Iris, the grande dame of summer gardens, is perhaps best known by its bearded varieties. Growing from 2 all the way through 40 inches tall, these graceful beauties have several distinct parts. There are typically three standards, or flag-like, upward reaching petals, and three falls that tumble down, lined with the famous fuzzy beard. Other types of iris include the beardless Japanese, Siberian and Louisiana Iris, as well as the Dutch Iris, which is popular in rock gardens.
Irises are susceptible to a host of foliage diseases and serious insect problems. With care and early attention, though, you can maintain the health of your iris population for continual summer color.
The most serious pest threat to this class of plants is Macronocutua onustairis, the iris borer. Determined moth larvae eat away at young foliage under a foot in length and enter the rhizome. Insects carry a bacteria with them that leave the plant highly susceptible to bacterial soft rot. Spring is the time to treat infected plants with a chemical spray specifically for borers. Remove all visibly infected plant materials promptly to stop the spread of the associated bacteria and reduce the need for insecticides.
Bacterial Soft Rot
Soft rot is typically introduced to the plant through disease carrying iris borers that infect the rhizome. Flesh of the rhizome, or thick finger-like root segments, will become pulpy, reduce to mush, and produce an unpleasant odor. To stop the spread of bacterial soft rot, dig up the infected rhizomes with clean tools. Cut out the infected flesh parts and dip the entire rhizome in a solution of 10 percent bleach. Allow the root to air dry, then re-plant and cut foliage back to below 6 inches. Dip tools in a similar solution after working with infected plant material.
Fungal Leaf Spots
A cosmetic but common ailment, leaf spots on the iris are caused by the fungus Didymellina macrospora. Deep brown spots with a watery yellow looking ring will morph into ovals with a red brown boarder if not destroyed. Leaf spot can spread easily from leaf to leaf in a plant, as well as to neighbors sharing the same space and touching foliage or where water is splashed between them. When foliage shows signs of infection, remove promptly and destroy.
Puccinia iridis, a fungus that causes rectangular or oval shaped, brown wounds. These infected areas can spread and eventually will damage or kill the host foliage if not treated. Avoid fungus spread with proper sanitation. Provide adequate space between plants for air circulation and to avoid water splashing the infection from plant to plant during irrigation or rain. Keep the area around the base of the plants free from dead leaf litter. Do not compost infected foliage as disease can live and over-winter on dead plant material.
Less than 1/8 inch in length, this soft bodied, sucking insect gets sustenance from the juicy flesh of iris foliage and stems. Aphids produce generations of winged and wingless spawn in large numbers through the entire growing season, varying from green to red, black, yellow, brown or even gray. Some female species and eggs can over winter, surviving to plague your garden next season. Aphids and eggs can be removed with a strong spray of water from a garden hose. Chemical treatment may also be applied in severe cases, but be sure the insecticide you choose lists aphids as a targeted pest and iris as a host plant for best results.