The fungus Nectria invades wood damaged by freezing, hail, animals and insects.

Nectria fruiting on a twig

Most hardwoods with such injuries are susceptible to attack. Slightly sunken areas (cankers) develop around wound or damaged tissue. Smooth barked areas are generally darker brown than surrounding bark. If the bark of the tree is rough and dark, the canker is very difficult to see.

This fungus is an opportunistic pathogen readily attacking damaged wood. It is active at all times of the year when temperatures are above freezing and sufficient moisture is present.

Trees weakened by recent transplanting, root pruning by nearby excavation, and especially by freezing are most susceptible to Nectria attack. If limbs are pruned during wet autumn weather, Nectria can readily invade the wound. Vigorous trees usually form a callus, wall off the fungus within one year, and recover. If a tree is not vigorous, invasion may continue and ultimately girdle and kill the branch or, in the case of small trees, the trunk.

Both the asexually formed spores (conidia formed all year) and the sexual spores (ascospores) formed in the late summer are capable of causing disease.

Symptoms And Signs

• Random branches and twigs do not leaf out in the spring.
• Branches of trees and the trunks of small trees are girdled and killed.
• Areas around wounds are slightly sunken.
• Salmon, pink, or cream-colored fungal fruiting structures, shaped like small cushions, form in the spring and early summer where there are cracks or natural openings in the bark over the canker. These asexually-formed structures turn brown and black with age, especially after freezing.
• In the summer and fall, red to orange-red sexual fruiting structures form in the cankered area.

Management

• Avoid injuring the tree in the autumn. Avoid pruning in the autumn, especially if the weather is wet.
• During dry weather, prune off infected branches.
• Protect trees that have been transplanted within 2 years from drought and other stresses.
• Protect young trees from freeze damage where possible.