Daylilies do not generally have serious disease and pest problems in the north. The few diseases they do have can be easily controlled by proper culture. Always pull summer mulch away from the crown at least an inch in early spring and again in late fall to prevent bacterial and fungal infection of the crown when sudden freezes damage the daylily foliage and crown. If chemical treatment is necessary, use a soil drench fungicide labeled for ornamentals. It should be applied twice about ten days apart.
Some serious growers use a soil drench fungicide applied in late fall as a preventative.
Watering your plants early in the day when it is humid will prevent leaf spot fungi. Pests include aphids, thrips and spider mites. They are rarely a problem in the north. If they are, chemical sprays labeled for the particular pest and ornamentals will control them. In all cases, follow the directions on the pesticide label.