Victoria Plum pruning

Just adding on from an earlier post really, I planted a Victoria Plum last year and a new shoot developed over winter, that shoot is further ahead than the main part of the tree, (flowered and leafed earlier), I naively thought it was a rogue plant that had hijacked the pot only when i dug down to remove it did i notice it was joined. Should I cut it off, its a good 3- 4 feet tall and probably about 3/4 inch diameter and if i do, should i paint where i cut it off, i read somewhere about a disease victorias can get and you should treat it with something if a branch snaps or anything. The extra shoot does look nice but i dont want it to weaken the main tree part.

 

If it is below the surface it's probably a sucker (unless you planted the plum below the graft point) and should be removed.
Try to tear it off rather than cut it as this will help prevent further suckers sprouting from around the cut.
If it is below ground I dont think you ned to worry about painting it.
The disease you mention is probably silver leaf and you are correct about painting breaks or large wounds.