What happens if you put off road diesel in a diesel tractor engine?

Depends...by "diesel tractor" do you mean "farm tractor" or "semi tractor"?

You're SUPPOSED only to put off road diesel in a farm tractor. That's why they make it.

If you have put it in a semi tractor, get hold of your dispatch/brief/lawyer right away for advice on what to do.

If you have a pre-2007 engine your trouble is almost exclusively legal. If your engine is 2007 or 2010 compliant, you're probably going to screw up your EGR and diesel particulate filter, which will break your engine.