European canker

Symptoms

The disease mainly attacks apple and pear. Sunken, concentrically cracked and gradually enlarging wounds, known as cankers, form on the shoots and branches, and within them the bark and wood die back. Around the wound the tree produces ridges of callus tissue in an attempt to heal it over. In autumn and on damp days, small, brick-red fruiting bodies of the fungus appear along the edges of the cankers. Branches above the canker die back, while young shoots become infected through the leaf scars and quickly perish. A canker is a perennial wound — the fungus overwinters in the infected bark and each year widens the margin of necrosis, so that the edges of the wound form characteristic concentric rings that alternate between dying tissue and ridges of callus (an image resembling a 'target'). The fungus produces two types of spores: in summer, in moist conditions, creamy-white, slimy cushions of conidial spores appear on the surface of the canker, while in autumn and winter the aforementioned brick-red, spherical fruiting bodies (perithecia) with ascospores form. Infection occurs mainly through injuries and natural points of entry: fresh scars left by fallen leaves, stipular scars and lenticels, as well as wounds from pruning and fruit picking, frost cracks and damage from woolly apple aphid feeding. A shoot or branch completely girdled by a canker dies back above the wound. The same fungus can also cause fruit rot — most often developing from the calyx end and only becoming apparent in storage.

Treatment

Infected thin shoots should be cut out with a margin of healthy wood, while extensive cankers on thicker branches should be smoothed with a sharp knife down to healthy tissue and sealed with a wound dressing. It is important to remove fruit mummies and infected debris from the orchard, as they are a source of spores. Preventive sprays with copper-based products during leaf fall, when the leaf scars form entry points for infection, are helpful. Carry out pruning in dry weather, because rain and moisture spread the spores — cut at least 15-20 cm below the visible edge of the canker, and disinfect tools between successive trees. Remove the cut shoots and the bark scraped from the cankers from the orchard and burn them, because if left in place they continue to release spores. Copper sprays are best repeated several times during leaf fall (roughly at about 10, 50 and 90 % of leaves fallen), and additionally in spring at bud burst, when further fresh wounds are exposed. It has to be borne in mind that in a heavily infected tree the disease can no longer be completely cured — management takes the form of long-term suppression: systematically cutting out new cankers, protecting every fresh pruning wound and keeping the tree in good condition.

Prevention

It is worth planting less-susceptible cultivars and avoiding bark injuries during pruning and maintenance, because every wound is a potential gateway for infection. The trees' health is supported by an airy, thinned-out crown, balanced fertilization without excess nitrogen, and by avoiding waterlogging and standing water at the site. It is crucial to establish the orchard with healthy, certified nursery stock — young trees can be a source of latent, symptomless infection that only becomes apparent after years. Avoid heavy, waterlogged soils and cold hollows in the terrain (frost pockets), where the bark cracks more easily and wounds heal more slowly. Limit woolly apple aphid feeding, whose damage opens the way for infection, plan pruning for dry weather and seal larger wounds. It is also worth removing infected branches from neighbouring, neglected trees — a single untreated orchard can be a constant source of spores for the whole area.

How to tell it apart from look-alikes

Control — organic and chemical methods

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