White rug now has ugly stripe!!!

I washed my white bedroom rug and hung it outside over the deck railing as always. I forgot about it and left it out there for a few days. When I went to get it I found a yellow stripe the same size as the railing, (about 4 inches wide) across the middle where it was laying over the railing. The rubber side was touching the rail not the top. I think it is nylon and would appreciate any advice to fix it. I just washed it again with Rit white-wash but it didn't help at all. Meems