infect. When this happens we see shoot dieback.
Recognition of the early stages of infection depends on careful examination of the tree for signs of leaf scar tissue infection. This is best done in late winter/early spring, before leaves are present to hide the lesions. Pruning wound callus should also be thoroughly checked. Pruning wound infections will first show up as failure to give good callus development around the cut. Cankers often develop in branch crotches, so examine these carefully as well.
Any European Canker infections found should be carefully pruned off, or if they are on the main framework, scraped clean of any discoloured tissue and then treated with a suitable fungicidal wound protectant.
The infection from well established cankers will enter the woody tissue and express as a dark brown staining which may extend beyond the bark lesion. Such cankers may continue to be active after treatment if the wood infection is not cleaned out.
Fireblight cankers on apple are similar in appearance to European Canker lesions and could be mis-diagnosed for one or the other. The major difference between a fireblight canker and a European Canker lesion is that in the case of a fireblight canker which is no longer active the tree will develop strong callus tissue around the margin of the fireblight lesion and eventually heal itself, whereas the European Canker lesion will not show a determinate healthy margin between healthy and infected tissue. Active fireblight cankers will produce bacterial ooze, whereas European Canker lesions do not.
There is also some suggestion that European Canker infection may be linked to fireblight strikes in districts where it is present.
Black rot, Botryosphearia spp, cankers are also similar to European Canker.
In New Zealand, European Canker is found in most western districts, including Auckland, Waikato, Taranaki, and the wetter parts of the Nelson district. It is well established in the Bay of Plenty and there are isolated instances of infection in the Gisborne area.
It is totally absent from the drier Eastern districts including Hawke's Bay, Wairarapa, Marlborough, Canterbury and Otago. Occasionally lesions have been found in Hawke's Bay on the trunks of trees that have been imported from districts with endemic infection, but there has been no evidence of the disease spreading from these initial lesions under Hawke's Bay conditions. The climate over the critical leaf fall period is too dry for scar tissue infection.
In districts where the disease is a problem, good fungicide protection over the leaf fall period is the key to its control. For control, the benzimidazole fungicides are the most effective, with tolyfluanid almost as effective. Captan and persistent copper formulations also give good control.
Good descriptions and pictures of European Canker lesions can be found in "Diseases of Tree Fruits in New Zealand" by J D Atkinson, published by DSIR in 1971.
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