Iris Diseases

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Iris Diseases

Scorch:

A disease that slowly kills Iris plants. They slowly turn brown over a time period, with the rhizomes remaining hard. A prevention from death can involve cutting the rhizome into sections. Thus an infected clump will usually have one part survive. It is necessary to dig up the clump to examine it closely.

The Iris borer can also eat out a clump and the leaves will also turn brown from the tips down. This can be quite fast as the borer is in the rhizome before this effect is noticed and it proceeds quickly. If you dig up the plant you find a chewed out area (usually quite large) and sometimes the borer is still present. Soft Rot can also cause similar problems. In this case when you dig up the plant you will find a mushy plant, or what remains of the rhizome.

A recent suggestion for cure of scorch involves heating the plant to 104 degrees F. for a few days. This seems to kill the scorch, leaving the plant unharmed. This can be done with a covering over the plant, but care must be taken that the temperature doesn't get too hot and cook the plant.

Borer:

An insect that can be very damaging to the Iris. Not found in western Canada or the U.S.A. In mid and eastern sections of Canada and the U.S.A. it can be very devastating.

The moth lays it's eggs late fall and they overwinter on the leaves. They hatch in early spring, around the time the daffodils come into bloom. They are initially near the top of the leaves, usually where the leaves separate from each other. They gradually chew their way down the plant and three weeks after the TB's stop blooming, they are in the rhizomes. About the end of August (here in southern Ontario) they have left the plant and are in metamorphosis.

Several chemical treatments are effective. Cygon 2E (or 240E as it's sometimes labeled) is very effective. This is a systemic poison. This means it is absorbed into the plant to be digested by the borer and it finishes it off. If the growth of the plant is too advanced the poison is not absorbed into the foliage and is not effective. The rate of application is 2 ml per liter of spray. Take lots of precautions as this is a nasty chemical. Malathion is also supposed to be effective, but I haven't tried it.

The organic method is to hunt and squeeze. Chew marks are often seen at the place where the leaves separate from each other. By opening the folds you can often trace the chewing damage until you find the borer. They can be removed and disposed of. This can also involve squeezing the borer between the leaves.

Leaf Spot:

This is an annoying and unsightly condition involving small brown spots on the leaves. This is a fungus that, though unsightly, does little damage to the plant. A spray of Benelate early in the season can help with this, although it is often necessary to repeat several times in the season. Often the easiest way to deal with it is to just remove the unsightly leaves and dispose of them (but not by putting them in the compost). An increase of nitrogen in the fertilizer can often be helpful, but too much nitrogen can make the plant more susceptible to soft rot.

Soft Rot:

This is most often see in the spring. An inspection at that time would be advisable. Any rot should be removed. I use an old tablespoon that has its edge sharpened. Then I spray the cleaned part with a mix of half javex and water. Other possible sterilizer/cauterizer are Lysol and powdered sulphur.

The method of dealing with this in warmer climates involves digging out the plant, cutting out all the rot, and leaving the rhizome to dry in the sun. For us in Canada, particularly southern Ontario, the springs are cold and cloudy so this method doesn't work (no hot sun to dry out the rhizomes). This can work later in the season when the weather is warmer and there is more sun.

2005