Could a rootstock sprout occur above Meyer lemon graft line?

I recently brought my young Meyer lemon tree in for the winter. One branch is going nuts! It has grown some huge, healthy, shiny leaves. The rest of the tree looks fairly placid, although there were two blossoms, which seems like a good sign - this would be its first year to fruit.

The branch that is going like gangbusters seems to be growing from *above* the graft line (see picture... sorry, the site apparently won't let me upload more than one photo, so I'm giving you a shot of the whole tree - not sure you can see the graft line all that well, but the branch is definitely coming out of the trunk above the graft line), so I hadn't pruned it originally. Should I? Could it possibly be coming from the rootstock?

Thanks for any experienced comments!!

Julia in Va.

I have an Improved Meyers Lemon that sends out new growth above the graft point vigorously with thorns. Only certain shoots do that. My Seedless Washington Navel does the same thing.

I have seen some other trees do the same thing. The original trunk slows down its growth and a new branch becomes the primary. I have had to lop off the top of some trees and let the more vigorous shoot take over. As long as it is above the graft point, you should be okay.

HOWEVER, sometimes wood above the graft point will take on some characteristics of the rootstock.
There is even something called a bud sport which is a natural mutation on a particular bud that differs from the host tree. Usually it is very similar to the grafted tree but has one particular different quality.