Shatoot

Manfred says...
That's an interesting situation. It sounds like a minor graft incompatibility, with the scion being too vigorous for the rootstock, though it could be the other way around. In those situations, the question needs to be asked - why graft a vigorous scion like that at all. Shahtoot grows well enough on its own roots, and (as far as I know) all mulberries grow well and readily from cuttings.

If I am right (toss up a coin) the plant will be weak and unreliable, liable to break either in a storm, or just when it can't carry its weight any more, on a graft the scion is trying to overgrow. It may even be trying to grow roots into the rootstock as an opportunistic parasite.

If it is only a small juvenile, I'd be taking a cutting from the scion as insurance. Keep it in a pot and if the expensive master fails plant the simple, cheap replacement.

Alternatively, since they do grow so redily from cuttings, if it is small enough, replant it with the graft at or below ground level. Read up on lilac production for a rundown on how that works.