Iris (Iris)

Plant Health Problems

Diseases caused by Fungi:

Leaf spot, Didymellina.
This common disease is usually found on bulbous iris. Characteristic symptoms appear as irregular, tiny brown spots with yellow margins scattered over the leaves. When infection is heavy, the leaf spots coalesce and appear as streaks in the leaves. These enlarge rapidly after flowering and the leaves may turn completely brown and shriveled. Although the fungus does not invade the rhizome, successive infections which destroy the leaves can severely weaken bulbs and rhizomes.

Management of this disease includes good sanitation since the fungus overwinters on old leaves. Once frost has killed the tops in autumn, leaves should be cut and removed from the planting bed. Efforts to maximize plant vigor by fertilizing and watering are helpful. It is important to avoid overhead irrigation or to water early in the day. Good spacing to avoid crowding is also effective. Although not usually necessary, except in very wet seasons when infection can be serious, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut are chlorothalonil, thiophanate-methyl, and mancozeb. Consult the label for dosage rates and safety precautions.

Rhizome rot, Botrytinia convoluta.
Symptoms are recognized when plants fail to grow in the spring. When the rhizomes are dug, they appear rotted but don't have a foul odor. Gray, fuzzy mats may be present along with shiny, black sclerotia on or in rotted tissues.

Prevention is the most effective control for this disease. Roguing and removing infected plant parts or rhizomes is necessary.

Diseases caused by Bacteria:

Soft rot, Erwinia carotovora.
This is the most common problem of iris in Connecticut. This bacterium attacks leaf bases and rhizomes and is easily identified by its foul odor. The bacteria gain entrance through wounds, usually those made by iris borers. Infection first appears as water-soaked margins around the borer hole in the leaf. These progress as water-soaked streaks down the leaf. Once in the rhizomes, the bacterial rot progresses rapidly. The characteristic foul odor is one of the most diagnostic symptoms.

The most effective control of this disease focuses on control of the iris borer. Additionally, setting the rhizomes shallow is helpful since this permits the sun to get to part of the rhizome. Crowded or shaded situations should be avoided since they are conducive to development of soft rot. Well-drained soil is better for iris than heavy or poorly-drained soils. Sanitation is also very important. Cleaning up all plant debris on which the borer eggs overwinter in the fall or very early spring will cut down materially on borers. Leaves with early symptoms of disease can be cut well below the water-soaked areas and must be done before the bacteria reach the rhizome. All equipment should be disinfested between cuts with 10% household bleach, 70% alcohol, or one of the commercially available compounds.

Leaf spot, Xanthomonas.
Symptoms appear as translucent, irregular spots on the leaves. These are dark green and water-soaked during wet weather. When many spots develop, they coalesce and the entire leaf appears to be involved. This symptom is often confused with Didymellina leaf spot. The bacterium does not invade the rhizomes as is the case for soft rot.

Sanitation is very important to minimize the impact of this disease. This includes cleaning up all plant debris in the fall after the tops have been killed by frost. Leaves with early symptoms of disease can be cut well below the water-soaked areas. All equipment should be disinfested between cuts with 10% household bleach, 70% alcohol, or one of the commercially available compounds.

Diseases caused by Viruses:

Mosaic, Iris mosaic virus.
Symptoms appears as light and dark green mottling of leaves, early flowering, and color breaking in flowers. Plants often develop a general stunted appearance and bud sheaths may be marked with bluish-green or yellow streaks. This virus is transmitted by aphids.

The most effective controls involve roguing and destroying infected plants since the virus persists in these. It is also important to control aphids since they transmit the disease.

Insect Problems:

Aphids.
Aphids occasionally infest iris but cause little injury. If they are abundant, a spray of malathion or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, will kill them. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.

Iris borer, Macronoctua onusta.
The larval stage of this insect is pinkish with a brown head. It tunnels in the rootstock of German iris, injuring or destroying many plants. The larva reaches a length of about 1 1/2" in late fall and pupates in the ground. The adult moth emerges in October. The moth has a wing spread of about 2" and is brown with black markings on the forewings. The females lay eggs on the iris leaves, which hatch in the spring and the young larvae tunnel in the stems and soon find their way into the roots. There is one generation each year. In early spring, the leaves may be gathered and burned, if allowed, to destroy the eggs. Spraying with malathion or acephate, which are among the compounds registered for control of this pest in Connecticut, once a week during April and May may kill the young larvae before they bore into the plants. Consult the labels for dosage rates and safety precautions.

Iris weevil, Mononychus vulpeculus.
This insect breeds in the seed pods of blueflag iris (Iris versicolor) and may be found in European, Japanese, and native cultivated iris. It creates many small holes in the seed pods. Eggs are deposited in the iris ovary. The larvae feed on the seeds. The adult weevil is black and 1/5" long. It emerges when the pods open. One generation occurs each year from overwintering adults. Removal and destruction of all flower heads as soon as the blossoms wither should reduce the infestation.

Lesser bulb fly, Eumerus tuberculatus.
The maggots of this insect occur in the rhizome or root of weakened plants. They are secondary invaders as they generally feed on injured or decaying tissue, some of which may have been caused by the iris borer. The maggots are a dirty white color and about 1/3" long. The adult resembles a housefly in appearance.
Control may be expected when infested rhizomes are cut out and destroyed.

Iris thrips, Iridothrips iridis.
Small, milky-white larvae feed on the inner surface of leaf sheaths and on the young leaves of many varieties of iris, causing a rusty or soot-like blackening. The adult is a glistening dark brown and overwinters in the crowns of the iris. Spraying with insecticidal soap, malathion, or spinosad, which are among the compounds registered for use against this pest in Connecticut, will give satisfactory control. Consult the labels for dosage rates and safety precautions.

Slugs, Limax maximus.
Garden slugs, sometimes infest greenhouses, as well as moist, shaded outdoor gardens. These molluscs feed mostly at night, eating notches in the margins or interior of tender leaves, leaving a slimy, iridescent trail wherever they crawl. During the day, they hide under rubbish.

Slugs can be controlled by lightly cultivating the ground in the spring to destroy dormant slugs and their eggs. A band of diatomaceous earth put around newly planted geraniums will control slugs by rupturing their epidermis. Placing a small board in the flowerbed for the slugs to hide under during the day makes it easy to destroy many at one time. A bowl of beer, sunk into the ground, with a roof to protect from sun and keep large animals out, will act as a bait and the slugs will drown. If necessary, chemicals such as metaldehyde, which is among the compounds registered for use against this pest in Connecticut, will control slugs. Consult the label for dosage rates and safety precautions.

Twospotted spider mite, Tetranychus urticae.
This pest infests the leaves, which become light yellow in color, and the plants have a generally unhealthy appearance. Sometimes the mites form webs, which more or less enclose the leaf. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give sufficient control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1/2 - 1% dilution) can be effective if predatory mites are present. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Abamectin is an effective restricted use product. Consult the label for dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to the predators than to the pest spider mites.