I have a cherry tree that is about 2 years old. Strangely, the width of its trunk above the grafting joint is much thicker than that of the rootstock (mazzard) below and I have not seen such a variation in any of my other young trees. Should I be concerned that this might be a weak spot as the tree grows bigger? Will this even out eventually or is there something I can do to "correct" it?
I'm not an expert, but this suggests to me that the twig/ bud that was used for the graft (the scion) was taken from a far more vigorous variety than the rootstock, and therefore their growth rates are incompatible or, perhaps, that the graft was not well done (see photo).
I doubt that you can do anything to "correct" it. If the tree crops heavily, it would be advisable to give it some support, to compensate for the increased strain on the union which is almost certainly weak.