HYG  Pest newsletterInsectsHorticulturePlant DiseasesWeedsSearch
{short description of image}

Issue Index

Past Issues

Honeylocust Knot

October 18, 2005

Not a great deal of information available on this problem; but as it has been seen in the Midwest, I thought you might want to keep an eye out for affected trees. Honeylocust knot was discovered in Ohio in 2000 by Hannah Mathers, Ohio State University Extension state specialist for commercial nursery and landscape. It has since been reported in Michigan, Maryland, Kentucky, Illi-nois, Indiana, and Iowa.

Honeylocust knot is a problem that may be easier to see now that leaves are beginning to fall. Still, symptoms first appear in the spring or early summer. Honeylocust appears to be the only host; and so far, ‘Skyline’, ‘Shademaster’, and ‘Imperial’ cultivars have been affected. The condition causes small, knotlike galls at the nodes of stems. Not all stems are affected. Given the normal swelling of honeylocust nodes, there may be some confusion over symptoms. We saw one case at the U of I Plant Clinic in 2004 (none in 2005), and it was easy to see that something was amiss. There were abnormally large nodes with swellings on both sides. Some research has been done on this disease at the Ohio State University by Pierluigi (Enrico) Bonello and others. If you visit his 2002 report, you will find pictures of symptoms at http://ohioline.osu.edu/sc189/sc189_55.html. He states that besides the galls, symptoms include shepherd’s-crooks on stem terminals, wiltlike leaflet cupping, yellowing of the canopy, defoliation, and sometimes death of the tree. Often, the stem tips have a fire blight look. The knots that give the disease its name are 1/4” to more than 2” in diameter on the nodes of twigs and branches.

This disease is thought to be caused by a bacterium because of reasons stated in the Bonello report. Future research should help confirm this suspicion. Once the bacterium is positively identified, we can provide information on control. If the pathogen is a bacterium, strict sanitation with regard to any pruning is important to avoid spreading the disease. Michigan State specialists suggest “spraying all pruning tools between cuts with Lysol, dipping them in Greenshield, 10 percent household bleach (one part household bleach to nine parts water) or other disinfestant to help keep bacteria from spreading on pruning tools. Applying 70 percent alcohol to disinfest tools works if the tools are flamed; it’s the flaming that kills bacteria more than the alcohol itself.”

If you find any suspicious-looking galls on honeylocust trees, and you are an Illinois resident, samples can be sent to Nancy Pataky, Plant Clinic, N533 Turner Hall, 1102 S. Goodwin, Urbana, IL 61801. Do not send samples from out of state; contact a plant lab in your state.