This unusual growth is called a burr knot. It is a form of aerial root growth that is a genetic disorder. Burr knots can occur in some types of grafted apple trees, especially in wet, high humidity conditions and if the tree was planted too deeply. Burr knots can weaken a tree structurally if present in large numbers. For grafted fruit trees, the selection of a rootstock that does not produce burr knots is ultimately the best method of control.

This is not something that would spread to any other trees, since it is not a pathogen. There is a little more information on our web page Apple and Pear Problems and on the Extension Foundation's apple content page: https://apples.extension.org/burr-knots-on-apple-trees/


Miri