Irises are a beautiful and versatile flower that can be grown in many different climates. In Minnesota, planted iris bulbs are susceptible to a variety of pests and diseases. The most destructive insect pest is the iris borer (Macronoctua onusta), which cuts holes in the leaves and tunnels through them. The most serious disease is bacterial soft rot, which causes rhizomes to become soft and smelly.
Other common diseases include flower or leaf blight, crown rot, rhizome rot, leaf spot, and mosaic virus. To prevent the spread of disease, it is important to remove yellowed leaves right away. In the fall, all leaves and other debris should be removed as pests and diseases often overwinter in old foliage. Aphids, iris borers, and iris thrips may also be present.
Irises come in a variety of shapes and sizes, from low ground covers such as Iris cristata, which is only 6 inches tall, to large Japanese irises that can reach 3-4 feet tall. The Nordic Blue Flag (Iris versicolor) is a member of the iris family that is more commonly found in the wild than in gardens. When planting irises in your garden, consider adding them in the middle of a perennial garden so that plants that bloom later can hide the iris foliage. Irises tolerate most common garden soils and are among the easiest lilies to grow in most regions.
The tall bearded iris has wavy edged petals or other ornaments more often than other groups of irises. Bearded irises are susceptible to several diseases, including bacterial soft rot, iris burn, and fungal leaf spot. All of these diseases cause the tips of the leaves to turn brown and the plants to weaken, similar to the symptoms caused by iris borers. Siberian irises are more tolerant to attack from iris borers than other types of irises.