Daylily leaf streak is a common fungal disease caused by Aureobasidium microstictum. Symptoms begin as chlorosis along leaf midveins, often starting from the tip and moving down the leaf. Necrotic tissue follows. Small, reddish brown flecks or spots develop in this tissue. The effect is yellow and brown streaks and specks on the leaves. Daylily cultivars vary in susceptibility to streak, but generally the most severe result is streaking and death of infected leaves. Fruiting bodies of the fungus are difficult to see even with a hand lens but appear as small white spots on either leaf surface. The fungus develops most quickly when temperatures are warm but not hot. Look for daylily leaf streak in susceptible daylily beds now. It spreads by splashing spores or spores spread on animals (including us). To avoid spread of this disease, try to irrigate the soil rather than the foliage, and avoid working with wet plants. Also try to keep plants thinned to improve air movement. No fungicides are listed specifically for this purpose, but general use fungicides could work as preventives where this problem is chronic.
Daylily rust became a problem in the United States in 2000. It is a fungal disease caused by Puccinia hemerocallidis. Daylily cultivars reported as susceptible to rust include Attribution, Pardon Me, Gertrude Condon, Crystal Tide, Colonel Scarborough, Starstruck, Joan Senior, Imperial Guard, Double Buttercup, and Stella D’Oro.
Daylily rust causes yellow to brown streaks on the leaves, much as with daylily leaf streak. Rust causes raised pustules on either surface of the leaf, but especially the underside. The yellow–orange to reddish brown pustules produce abundant spores that rub off when touched. Resistant varieties may produce only yellow flecks. There are a few very good Web sites that discuss daylily rust and show pictures of the disease and pathogen. A site to start with is the daylily rust information page, which has links to many other sites and includes photos of both rust and leaf streak. You can enter “daylily rust information page” in your search engine to find this starting point.
The big concern over daylily rust is that it spreads very rapidly (new infections arise in 2 to 3 days on the more susceptible cultivars), and daylilies have become one of the most popular and widespread perennials in the Midwest. We do not believe daylily rust kills entire plants, but it may kill infected foliage on susceptible plants. If found, infected foliage should be removed and fungicides considered to protect new foliage.
Although daylily leaf streak and daylily rust look similar, there are no pustules or rusty spores with leaf streak. Aphid feeding can cause similar symptoms. Try rubbing the streaks with your finger or a piece of white paper. Rust leaves an orange streak. Use a hand lens or send a sample to the Plant Clinic for identification. A positive ID is done by examining leaf tissue with microscopes to find the fungal fruiting bodies and spores. Although live tissue is best, even dead foliage is fine in this case because the fungus remains on dead or dry foliage. The Plant Clinic has not yet received a positive daylily rust sample in 2004.