Cankers on Dwarf Peach and Chokecherry

(Question)

Hello, cankers have started to develop on these two trees. I first noticed it this summer with the Chokecherry. It was a beautiful tree approximately 14 feet high. One evening the main trunk collapsed into a neighbours garden. When we went to see what happened we noticed black globs on the limbs of the chokecherry. We just cut off the broken trunk and limbs. The tree is now the size of an understory bush. I have just started to notice very small black globs on the Dwarf Red Haven Peach tree. This tree is only 2.5 years old. What can I do to stop and prevent the cankers. I’m not familiar with why cankers occur. So far I have not done anything. Thank you.

(Answer)

Many different types of cankers can attack trees, and fruit trees are particularly vulnerable. Cankers – the dark, glob-like or sunken areas you describe – are the visible result of diseases caused by both fungal and bacterial infections that enter the tree’s vascular system. These infections may take hold because of improper pruning cuts, or injury to the tree during the winter, or damage done by insect borers, for example. The spread of infection can do great damage to the tree’s vascular system, including the death of branches and even of the tree itself. As the disease weakens the tree, it leaves it even more vulnerable to damage from other diseases or insect pests.

Peach canker, or perennial canker, is a fungal disease that causes the blackened, calloused cankers that you have seen on your tree. All peach trees are susceptible to this disease, which will at its very least inhibit good fruit production, and at its worst will kill the tree altogether.

What can you do? Your instinct with your Chokecherry tree was correct: removing all affected branches by pruning.  You should also make sure that the debris under your tree is disposed of separately, as opposed to composting it, and be sure to sterilize your pruning equipment after you have used it (indeed most experts recommend sterilizing following each cut). In planting your new peach tree next to a tree with an existing fungal disease you may unwittingly have allowed it to spread. (The Chokecherry, Prunus Virginiana, is also susceptible to the same canker).  If your Chokecherry no longer seems viable or attractive in its current shrub form, you may wish to consider replacing it with another species, perhaps not a fruit tree – or at least keeping your eye closely on it to ensure it is in fact completely disease-free.  The fact that the main trunk was so severely affected makes replacement seem the wisest option.

This website provides a very clear description of perennial canker, and although it is written for fruit growers on a larger scale, it contains good information on how to prune your tree: https://nysipm.cornell.edu/factsheets/treefruit/diseases/pc/pc.asp.

Good tree health in general is the best defence against pests and diseases. Here is a previous Toronto Master Gardeners post that you might find useful: although the fruit trees in this instance are pears and plums, the advice is valid for all fruit tree species, especially in protecting young fruit trees which are most vulnerable: https://www.torontomastergardeners.ca/askagardener/fruit-trees/

Last but not least, if you should lose your peach tree, you may be interested in researching, or inquiring at your nursery, about new cultivars that may have increased resistance to pests and diseases. The Ontario Ministry of Agriculture, Food and Rural Affairs has produced a very good fact sheet on this: https://www.omafra.gov.on.ca/english/crops/facts/07-041.htm.