Apple Canker

I planted a row of six apple cordons three years ago, different varieties. They've grown well except for one, Sunset variety. This year the leaves were yellowish from leaf burst. It set fruit but they dropped off when very small. I recently examined it closely and it clearly had two bad areas of canker low down on the trunk, a few inches above the graft union. I've dug up and burnt the affected tree and sprayed the others with fungicide after the fruit was harvested. I plan to spray again at leaf fall.

I need advice about whether I can do anything else to protect the remaining trees. Can canker get into the soil? Will it be safe to set a new tree in this position during the winter? Any advice much appreciated.