The Iowa Gardener: How to treat dreaded daylily leaf streak

In daylily leaf streak is caused by the fungus Aureobasidium microstictum. Symptoms start with yellow or reddish streaks on the leaves, usually starting at the tip and then work downward. Eventually, the infected leaves wither and die completely. (Michigan State University)

This time of year, my daylilies (Hemerocallis spp.) are looking pretty sad. The leaves, which this spring were so pretty and green and fresh-looking, are now streaked with yellow and brown and many, especially near the ground, are completely brown and dead.

I have loved daylilies for many reasons, but top among them was that they have been extremely disease resistant. Daylilies are one of the few perennial flowers that require almost no tending, which is why you see them growing wild in ditches or in country gardens without any additional water, weeding or fussing.

As with any fungal disease in plants, daylily leaf streak is worse in wet years and usually less bad in dry years. (University of Minnesota Extension)

But about 10 years ago, I noticed that a couple of weeks after my daylilies were done blooming, the leaves started to turn yellow and then brown, dying out altogether. It was so bad that by the end of August, I could gather up handfuls of dead brown leaves and pull them out.

It turns out the culprit is called daylily leaf streak, caused by the fungus Aureobasidium microstictum. Symptoms start with yellow or reddish streaks on the leaves, usually starting at the tip and then work downward. Eventually, the infected leaves wither and die completely.

As with any fungal disease in plants (there are many), the problem is worse in wet years and usually less bad in dry years. But even though this year has been dry, I have been consistently watering my daylilies, which I now realize encourages the problem, since fungal diseases thrive in damp conditions.

But that’s not the only reason. Other gardeners in my area who do not water their gardens have been having similar problems. I’m also intrigued that still other local gardeners say they have had this problem for decades, as long as they’ve been growing daylilies, and others say it’s a new problem to them. All of which suggests to me that some gardens have not been infected by the fungus and others have. Most likely, my garden got infected about a decade ago.

My garden is pretty extensive, and I have several dozen if not hundreds of daylilies. So it’s worth it for me to put considerable effort into minimizing this problem. But I’m realistic, too. I probably will not be able to cure it completely, since fungal diseases are so prevalent in Iowa with its ample rainfall and warm, humid summers.

To minimize leaf streak in daylilies, follow the same tried-and-true techniques that prevents fungal disease in other garden plants:

Remove infected leaves as best you can by pulling or trimming them off. Do not put the leaves in the compost pile or elsewhere in the garden since that just spreads the disease further. I put mine in a yard waste dumpster I rent from my local waste management company, which they come and pick up once a week. I find that with some of my daylilies, this means pulling out almost the entire plant, which is OK. They will come back fine next year, or in some cases, even regrow into a tidy little tuft by fall.

Do not water overhead with a sprinkler, wetting the leaves, unless you have few other options. Instead, water the soil directly with a soaker or a drip hose or a drip emitter system.

If you must water overhead, do so in the very early morning hours, before it gets hot and the water from the sprinkler evaporates (as much as 50 percent on hot, windy ways). Unlike watering at night, morning watering allows leaves to dry off quickly to make it harder for fungus to take hold.

Avoid working with daylilies when they are wet, like in the morning when the dew is heavy or after a rain. You’ll likely spread the pathogen from plant to plant.

It may help to thin out daylily plantings. I have masses of daylilies, planted closely together. This fall, I’ll be digging many of them up to make more room between plants, which allows better air flow and penetration of sunlight that prevents fungal growth.

In the fall, cut back daylilies to the ground and dispose of the leaves somewhere else than your garden. This prevents the fungus from overwintering in your garden.

Because I am having ongoing problems with leaf streak, I also will be seeking out those daylily cultivars that are resistant to the fungus. Certainly, I have noticed that some types of daylilies in my garden are more badly affected than others. Resistant varieties include: Betty Bennet, Edna Spalding, Ella Pettigrew, Globe Trotter, Nancy Hicks, Pink Superior, Ron Rousseau, Sudie, Tropical Tones, Upper Room, Winsome Lady.

The problem is bad enough that it is disfiguring my daylilies to an extent that I am considering applying a garden chemical. I try, as much as I absolutely can, to avoid synthetic pesticides, herbicides and disease controls in my garden. Not only do I think they have a negative effect on beneficial birds and insects and the environment, I hate the expense and hassle.

I will ignore some of the home remedies that are not based on science and research. But university horticulture extension services recommend, if you choose to treat the problem, to apply fungicides containing thiophanate-methyl or myclobutanil.

Fungicides containing thiophanate-methyl include Cleary’s 3336-WP Turf & Ornamental Fungicide and Southern Ag Thiomyl Systemic Fungicide. Fungicides containing myclobutanil include Ferti-lome F Stop Lawn & Garden Fungicide, Monterey Fungi-Max, and Spectracide Immunox Multi-Purpose Fungicide.

As with all fungicides, the time to apply them is not later in the growing season when you can start seeing the damage and streaking. It’s in the early spring, before the fungus has a chance to get started. You’ll minimize environmental harm if you follow product directions exactly, measuring carefully and using proper equipment and following disposal directions. Generally, however, most of the products instruct you to begin spraying as soon as new growth appears, and making a few repeat applications, probably three or four, at two-week intervals.

I’m excited at the prospect of getting rid of this disease in my garden. Daylilies have been my great joy, brightening up the whole yard in July and then retaining a pleasant, green, grasslike shape until fall, when the leaves have turned an attractive straw yellow. I look forward to enjoying their monthslong beauty once again.

Veronica Lorson Fowler is co-publisher of The Iowa Gardener website at theiowagardener.com.