There are several leaf diseases on day lilies. One is anthracnose. but the most common one in the midwest is leaf streak. It is not fatal to the plants but does cause poor aesthics. I do not use any fungicides on my plants. I just let them die back every year. however if you want to spray every 10 to 14 days all summer, try captan, daconil 2787, maneb or mancozeb.
