Tenant says plumbing is backing up; plumber says it is roots in the sewer line. Line has been roto-ed as best as possible. Plumber says we 'may' need to remove the tree and replace the line. 'May'?? What are the names of the product(s) that can be put into the line to kill/destroy the roots? How effective are they? Will it only attack the roots in the line or will it kill the tree too? Advice is appreciated.

Going in, let me say that sooner or later you're going to have to bite the bullet and replace that old sewer line with PVC. The very first thing that must be done is to send a snake with a cutter tip,(see image) to cut and clear the line. Then If you wish to control roots RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Hope this helps Tom

Truthfully said, and based on many years of experience, once you have roots in your sewer line, they are there to stay, and prosper, as long as you live in the house!

Roto-Rooting and putting "root killer" agents into the line is only temporary solution. The only solution is to excavate your main and replace it with new one. Once you installed new ABS or PVC sewer, tree and its roots won't matter...

Sewer replacements are very expensive work. So, you may do two things:

1. Learn to live with it. Roto-clean your sewer once a year preventively and you should be fine - maybe even twice a year if you have serious problem- and forget replacing sewer.

2. Hire plumber and do sewer video inspection. Stand by the plumber's monitor and see the roots in the sewer. Ask him to give you recorded CD of the inspection. Take that CD and show it to other plumbers for "second" opinion before you commit yourself to replacement work.