Most daylilies are sturdy plants that require little attention to thrive given enough sunlight and water. Daylily leaves may turn yellow as the result of too little or too much water and several more serious problems.
Daylilies are undemanding in terms of fertility, most requiring little or no fertilizer during their lifetime. If, however, they are planted in sandy or clay soil and leaves begin to turn yellow, it may indicate they need more organic matter in the soil to provide nitrogen for growth.
Yellow-streaked leaves that turn brown and die may result from an infection of Collecephalus hemerocalli or Aureobasidium microstictum fungi. Leaf streak responds to a fungicide, but daylily rust, a recently discovered infection caused by the Puccinia hemerocallidis, does not. It begins with a central yellow streak on the leaves and soon progresses to rusty spots along the length of the plant.
Root Knot Nematode
Root knot nematodes are parasites that attack plant roots. Early symptoms resemble those of lack of moisture, according to Clemson University. Leaves turn yellow and the plant becomes stunted before wilting and dying because the parasite is destroying the plant’s roots. Nematodes are more likely in sandy soils.