Well, don't ignore it (if you want to keep the car). At the minimum, you should go after it with rust converter. Go under there with a wire brush & rust converter. It may take considerable scraping to find out how extensive the rust is. I would usually expect to coat it with rust converter and keep checking it to make sure it doesn't spread. After some point, it would be body shop time.
Note that a lot of body shops hate rust converter--they think it makes their job harder.