Cytospora Canker

Cherry, apple, peach and plum trees are susceptible to a fungal disease called Cytospora canker.

Cytospora canker of peach (Curtis Swift) Public Domain

Cytospora canker is a collection of symptoms caused by several species of Cytospora fungi. This disease also occurs on ash, birch, cottonwood, elm, maple, willow, spruce, and other conifers. Some Cytospora fungi are host-specific, while others can infect multiple tree species. Sadly, Cytospora canker can be fatal.

Cytospora canker lifecycle

Cytospora canker fungi infect trees and shrubs that are stressed or weakened by injury, frost damage, drought, or pests. Spores enter your garden on wind and rain. Infection can occur at any time of year, but trees are most vulnerable during dormancy. Fungal spores enter through tiny wounds in the roots or bark and begin growing in the xylem and phloem. This fungal growth blocks the flow of water and nutrients. If infection occurs in the trunk, the tree will die.

Cytospora canker symptoms

The first sign of Cytospora canker is often the random dieback or flagging of tree or shrub branches. You can see long, narrow cankers on infected stems and branches. These fungi grow so rapidly that cankers may or may not be sunken or discolored. You might observe the bark split along the edge of these cankers as the tree tries to defend itself. These cracks allow for the formation of a callus that blocks the fungi from entering the rest of the plant. Sometimes this works, and sometimes it doesn’t. Sometimes, this girdling occurs without any visible cankers. Gumming is another defense trees use. Gumming is when stems and fruit ooze out a sticky sap.

If you cut into a diseased stem, you may notice discoloration and a funky smell. If you see tiny black spots, you are looking at the fruiting bodies of the fungi.

Cytospora canker prevention and control

The easiest way to prevent Cytospora canker is to keep your trees and shrubs healthy in the first place. Healthy plants are less likely to become stressed enough to be vulnerable to infection by fungal spores in the first place. 

Since drought and flooding are the most common conditions that make trees susceptible to Cytospora canker, regular irrigation during summer and proper drainage in wetter months can prevent infection. These other tips can help you prevent Cytospora canker in your landscape:

• Prune for structure and good airflow and to prevent broken branches.
• Avoid digging around trees and shrubs.
• Prevent injuries from weedwackers and mowers.
• Control insects, such as scale and borers.
• Select species suitable to your microclimate.
• Conduct a lab-based soil test to see if there is a potassium deficiency or other nutrient imbalance.
• Whitewash bark to prevent sunburn damage.
• Space plants at proper distances from one another.
• Only install healthy, disease-free plants.

Once infection occurs, remove any affected stems and branches by cutting close to, but not damaging, the branch collar. Be sure to disinfect your cutting tools with a household cleaner or ethyl alcohol between each cut. Then apply a fungicide to each cut. Do not use sealants, as these treatments can trap spores and moisture where you least want them.

There are no known chemical controls for Cytospora canker, so keeping those trees and shrubs healthy is your best bet.