Iris, mainly the rhizomatous kinds, have fewer pests and diseases than any other significant group of ornamental plants. This fact, coupled with the ease with which Iris may be grown under widely varying conditions, largely accounts for their great popularity among home gardeners.
Occasionally a few insects and microorganisms attack them, particularly in old or neglected plantings, and control or preventive measures must then be practiced.
Public enemy number one of the iris is the borer, a pinkish caterpillar with a brown head that grows nearly 2″ inches in length before ending its feeding cycle. Inexperienced gardeners rarely detect it before it has hollowed out the rhizome and caused the leaves to wilt.
Borers get started in early spring (April) after hatching from eggs deposited the previous fall by moths on dead iris leaves and other nearby debris. The tiny caterpillars move up the green leaves and chew on the inner edges of the iris fans. They soon bury themselves in the folds of the leaves and eat their way down inside the leaves,
Finally reaching the rhizome. Due to the exuding drops of leaf sap, leaves infested with young borers have tear-stained or water-soaked markings.
Once in the rhizomes (early July), the caterpillars feed voraciously and increase in size rapidly. They then leave the hollowed-out rhizomes and bury themselves in the soil a few inches below the plant, where they change to brown pupae.
After about a month, the pupae wiggle to the soil surface and split open to release the adult (the moth stage).
Moths fly at night in late August and September. At twilight, they can be seen crawling around the base of iris plants. They are brown with black markings and have a wingspread of 2″ inches.
Water-soaked areas resulting from the mines of the young caterpillars in the leaves are good indicators of the presence of borers. Such areas should be squeezed between the thumb and forefinger, commencing at the leaf’s base and pulling upward to crush the young borers.
A second practice is to remove and destroy old iris leaves and other debris in the fall. This will eliminate many of the overwintering eggs.
Where extensive plantings are involved, pinching the tunneled areas in leaves is impractical. In such situations, it is best to protect the new growth with weekly applications of an insecticide like Malathion or natural Neem Oil.
The first application should be made after the first warm spell in April and repeated once a week until blooming. Because some of the tiny borers may hatch from eggs deposited on nearby debris other than iris leaves, it is wise to spray the areas immediately adjacent to the iris plants also.
Gardeners should remember that if the insecticide application is delayed until after the young borers have entered the leaves, the treatment will be ineffective.
In old, heavily infested plantings, the iris should be lifted in July or August, and all unsound portions and any larvae contained therein should be cut away and then destroyed (burning is the preferred method). Such material should never be thrown onto the compost pile.
The common stalk borer is the second pest that occasionally infests iris and many other kinds of plants.
This borer has alternating white and chocolate brown lines down the length of its body with a median girdle of solid brown. Unlike the iris borer, it is agile and moves readily from plant to plant. In May, it leaves other host plants such as ragweed and moves to the succulent iris flower stalks.
It climbs up the branch for a short distance, bores a hole, and enters. The frass this borer throws out from the entrance hole is a dead giveaway of its presence. The Malathion treatment is recommended for the iris borer will also control this borer.
The rose chafer is a long-legged, yellowish-brown beetle about one-third inch long. Occasionally feeds on iris flowers. It frequently appears in swarms rather suddenly in June or early July and continues to provide for several weeks.
Only a few iris are involved; the beetles should be hand-picked in the early morning when they are least active and smashed between a couple of 2×4s, although a shoe heel works great too! Where a large number of plants are concerned. Spraying with Malathion is sometimes effective.
Leaf spot is the most prominent, though not the iris’s most destructive, fungus disease. Brown, more or less circular holes appear mainly on the upper portions of the leaves from flowering time to the end of the growing season.
The fungus responsible for leaf spot overwinters in the old leaves. A crop of spores forms on these leaves in the spring to initiate infections on newly developing leaves.
Usually, this disease can be kept in check by removing the old infected leaves and cutting back the new leaves to 5″ or 6″ inches where the spotting is severe and persists, despite the above practices.
The gardener should spray at weekly intervals with zineb (trade names Dithane Z-78, Ortho-Zineb ) until the leaves are fully grown. The zineb can be mixed with the Malathion to control the leaf spot. And borers in one operation.
The most destructive and fortunately least prevalent fungus disease, crown rot, can destroy an entire planting of the iris when conditions for its development are favorable. The fungus invades the leaves near the soil line and quickly causes complete collapse and death of the top.
It produces small, round resting structures (sclerotia) which resemble cabbage seeds. They are creamy-white at first and cinnamon brown at maturity and form at the base of infected plants and on t.he surrounding soil. These structures are resistant to freezing, high temperatures, dry spells, and chemicals. For these reasons, the fungus is tough to eradicate.
Infected plants together with adjacent soil should be lifted carefully to avoid spreading any of the diseased material and then burned or buried deeply at a reasonable distance from the garden area.
Where the crown rot fungus is known to have been present, iris should be planted on ridges so that water drains away from the bases of the plants, thus avoiding an excessively wet condition at the soil line, which favors infection.
A soft decay of the rhizomes is caused by bacteria that enter through wounds made by borers and other agents. This disease is always most severe where the rhizomes are planted too deeply and where air circulation is poor.
When soft rot appears in an iris planting, drastic measures must be taken. First, severely rotted divisions should be destroyed. Choice ones may be salvaged by cutting out the rotted areas and thoroughly drying the sound portions before replanting. In replanting, allow the “back” of the root to remain above the soil. Controlling borers will go a long way towards preventing soft rot.
Among the less acute iris diseases are:
- Mosaic is caused by a virus that is spread by aphids.
- Bacterial leaf spot appears in late June after a showery spell or on plants grown in damp, shaded places.
- Root knots., caused by nematodes or eelworms, which form abnormal swellings on the roots.
This list of the pests and diseases to which iris are susceptible may appear formidable. As we said at the start, however, iris are less subject to pests and diseases than are most other garden plants. Rarely do more than one or two of these pests and diseases attack the plants at any one time. Best of all, nearly all of them can quickly and easily be brought under control.
1783 by NA