I think that the tree is trying to replace it's diseased trunk by sending up new replacement trunks from its
roots. As the old trunk dies, it's ability to transport
energy from the roots is constricted, so that
root energy must make a new way to reach for the sun.
I would consider allowing the root suckers to grow and see if they will succumb to the same disease. You could either leave the old trunk to die of its own, or use a sterilized pruning saw to make an angled heading cut two feet below the lowest canker. I would remove the diseased
wood and burn it.
If the new leading stems also get the same disease, then I would try to grow a genetically different individual. Either from seed, or a cutting from a different plant. you might even consider planting seeds from this diseased tree.
these are just my thoughts, hope this helps.