Bur Oak Galls

Asked March 10, 2016, 3:25 PM EST

Roughly 2 bur oaks per acre (over my 20 ac property) have numerous, dark galls that are the size of a golf ball on average. They start on lower branches and move to the higher branches over a few years. Pruning them off in the winter months seems to slow, but not stop, them from covering the tree. The closest description i could find online was are phomopsis galls, but I am not sure if I am in the right area. I'm certain it's not the oak bullet or apple galls. I have attached a photo of a dormant tree to show the extent of the infestation on a mature oak that's been overrun. Any help would be appreciated.

Mille Lacs County Minnesota oak galls

1 Response

It sounds like you have done some research on this so you know that the galls, while pretty ugly, don't normally seriously harm the trees. The galls are an overgrowth of plant tissue caused by small insects or mites. The insects leave the gall after a short period of time, so most of the galls you are seeing no longer have any insect activity.

Because the galls normally don't hurt the trees and insecticidal control needs to be precisely timed to kill the bugs when they emerge, controls are normally not used. Cutting off the galls before the insect emerges will prevent them from reproducing and causing more galls. Raking fallen leaves and branches can also help, depending on the insect's life cycle. These options may be difficult for you due to the size of the trees and your property. A certified arborist (http://www.myminnesotawoods.umn.edu/2009/01/finding-tree-care-help-certified-arborists-and-mn-tree-c...) may be able to assist you with pruning and spraying options. Or, you could just leave them be. Although there are a lot of galls on your tree, it looks a lot worse than it is.

More information on galls can be found at:
http://www.extension.umn.edu/garden/insects/find/insect-and-mite-galls/
https://web.extension.illinois.edu/ccgms/downloads/42438.pdf