The following is information on daylily leaf streak, a disease of daylilies now present in the landscape.
Daylily leaf streak has made a real resurgence during this hot, humid, rainy period the last couple of weeks. The foliage will get diffuse brown spots and develops into long steaks of dead tissue bordered by yellow tissue. The old dead leaves will make the plants look unsightly until the weather changes and new healthy leaves replace them later in the month. This is a fungus disease caused by Aureobasidium microstictum. Avoid damage to the leaves, which is one of the ways it enters the plant. Avoid highly susceptible varieties and limit overhead irrigation. Fungicides can be helpful if applied preventatively (before symptoms appear). I have had success with some of the triazole fungicides such as myclobutanil (Systane, Eagle, Immunox) and Banner MAXX.
Daylily leaf streak. Photo by Tom Creswell, Purdue University.
Information from Bob Mulrooney, Extension Plant Pathologist, UD