What is apple canker?
Apple canker is a disease caused by a fungus, Neonectria galligena, which attacks the bark of apples and some other trees, causing a sunken area of dead bark and, eventually, death of the branch. New cankers form from mid-spring, and once formed are present all year.
Apples are the most important hosts, although Sorbus species are also attacked and, less frequently, ash, beech and some other trees.
Cankers are round or oval areas of dead, sunken bark, often starting at a wound or a bud.
You may see the following symptoms:
- On small branches and fruiting spurs: The infection may girdle the stem and kill it in a single season
- On larger branches: Cankers are perennial, with the affected area covered with dead bark that shows concentric rings indicating periods of active spread. Older cankers lose the bark, exposing dead wood in the centre. Perennial cankers develop raised edges as the tree’s bark attempts to grow back over the exposed area. Eventually the branch will die above the canker, being progressively weakened as the bark is killed
- On fruit: Developing fruits are sometimes attacked, and will rot and fall
Non chemical control
Canker is said to be more serious on wet, heavy and/or acid soils, so pay attention to drainage and raise the soil pH by liming if needed.
Completely cut out all affected smaller branches and spurs. With the larger branches, try to cut out all infected material. All such pruning should remove all brown, infected bark and wood, cutting back to fresh green tissues. Paint immediately with a protective wound paint such as Medo, Prune and Seal or Arbrex Seal and Heal, to prevent the wounds becoming reinfected.
No variety is completely resistant under all conditions, but the following all show some resistance: ‘Alfriston’, ‘Annie Elizabeth’, ‘Brownlees Russet’, ‘Cockle Pippin’, ‘Crawley Beauty’, ‘D’Arcy Spice’, ‘Emneth Early’, ‘Grenadier’, ‘John Standish’, ‘Katy’ (‘Katya’), ‘Lane’s Prince Albert’, ‘Lord Derby’, ‘Merton Russet’, ‘Newton Wonder’, ‘Orleans Reinette’, ‘Reverend W. Wilkes’, ‘Rosemary Russet’, ‘Winston’.
Some varieties are particularly susceptible to canker and these include ‘Cox’s Orange Pippin’, ‘Lord Suffield’, ‘James Grieve’, ‘Ribston Pippin’, ‘Worcester Pearmain’, ‘Ellison’s Orange’ and ‘Spartan’.
Spray with the copper-containing fungicides Bordeaux mixture or Bayer Fruit and Vegetable Disease Control, once after picking but before leaf fall and a second time when about half the leaves have fallen.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The fungus produces white, water-dispersed spores from the edges of the lesions in summer and airborne spores from small red structures in the winter and spring. Both can initiate infections, usually through wounds including pruning cuts. Damage left by falling leaves (leaf scars), frost, scab disease, and woolly aphid are also routes for infection.
In perennial cankers there is an interaction between fungus and tree. At some times the canker spreads and, at other times, the bark grows back over infected tissues. Sometimes the canker stabilises or heals but, more usually, it spreads gradually until the branch is girdled, or so weakened that it breaks.