You may have read something recently about a "new" rust disease on daylily that is spreading rapidly in the United States. The rust is daylily rust, discussed in issue number 15 of this newsletter. We've talked about rusts before on many landscape plants, and they were never considered major problems, so wha's the big deal?
Daylilies have not been plagued with any serious disease or insect problems in the past. They are easy to grow and have been bred to provide hundreds of selections that thrive in our gardens. Many growers have become daylily collectors and have invested considerable time, plants, and space to develop splendid displays of these plants. Along comes a rust disease that causes yellow to brown spots on leaves, poor plant vitality, and leaf death in susceptible varieties. In addition, this rust can infect in 2 to 3 days, and spores are spread easily by wind and water. The pathogen can move easily between nurseries because spores can be present on leaves and tubers of symptom-free plants. The good news is that other species in the garden are not hosts of this rust. It is found on daylily and a perennial called Patrinia<, but so far there have been no reports of rust on Patrinia in the United States.
Since my first article on this disease, the Plant Clinic has confirmed three cases of daylily rust in Illinois. It appears that all three cases involved plants that had been ordered by mail from out of state. Illinois Department of Agriculture inspectors have not seen daylily rust in Illinois nurseries. The general prediction is that this rust will soon become fairly common in our gardens, but meanwhile we need to do our best to keep it contained. Inspect new plants carefully. Because plants could be infected but not show symptoms, consider isolating new plants until you are certain that rust is not present. If rust appears in your daylilies, remove and destroy infected foliage. Consider using a fungicide to control this rust.
There is currently no fungicide that specifically lists daylily and rust on its label. It is likely that chemical testing will provide more information on efficacy in the next year. Meanwhile, the Florida Department of Agriculture and Consumer Services makes the following recommendation to Florida commercial nurseries and shippers of daylilies. They say to protect new growth by applying alternately two of the following four fungicides at the label rate and interval: propaconizole (Banner Maxx), azoxystrobin (Heritage), flutolanil (Contrast), myclobutanil (Systhane).
I have two concerns with this chemical information. One is the availability of the products to home-owners. The names in parentheses are commercial trade names of products containing the active ingredient listed. These products may be used by home-owners, but commercial products usually come in large packages at considerable cost. Propaconizole is available in a homeowner product called Fertilome Systemic. Myclobutanil is available in a homeowner product called Immunox. The other two chemicals are not available in homeowner-sized packages.
My second concern is the suggestion of alternating two products. Many of the new systemic products (all of those suggested) are more effective than protective contact fungicides because they are systemic and because they have very specific ways in which they stop the fungus. When they are used repeatedly, the fungal pathogen (rust) may develop resistance to the fungicide, making it ineffective. The recommendation of alternating two products is intended to try to prevent fungicide resistance from developing. Modes or sites of action of chemicals are given names. Propaconizole and myclobutanil are DMIs, azoxystrobin is a STAR, and flutolanil is an Oxathiin (not understood). These labels categorize the fungicide according to how it works on the fungus. Because propaconizole and myclobutanil are both DMIs, rotating them will be ineffective. Nothing is gained when a homeowner alternates between Fertilome Systemic and Immunox. As more information develops on management of daylily rust, we will pass it along to our readers. Meanwhile, keep an eye on the daylily rust Web pages referred to in issue 15 of this newsletter.