Apple tree canker is a fungal disease of apple trees caused by Nectria galligena. The disease is also referred to as European canker and is characterised by the appearance of sunken areas on the bark. The fungus can infect apple trees during all stages of growth. Infected trees grow poorly, and the wood becomes highly prone to fracture during periods of high wind. There is an appearance of red spores in the cankers during fall. These resemble the eggs of red spider mites. The fungus usually enters the tree through wounds or pruning cuts made during fall. You can manage and treat apple tree cankers through a number of ways.
Prune out all infected tree areas. Use a sharp, clean pruning tool and do not use an infected tool on healthy tree areas or other healthy trees without proper cleaning.
Cut out the smaller shoots entirely if cankers are on shoots. If cankers occur on larger branches, carve out all the brown infected tree tissues with a sharp knife. The branch is then able to regain its health.
Burn all pruned, infected tree parts. This minimises spread of fungal infection.
Treat infected trees by spraying affected areas with fresh bordeaux mixture at the rate of 10-10-100. Use the treatment during early fall prior to rains.
Apply fungicide again after the tree has lost three-fourths of its foliage during fall. This is often necessary when trees are seriously infected. Bordeaux is an organic fungicide for treatment of apple canker.
Other treatment options include the use of fixed copper materials. This is also an organically accepted treatment. Use copper-based fungicides according to rates indicated on label. Do not confuse tree canker disease with fire blight. Make careful inspection to establish presence of fungal spores and reddish brown lesions just below leaf scars prior to starting treatment.