An added note to the helpful gardeners reply:
If the rootstock sucker is a new shoot it should snap off cleanly by hand. If the sucker is woody it will have to be pruned off. If you cannot remove all of the sucker, carefully scrape off the bud node from the rootstock. Best to use a knife to do this and take your time. When done properly you will see a slight concave dent in the root stock. Avoid damage to the rootstock in the process. If the bud node is not removed it will keep suckering. Check for suckers on a weekly basis and remove them while young and soft. Suckers can quickly sap the strength from a rosebush
and rob you of blooms.
Should you want more specific information contact the American Rose Society at www.ars.org On the home page is a link to ask questions. It links you to rose growers in your state.