I use a product called Incredible Pink. I have used this amazing product for years on just about any tough stain you can imagine from the mildew in a boat to the toughest laundry stain. I have yet to have a situation where this stuff doesn't work. It is generally available at Aubuchon Hardware and other retail outlets but I have heard stories that isn't widely available.
It is made locally in Gorham Maine
http://chem-quest.com/incredible-pink.htm
BT