Honeylocust knot or galls - Call for more research

Question: "I was wondering if you know of any treatment for Honey Locust Knot? Or what is your suggestion to deal with that problem. I have a client that has 7 Honey Locust tree's that are clearly infected with this Knot" -Arborist from Maryland

Picture of a honeylocust sample received at the U of I Plant Clinic with galls on branches.

Picture of a honeylocust sample received at the U of I Plant Clinic with galls on branches.

You recently emailed Nancy Pataky with a question about honeylocust knot.  She is now retired and she has forwarded your email to me.  Honeylocust knot can be tricky, as to my knowledge, the cause has not been proven by research.  In my recent investigation on this topic, I have found that there could be many different causes for “knots or galls” in honeylocust.  You first need to rule out chemical/herbicide use near these trees.  For example, the herbicide, Imprelis (no longer sold), has been found by Midwest plant diagnosticians to cause knots in honeylocust.  Insects and bacteria are also some other causes for honeylocust galls.

We rarely see these honeylocust galls in Illinois, but when we did, we did not find any evidence of chemical or insects in the galls; therefore, Nancy Pataky suspected a bacterial cause.  She wrote about this topic in the U of I Home, Yard, and Garden newsletter:  http://hyg.ipm.illinois.edu/pastpest/200805a.html . Nancy Pataky’s theory of bacterial (Pseudomonas savastanoi) formed galls was based on research from the Ohio State:  http://ohioline.osu.edu/sc189/sc189_55.html . If you read this bulletin from the Ohio State, they had reports of this disease mainly in the Midwest, but also in Maryland.  The spread of the bacterium was thought to be caused by the lack of sanitation during hand pruning and the trees eventually died.  In this newsletter article, Nancy Pataky mentions that a plant pathology student at the Ohio State was beginning a new project on this problem; however, we have since learned that the funding was lost. 

We have an Agdia plant diagnostic listserve, a great resource for plant diagnosticians across the US to discuss problem plant samples. Here are some recent posts about honeylocust galls:

“Several years ago (2003ish) while working in Maryland, we found galls on honeylocust.  I'm surprised that this link is still active, but you can look at some images:  http://www.mda.state.md.us/sc/plants-pests/plant_protection_weed_mgmt/new_pest_detections_md_nurseries_-s2.php .  We called it honeylocust gall or knot.  The problem was on Shademaster and Skyline honeylocust and we found it both in the landscape and at a nursery.  It was present on the landscape trees for multiple years and eventually killed many of the trees.  At that time, I did some research and there was an article on the internet that suggested that Pseudomonas savastanoi was the cause.  I sent samples to Ohio State.  I do not know if they ever confirmed if this was the cause or what the status of the research is at this time.”

Another plant diagnostician that had a honeylocust sample with galls reported the following on the diagnostic listserve: “We had a similar case and could not rule out gall midge.  An entomologist thought it might be Neolasioptera brevis, but we had no direct evidence.” 

Since the release of Nancy Pataky’s article, I have received inquiries and reports about galls on honeylocust.  For example, a Canadian plant diagnostician has reported, “We had one honey locust sample came in the middle of July showing exactly the same gall symptoms. After one month's moist chamber incubation, some Lepidopteran larvae (Family Pyralidae,snout moths) came out from the galls. I have seen reports on locust twig borer (Ecdytolopha insiticiana) but our entomologist insisted that what we got were not the same.”

Therefore, if we are not sure of the cause, we can’t be sure of a treatment.  I would suggest that you send a sample to your local Plant Clinic.

I would also suggest that you rule out chemical/herbicide issues.  Next, I would try to cut into a gall and check for signs of insects.  If you suspect that this is a bacterial induced gall problem, please be sure to sanitize your pruners after each cut.   I am sorry, but, unfortunately, I have no other management suggestions besides keeping the tree in good health by fertilization and watering during times of drought.   

Please let me know if you know of any recent research done on honeylocust knot or gall. 

Stephanie Porter