You shouldn’t have to dig them up and start over, though if you only have a few day lilies that is an option. Clean up any foliage out of the area as that’s how the fungus that causes leaf streak over winters. You should also replace your mulch to prevent it from harboring the fungus. Spraying with fungicides containing thiophanate methyl would also be helpful (sold as Southern Ag Thiomyl, Clearlys 3336, and others)