Beautiful Iris are Healthy Iris… Keep Them That Way Part #2

In part 1 on Beautiful and healthy Iris we talked about pest control on Iris now we will look at diseases on flowering Iris plants.

Iris Diseases

Leafspot is the most prominent, though not the most destructive, fungus disease of iris. Brown, more or less circular spots appear mostly 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 ) at least until the leaves are fully grown. The zineb can be mixed with the malathion to control leaf spot. and borers in one operation.

The most destructive and fortunately least prevalent fungus disease, crown rot, can destroy an entire planting of 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 (selerotia) which resemble cabbage seed.. 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 t.he fungus is very difficult 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 good 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 which 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 important iris diseases are: mosaic, caused by a virus which is spread by aphids; bacterial leaf spot, which appears in late June after a very rainy spell or on plants grown in damp, shaded places; and root knot., 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.

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