Day lilies (Hemerocallis spp.) are known for being generally disease-resistant, but they occasionally develop fungal problems in conditions of high moisture. Most of these diseases attack the crown and roots of the plant, and one of them attacks the foliage itself. No fungicides are specific to these ailments, and they should be treated using cultural practices. Occasionally, day lilies are attacked by a bacterial soft rot. To avoid these diseases, maintain well-draining soil, and divide day lilies as needed to prevent overcrowding. Day lilies grow in U.S. Department of Agriculture plant hardiness zones 3 through 10.
Leaf streak is caused by Aureobasidium microstictum, the most common foliar fungus. Dark green, soggy spots appear on infected plant leaves, which turn yellow, then red-brown and leave a streaking pattern down the center of the leaves. Leaf spot or scorch, caused by the fungus Colletotrichum, is less common but similar in appearance. Leaf spot causes streaks of bleached or browned tissue down the center of the leaves. For both, water at the base of the plant to prevent spores from traveling through the water that drips from the leaves. Remove infected leaves, and cut back foliage during the winter to keep spores from overwintering. Do not compost infected plant material, which can spread through compost.
White mold is a fungal disease that causes the crown and lower leaves of the plant to rot. A white, cottony growth called mycelia covers the crown and soil areas surrounding the plant. This fungus remains dormant in the soil for several years and is difficult to control. Avoid infections by maintaining good cultural practices that prevent high-moisture conditions. Remove and throw away diseased plants and all soil within a foot of the plant. White mold affects many plants, so choose only plant types that are immune to white mold fungus.
Rhizoctonia Crown Rot
Rhizoctonia crown rot is a fungal disease that occurs during especially wet years and in poor-drainage conditions. This disease causes the crown of the plant to rot and a dark, fuzzy fungal growth to appear near the soil surface. Plants can recover from this disease once conditions improve. Move plants to a location with better soil drainage to improve plant health.
Bacterial Soft Rot
This disease, caused by various soil bacteria, results in areas of soft, mushy plant tissue and foul odor. The bacteria enter through plant wounds and openings near the plant crown. It spreads quickly in moist conditions and can kill an entire clump of day lilies. Severely damaged plants should be thrown away. If a plant is only partially affected, cut the infected portion off and discard it. Allow the cut to dry, then plant the healthy portion of the plant in an area that has good drainage.
Rust is caused by the fungus Puccinia hemerocallidis and results in bright yellow-orange streaks on the top side of the leaf and bright spores on the underside of leaves. This fungal disease spreads rapidly and should be contained using cultural practices and fungicide. Remove and throw away all infected leaves to prevent the spread of the fungus. Apply fungicides of flutolanil, azoxystrobin, mancozeb or chlorothalonil weekly, according to label instructions, until all symptoms of rust are gone.
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