What's up with this apple rootstock?

This is on g11. You can clearly see where the scion begins, with the vertical growth. The scion is an unknown local heirloom that appears healthy. I’ve got other apples on g11 in the same growing conditions that look normal.

maybe Crown rot? I’m no expert
http://extension.psu.edu/plants/gardening/fphg/pome/diseases/crown-and-collar-rot

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