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Perennial Canker

Perennial canker seems to have the most names of any fruit-tree disease. AKA "peach canker," "Cytospora canker," "Leucostoma canker," and "Valsa canker", it is a destructive fungal infection that primarily affects peach trees, but it can also damage sweet cherry, apricot, and plum trees. It is caused by two related fungi, which are now known as Leucostoma cincta and Leucostoma persoonii, but were formerly called Valsa cincta and Valsa leucostoma. As if that wasn't confusing enough, at a certain stage in their lifecycle these two fungi are given completely new names: Leucocytospora cincta (formerly Cytospora cincta) and Leucocytospora leucostoma (formerly Cytospora leucostoma). Whew! Under any name, both fungi will overwinter in woody material that became infected the previous season. In the cool, wet weather of early spring, fresh spores are spread by rain and wind to new infection sites. At this point, it is of key importance to know that perennial canker cannot create new infections in healthy wood. It can only gain a foothold in plant material that is dead, injured, scarred, or poorly healed.

The most recognizable symptom is a visible canker. An infected area on a scaffold or trunk will display dark and sunken bark, and it will ooze abundant amber-brown sap. When the weather warms, the tree will form a defensive circle around the canker, a sort of a callous. The disease spreads again in the fall, and the next season, the tree will form another circle. Aged cankers can present as a series of concentric rings. As the canker expands, dead interior material cracks and turns black, and the limb becomes girdled, withers, and dies. Because peach trees are usually pruned to two main scaffolds, a single canker can destroy half the tree.

There is no effective chemical treatment for cankers; cultural practices are the only way to protect your trees. First, remove infected trees before planting new trees. There is no point in trying to grow a healthy peach in an area where the fungus is well established. Planting sites should be well drained and you should choose trees with the appropriate cold hardiness for your climate. As the tree matures, it should be pruned to maintain an open habit with wide-angled branches. Narrow angles are especially prone to splitting and allowing access to pathogens. When pruning, stubs should not be left on the tree; branches should be cut to just above the "collar" where the smaller branch is joined to the larger. Pruning should never be done until after bloom. This is when the tree heals fastest and seals off its wounds. Active cankers must be removed when possible. If a canker cannot be safely removed, and there are healthy trees nearby, it is best to remove the infected tree entirely. Finally, damage from insects such as peachtree borer can create entry points for canker. If insect pressure is high, chemical control should be considered.

For further reading, see the Cornell Guide to Growing Fruit at Home and the Cornell Cooperative Extension IPM Factsheet on Perennial Canker.

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