Iris flowers are perennial flowers, meaning that they come back every year, and they grow from either bulbs or rhizomes, depending on the variety. While busy gardeners may appreciate the fact that the flowers will come up in the same place each season, it also means that different kinds of iris diseases and pests can become established as well. Some iris diseases are caused by different kinds of fungal infections, such as fusarium wilt. Other diseases are the result of insect activity that causes eventual bacterial infections.
When planting iris bulbs, it is best to avoid spots that have had these flowers in them for the last few years, unless the purpose is to expand an existing, healthy flowerbed. Many kinds of iris diseases can live in the soil for years, and new plants can easily become infected if exposed to them. Picking the wrong area is likely to result in plants that wither and die soon after sprouting, though some may live long enough to bloom before they succumb.
Fusarium wilt is an example of a fungal infection that can plague flowers for many years once it gets into the soil. This problem typically infects the roots of the plant first, causing dead brown spots that appear sunken and are soft to the touch. The first visible symptom may be leaves that are stubby and yellow; soon after putting out these leaves the plant will die.
Another one of the iris diseases caused by a fungus is mustard seed fungus, also known as crown rot and southern blight. Plants develop a brown slime that covers the base of the flowers and leaves, and may spread out along the plant, killing both healthy and unhealthy parts of the plant. This often attacks iris flowers native to the Pacific coast and the multicolored bearded irises. Mustard seed fungus can effectively be prevented, but if it once gets established both the plants and the soil must be removed from the area to prevent further spread.
Some types of iris diseases are brought about by insect activity. Bacterial soft rot is one such problem and occurs when bacteria enter wounds in the plant that have been caused by the activity of the iris borer. The rhizome becomes infected along with the leaves, and the plant will emit a foul odor, often without any obvious reason. Closer inspection will usually disclose the tiny holes left by the iris borer, and the only solution may be to dig up and discard the entire plant, especially the rhizome.
Bacterial leaf spot is one of the iris diseases that is not necessarily fatal to the plant, but it can still cause significant damage. Large, wet-looking spots appear on the leaves, first at the edges and then spreading toward the center and turning brown, eventually changing to white as the leaf dies. Strict sanitation can help control the spread of this infection, as can the removal of any infected leaves. Leaves must be completely removed from the area and should never end up on the compost heap or near the edges of the garden.