HI Robin,

Thanks so much for accessing ASKExtension at UVM.  Some experts weighed in on this and here is the response:

According to Terry Bradshaw, UVM Fruit Specialist, these are textbook burr knots. There are root initials that form at a node on certain cultivars or rootstocks. They are not damaging to the tree in most cases but can be a weaker portion of the tree and can be a site for insect invasion, but in most cases it is not problematic. Pruning them out would be more destructive than leaving them. Here is more info:
https://ipm.missouri.edu/MEG/2015/12/Getting-to-the-Root-of-Burr-Knots-on-Apple-Trees/

Thanks again for using UVM AskExtension and happy Fall!

Hi

Thank you for your response.  Burr knots make sense to me for the 1st and 3rd photos.  They match the appearance of the knots in the link you sent me.  But the bulbous structure in the 2nd photo is very different in appearance from the other two, and it is located on the underside of the branch.  Would it be possible for the expert to take a second look at that photo and confirm that it too is a burr knot?  Thank you.

Robin Foldesy 

HI Robin,

Thank you for your patience.  There was a second review of your second picture, and its characteristics still appear to those of burr knots. It is the tree initiating roots at various (odd) places in the tree as a response to environment, cultivar susceptibility, etc. If that specific area starts behaving differently than the other two (i.e.oozing, growing differently) you could send in a new picture showing these developments and we can reevaluate.  At this point, it is recommended to treat these as burr knots and leave them be.  Thanks again for utilizing AskExtension at UVM!