Daylily leaf streak is caused by the fungus Aureobasidium microstictum. Symptoms appear as small, reddish-brown flecks and brown spots that develop on infected tissue. You may then see a central yellow streak along the midvein that begins at the leaf tip. If severe, entire leaves can turn yellow. However, daylily leaf streak does not affect daylily flowers. Different cultivars will vary in how susceptible they are to this fungal disease. Nutritionally and water stressed daylilies are also more susceptible to this disease than healthy, vigorous plants.
Fungal spores are released when the foliage is wet and are spread by splashing water. Water early in the day so leaves can dry. Keep plants as well-spaced as possible to increase air movement around plants.
Fact Sheet: Daylily Rust and Daylily Streak