How To Deal With Cankers On My Honeylocust Trees?

Honeylocust Thyronectria Cankers

Over the last two years, the drought and polar plunges that have hit Denver have weakened the ability of Honeylocust trees to fight off disease. Thyronectria cankers are notorious for harming and killing Honeylocusts in the Denver area. Cankers are dried-up areas on the stems and trunks with a different color than the rest of the tree. Thyronectria cankers are elongated and can occur on young or old wood and become bumpy when the fruiting bodies of a fungus break through the bark. For more information, check out the Colorado State Extension bulletin on Honeylocust diseases. 

The best defense against Thyronectria and other canker diseases is tree vigor because there is no treatment. Homeowners should keep their trees healthy by prudent watering and pruning each year. Typical canker symptoms are:

• Dieback of affected branches, reduced foliage, yellow foliage, premature fall coloration, and early leaf drop.
• Areas of dead tissue or cankers are found at the base of trees, at branch crotches, around wounds, or on branch stubs.
• Cankers can range from slightly flattened surfaces to distinctly sunken areas with thick, rugged ridges at the canker margin.
• The thin bark areas on stems and branches have a red-yellow discoloration.
• Infected bark and cambium will be loose, and wood beneath them may have a dark, wine-red to yellow discoloration instead of white color.

A canker is a visible injury on the outer part of the tree and is usually the result of an open wound that has become infected. Any injury to the base of a Honeylocust is potentially an entry point for fungi. Most wounds come from careless use of lawnmowers and weed trimmers. Squirrel damage and improper pruning allow fungi to enter stem and trunk tissues. Canker disease tends to be species-specific. Thyronectria cankers are usually found on Honeylocust trees but are often confused with the following fungal infections:

• Cytospora cankers – Attacks Aspens, Spruce, Pine, Poplars, and Willows.
• Phomopsis cankers: Targets Juniper, Russian Olive, Blue Spruce, Douglas-fir, and Arborvitae.
• Nectria cankers – Found on Honeylocust, Oak, and Maple.

Canker diseases pose a severe threat to trees once established because they decrease the delivery of essential nutrients to branches causing dieback. Once the cankers appear on the trunk, it is just a matter of time before the trees die.

Tree Fungal Diseases Common to the Denver Area

Thyronectria Canker
Pleonectria austroamericana
Thyronectria cankers attack Honeylocust trees in Denver. The fungus presents itself as bumpy, disc-shaped asexual fruiting bodies that are light yellow-brown when fresh but blacken with age. Their perithecia or sexual fruiting bodies are reddish-brown in color. Both fruiting bodies start in lenticels or on branches with thin bark. Lenticels are the breathing pores on the bark of trees.

Black Spot Nectria Canker
Nectria nigrescens
Black Spot cankers are similar to Thyronectria but have raised asexual fruiting structures that are cream or peach in color. Their perithecia are flask-shaped and reddish-brown and grow in solitary or groupings. Sometimes the perithecia can be found clustered at the base of sporodochia which are compact stroma caused by parasitical fungi. These structures turn dark brown within a few days.

Coral Spot Nectria Canker
Nectria cinnabarina
Coral Spot Nectria cankers are similar to Thyronectria and Black Spot cankers, but they present as raised smooth sporodochia masses that are creamy colored and look like coral. Their perithecia are small bright reddish-brown round flasks that form in late summer singly or in groups.

 

 

Cytospora Canker
Cytospora
Some Cytospora fungi are host-specific, while others can infect several tree species. Cytospora cankers attack Aspen planted in Denver. Sometimes the fungi attack trees during the fall-winter spring seasons when temperatures are warm, but the tree is dormant and cannot defend itself.

 

 

Phomopsis Cankers
Phomopsis
Typically, Phomopsis usually attacks trees grown in nurseries and Christmas tree farms. For some unknown reason, this fungus is now affecting Evergreens found in yards.

 

 

Root Collar Rot
Several fungal pathogens can cause Root Collar Rot. The rot takes advantage of trees affected by Black Spot Nectria or Thyronectria cankers. The disease moves fast, so homeowners with trees that turn early in the fall probably need to call an arborist for proper identification. A tree with rot in advanced stages will need removal. 

Disease Cycle

Canker fungi overwinter on infected trees as fruiting structures or grow on dead matter as Mycelium. Mycelium is a fungi growth sometimes seen on the ground or on rotting material that looks like branches of a tree. Wind and rain help spread spores to other parts of the tree or neighboring plants. The spores enter trees through wounds where the fungus grows in the bark, cambium, and outer wood. As the fungi infection spreads, it kills adjacent tissues, eventually girdling affected trees.

Honeylocust Trees

Some Denver trees are more susceptible to cankers than others. The key Denver tree species to watch is the Honeylocust. Thyronectria cankers are now infecting Honeylocusts across the Denver area. Newly planted or transplanted trees are at high risk because they are still acclimating and are not yet fully established. Honeylocust trees have the following characteristics:

• Honeylocust varieties can be thorny or thornless
• The trees grow to 30′ – 70′ ft in height
• The crown is open-spreading and filters light well
• Compound leaves have 8 to 14 leaflets
• Leaves are bright green in summer and turn yellow in the fall
• In late spring and early summer fragrant, small flowers grow in clusters
• Mature Honeylocust trees have grayish-brown bark with long ridges and deep furrows

 

How Do I Protect My Tree from Cankers?