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One of the most beautiful perennials is the iris. It provides spectacular blooms, but it does have disease problems. The unusually wet weather of the past few weeks is the perfect condition for iris leaf spot. This fungus has caused severe outbreak of leaf spot across the state this spring. Spores produced in the spring are rain splashed onto new iris leaves where they begin the infection process. Leaf wetness, coupled with high humidity alows infection to occur. Overcrowded plants that need to be divided develop the disease earlier in the season, and often with greater severity.

Symptoms first develop as small brown spots with water soaked margins. Although you may not see much at first, soon after blooming, the spots quickly enlarge. This can cause death of the leaf from the tip back to the rhizomes. As the spots become larger they tend to become oval-shaped and lose their water-soaked margin. As the lesions age, these spots turn a yellow to reddish-brown with gray centers.

Premature killing of the leaves can weaken the rhizome causing the gradual death of the entire plant. Although infection is usually confined to the leaves; stems, flower stalks, and buds may become infected during severe outbreaks of this disease.

Plant irises in full sun and space plants at least eight inches apart. Providing iris with adequate spacing improves air circulation around leaves, allowing them to dry more quickly, preventing disease development.

This fungus overwinters in dead iris leaves infected the previous year, removing and destroying diseased leaves in the fall will reduce the amount of fungal spores available for infection the following spring.

Under severe infections a fungicide spray program is recommended for the following spring to prevent the disease cycle from repeating. Recommended fungicides include those containing chlorothalonil (Daconal 2787). Also one can use macozeb or dithane, or thiophanate-methyl.

If you have a problem keeping the fungicide on the foliage, add a commercial sticker or household liquid detergent to each gallon of spray to aid the fungicide in adhering to the plant.

The iris borer is a serious problem for iris growers. The borer hatches from eggs laid on leaves in early spring. A spring spraying of Orthene or Lygon when the leaves are five to six inches tall has been the usual defense. A sign of iris borer activity could be the premature yellow and brown foliage on the entire plant. When you lift up the iris rhizomes, trim away rotted portions or discard the entire plant. The iris borer causes a bacterial soft rot that eventually kills the entire plant.

One of the newer solutions to the iris borer problem is nematodes. They attack the borer as well as other pests such as squash vine borer and cut worms. Nematodes can be bought at large garden centers or ordered through the mail.

August is the time for planting new rhizomes and dividing those that have been in place for three or four years. If your iris did not bloom as well as they used to, the problem could be improper soil, not enough sun, or poor soil drainage. These problems can be correctly by moving the plants to a better site or improving the soil.

Use a garden fork to lift iris clump carefully. Use a hose to wash them. Have a sharp knife to trim the rhizomes, keeping plump, firm outer rhizomes with fans of leaves attached. Discard the woody center portions. Trim the leaves back to about six inches, then plant the iris rhizomes about eight inches apart in a sunny, well-drained location. They can go right back into the same spot if disease-free before.

Submitted by,

Jerry Vitalis

Chisago County

Master Gardener

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Ways to access

information

www.extension.umn.edu/county/chisago check out the “Hot Topics” box in the middle of the page for current Chisago County Master Gardener news and events.

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For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org.

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