In our southern part of the country, the Louisiana iris lives relatively disease and insect free. I have recently heard about possible insect problems in the Northeast, but as a south Louisiana grower, we do not take any special precautions against disease or insects. However, since the literature reports remedies to certain countrywide problems, it is prudent to share this knowledge.

On occasion, Louisiana irises encounter problems with leaf miners, cutworms, or a fungus disease known as iris rust. Cutworms are greenish-brown caterpillars that can damage tender new growth in short order. Many general purpose insecticides will repel these rascals, as will fertilizers which are fortified with insecticides.

Leaf miners spread their larvae in a weblike fashion in the leaf tissue. Active during the growth season, their channels are readily visible. Several new systemic insecticides are very effective against the leaf miner, but they may require a license to obtain and special precautions in their use.

Iris rust is the foremost problem on my farm. It is a fungus disease which shows itself as patches of reddish brown, powdery deposits on the foliage. Some varieties are more susceptible than others, and some years it does not appear at all. My personal opinion is that as the iris explodes into growth the month before bloom, this rapid growth makes the iris foliage susceptible to this fungus. Over-fertilizing with nitrogen may accelerate the problem. When feeding your plants, avoid placing fertilizer directly on top of the rhizomes. Irises grown in water are not as susceptible to the fungus; I suspect this is because the foliage growth occurs over a longer period of time due to heat retention in the water. In any event, the fungus doesn’t hurt the bloom or kill the plant — it just mars the appearance of the otherwise deeply green and lush foliage.

Less frequently the grower may find problems with the iris borer or leaf spot. The iris borer is not usually a serious problem in the Gulf South, but where they are planted in colder climates near the bearded irises, the borer may be a problem. Professional help may be needed if your garden has this problem, but the new systemic insecticides containing dimethoate are reported quite effective. You may try an October and a springtime application (after heavy mulch is removed) of "Cygon."

Leaf spot is a fungus disease that produces small tan or brown spots in dot-like patterns on the leaves. It is not prevalent in Louisiana like its look-a-like rust. The best way to control the fungus is to remove the leaves by cutting them about two inches above the ground and then burning the cut foliage. The rhizome should not be adversely affected.