Learn About Iris Leaf Spot

Iris leaf spot is the most common disease affecting iris plants. Controlling this iris leaf disease involves specific cultural management practices that reduce the production and spread of spores. Wet, humid-like conditions make the ideal environment for fungal leaf spot. Iris plants and the surrounding area can be treated, however, to make conditions less favorable for the fungus.

Iris Leaf Disease

One of the most common diseases affecting irises is fungal leaf spot. Iris leaves develop small brown spots. These spots can enlarge quite quickly, turning gray and developing reddish-brown edges. Eventually, the leaves will die.

Moist, humid conditions are favorable for this fungal infection. Leaf spotting is most common during wet conditions, as rain or water splashed on the leaves can spread the spores.

While infection of iris leaf spot generally targets the leaves, it will occasionally affect the stems and buds as well. If left untreated, the weakened plants and underground rhizomes may die.

Treatment for Iris Plant Fungal Leaf Spot

Since the fungus can overwinter in infected plant material, removing and destroying all diseased foliage in the fall is recommended. This should significantly reduce the number of surviving spores come spring.

Fungicide application may also help following the removal of infected plant material. Severe infections may require at least four to six fungicide spray treatments. They can be applied in spring to new plants once they reach about 6 inches high, repeating every seven to 10 days. Adding ¼ teaspoon of dishwashing liquid per gallon of spray should help the fungicide stick to the iris leaves.

Also, keep in mind that contact fungicides easily wash off in rain. Systemic types, however, should remain active for at least a week or two before reapplying.

Iris Leaf Spot

Leaf spot is a common disease of iris, especially rhizomatous species. It is caused by the fungus Didymellina macrospora. The first evidence of the disease is the appearance of small yellow-brown spots on leaves that are surrounded by a water-soaked border. The spots enlarge, turn brown and dry. The older spots are surrounded by a dark reddish-brown border. Large dead areas may occur when spots enlarge and merge and leaves may be killed prematurely. The disease is most severe in mild, damp weather, especially in sites where air movement is poor and diseased leaf debris has been allowed to accumulate.

Sanitation is important in controlling iris leaf spot. Remove all diseased leaves and flower stalks in the fall or early spring. (The causal fungus survives the winter on infected plant parts.) Space plants adequately, keep down weeds, and avoid wetting the foliage when watering. Do not work among plants when the foliage is wet. Plant iris in full sun in a rich, well- drained soil.

Fungicides are effective where cultural practices fail to check the disease. Fungicides such as Daconil 2787 or Bordeaux mixture can be applied at seven to ten day intervals to protect young growth. A spreader sticker or surfactant can be used to make the fungicide adhere to the foliage.

This article originally appeared in the August 12, 1994 issue, p. 127.

How to identify and Control Iris Leaf Spot

Iris Leaf Spot

While tidying up my iris garden I noticed that many of the leaves of the German iris had a spongy look and bore light green, yellow or brown watery spots. Some leaves had large brown-black watery areas and other leaves were dead. Unfortunately I have an Iris Leaf Spot problem this year and other people may too. We have had a great deal of rain lately with temperatures on the cool side making a perfect environment for the growth of the fungus (Mycosphaerella macrospora) that causes this disease. Severe infection gradually reduces bloom and weakens the plants by destroying the leaves that produce food for growth. In addition, infected plants are more likely to be infected by other pests and diseases. Irises are not alone in being infected by this fungus; daylily, gladiolus, freesia and narcissus can also be infected so control at this point is important for more than just my irises.

Knowing something about the life cycle of this fungus makes it easier to understand how to control it. The fungus, Mycosphaerella, exists as a minute thread-like mass (called a mycelium) when inactive during the winter or periods of unfavorable growth but produces two different kinds of spores that are spread by air currents and splashing rain when conditions are right; dampness with rain and temperatures between 50F and 77F (68F optimum).

Controlling the infection involves doing all you can to reduce the production and spread of the spores. You can’t control the weather but you can do some things to make the effect of the weather less favorable for the fungus and this may alleviate the problem enough so that you don’t have to take more drastic steps like spraying with a fungicide.

Use resistant varieties or other kinds of iris, like Siberian iris which are less prone to leaf spot.

Plant iris in full sun, in well drained soil, with good air drainage and divide the clumps often to keep them open. If soil is acid, add lime.

Remove all dead foliage and flower stalks during the year so that air can freely circulate and keep the leaves dry being especially diligent in the Fall. Remember, the fungus over winters on the dead foliage. Never put iris foliage on the compost pile because the fungus will remain in the debris ready to infect plants. Either burn it or put it out in your refuse bin.

Ditto for infected leaves or parts of leaves during the growing season.

Avoid working with the iris when wet and infected as you will spread those nasty spores.

If the infection is severe, spray the iris with fungicide. Start when the fans are 4-6” tall and repeat every 5 to 10 days. Use a contact fungicide (Daconil 2787 or Dithane both containing chlorothalonil) to knock down the infection that is already present. Use a systemic fungicide (containing propiconazole) to help make the plants more resistant to new infection. Combine the two fungicides and add a wetting agent (1/4-1 teaspoon per gallon) to overcome the waxiness of the leaves and really soak them. The contact fungicide is removed by rain but the systemic is not and remains active for 7-10 days. Adjust your spraying routine to the weather. Ask at your local feed store or garden center for specific brands as availability varies with states.

Radiation is one of the most effective means of killing microbes, but it has been used clinically to treat only cancer. That may be about to change. For the first time, researchers have turned radiation therapy against infection.

With antibiotic resistance on the rise among bacteria, medical researchers are looking beyond traditional drug therapies. One possible source of new weapons is radioimmunotherapy (RIT), a technique used to treat cancer patients. RIT works like radiation treatment, but instead of blasting tumors with radiation from the outside, individual radioactive atoms are injected into the blood and circulated throughout the body. The trick is to attach the radiation-emitting atoms to antibody proteins designed to latch onto the surface of the tumor cell. This ensures that tumor cells become coated with the RIT molecules while normal cells are spared from the DNA-blasting radiation.

To see if RIT could be useful against an invading pathogen, Kate Dadachova, a biologist at the Albert Einstein College of Medicine in New York City, and colleagues designed antibodies to bind to the surface of Cryptococcus neoformans, a tenacious fungus that plagues AIDS patients. In a paper published online this week in the Proceedings of the National Academy of Sciences, the team shows RIT’s mettle as a fungus fighter. When infected mice were injected with the antibody alone, they died just as quickly as untreated mice. But when radioactive bismuth-213 or rhenium-188 atoms were hitched onto the antibodies, the infections began to clear up and mouse survival increased up to 60% by the end of the trials. And the best news is that RIT doesn’t seem to kill bone marrow, a side effect that has limited the use of RIT in cancer therapy. Given this initial success in mice, Dadachova believes that any kind of infection can potentially be treated with RIT.

“The data clearly indicate that RIT can be effectively used to treat fungal infections,” says Gregory Adams, an oncologist at the Fox Chase Cancer Center in Philadelphia. Adams cautions, however, that there could be difficulty scaling the technique up from mice to humans, and there could be long-term tissue damage from the radiation.

Related sites
Adams’s RIT research
All about radiation therapy

Why Are My Iris Yellowing And Dying?

Answer #1 · Gardenality.com’s Answer · Hi Mary,
Sounds like your Iris plants either have a disease or have Iris Borers.
If it’s a disease it sounds most like bacterial soft rot. This disease is perhaps the most common found in Irises. The disease usually finds its way in by entering small holes made by Iris Borers, or through other damaged areas of the foliage. The symptoms are slow dying and yellowing of the leaves starting from the top of the foliage, and a soft greenish yellow or brown decay at the base of the foliage. This rot slowly moves to all of the rhizomes…roots.
The only method of control for bacterial soft rot is to catch it early. If spots and/or yellowing leaves are present immediately remove, selectively, those leaves. If the disease has reached the rhizomes at and below the ground level, remove the entire plant and all of it’s rhizomes/ root system and discard.
Iris Borers enter the rhizomes and slowly eat them away. The only way to check for these borers is to dig up the rhizomes and inspect them. If holes in the rhizome(s) are present then you will want to break open them open. If you see the nasty slimy critters inside you’ll need to remove all rhizomes that they inhabit. You can save and re-plant the rhizomes that have no borers present.
If you find neither borers or disease try fertilization with a high content of phosphorus. Phosphorus is the middle number in the three number analysis on most fertilizers. Example:10-10-10. The middle number is phosphorus. Many flower foods contain a high amount of phosphorus. Phosphorus promotes rapid root development in turn producing more foliage, more buds, and more blooms. Sounds like maybe your Irises need some of that. You could also get some Triple Super Phosphate (0-46-0) if you can’t find any other high phosphorus fertilizer. If you don’t understand what I’m referring to stop by your local independent nursery and they should be able to hook you up with the right fertilizer product(s).
Hope this helps you.
Brooks Wilson))

Iris borers turn lovely plants into yellow, slimy messes

Question: I have a lot of bearded iris in my garden, and they bloom beautifully in the spring. But then by the time July rolls around, the leaves get yellow and slimy. There are brown patches on the leaves and the entire plant smells bad. Do I have iris borers? If so, how do I get rid of them?

Answer: Though bearded iris are gorgeous spring bloomers, there’s no doubt that an infestation of iris borers can turn this beautiful plant into an icky mess by late summer. It does indeed sound like your plants are being attacked by iris borers.

The caterpillars of a species of moth (Macronoctua onusta), iris borers feed only on the leaves and rhizomes of iris plants. Adult moths fly around the garden at night, seeking their host plants. When a female moth finds an iris plant, she lays eggs on the leaves, usually in August or September. The tiny eggs sit on the leaves all winter long, waiting until the following spring to hatch.

Soon after hatching in early spring, the miniscule caterpillars burrow into the newly emerged leaves and spend weeks feeding inside of them. Over the course of the spring and early summer, the caterpillars make their way down toward the base of the plant. By the time July arrives, the borers have reached the rhizomes and they begin to feed on these fleshy roots.

The yellow, slimy leaves covered with brown patches are the result of their feeding. Eventually, these damaged leaves will turn completely brown, shrivel up and fall off the plant. Severe infestations can kill entire rhizomes and impact the following year’s flower production.

The unpleasant odor you detect when working with borer-infested iris is the result of the rhizomes rotting after the borers have tunneled through them. The smell is much like that of rotten potatoes or onions.

When early August arrives, the caterpillars are mature and ready to pupate. They exit the rhizomes and form an underground chrysalis. They pupate into a new generation of adult moths a few weeks later. Soon after that, they begin to breed and lay eggs on more iris foliage.

The simple trick to controlling iris borers is to cut the plants down completely in late fall and clean the trimmed leaves out of the garden entirely. Because the eggs overwinter on the foliage, it’s important to get all the foliage out of the garden. Cut each leaf all the way back to the rhizome and toss it into the garbage. Do not put the leaves in the compost pile. This process is best completed soon after we get a few hard frosts and the adult moths have stopped laying any more eggs.

Because your iris plants are already infested with borers, you can also dig them up and destroy any borers you find inside. Use a pitchfork or shovel to lift your iris clumps out of the ground and crack apart the rhizomes, seeking out the fat, pink caterpillars housed inside. Toss any soft or rotted rhizomes right into the trash and replant only the healthy ones. When you find a borer, squish it or put it into a jar of soapy water (our chickens are very fond of eating them!).

If you neglect to trim back the foliage in the fall, you can also apply beneficial nematodes (species Heterorhabditis or Steinernema) to your iris plants in the spring. These microscopic roundworms tunnel around the plant, seek out the immature borers and kill them. They typically come as a granular product that’s mixed with water and sprinkled over the plants. It’s best to do this when the iris leaves are fully grown but the plants have yet to develop flowers. You can buy these beneficial nematodes from online sources such as Planet Natural (planetnatural.com) and Peaceful Valley Farm Supply (groworganic.com).

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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