I would cover the root with a "Mound" of earth sloping away from the tree; no need to cover it more than an inch or two; but roots don't like light and exposed roots invite diseases and critters to chew them. If you post a photo of the trunk, we could give you a better idea of the graft/bud point. In general you can bury part of the trunk and sometimes it will even make the tree stronger/produce more roots from the trunk; but if you bury the bud union/graft, your tree will die prematurely.

I know exactly where the graft is - I can see it quite clearly. I'm not sure i can "mound" around the exposed root without reaching the graft point, since, as I said, the exposed part is UPHILL from the trunk (just a few inches), and the graft is just a couple of inches or less above soil level. But I will try and see if there is enough space to cover the roots so at least it's not right out in the air. Sorry no photo - it's a multi-step process to get it onto my computer, but maybe I'll try that next.