Hi,

This is a condition called Apple Burr Knot. It is not a disease or insect issue. They are caused by the progressive formation of aerial roots. It isn't a major health concern and there isn't a treatment for it. It is similar to adventitious roots on tomato stems. Burr knots occur frequently with dwarfing rootstocks M.7, M.9, M.26, MM.106, MM.111, and Mark. The problems associated with it is that burr knots can weaken a tree structurally if present in large numbers. Really the only preventative measure would be to select a rootstock that does not produce burr knots.

You can refer to this webpage for further information as well: https://apples.extension.org/burr-knots-on-apple-trees/

Let us know if you have further questions. 

Emily