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Iris Rhizome Rots

June 1, 2005

Iris flowers have been beautiful this season in Illinois. Recently, however, we have had reports of plants, or individual leaves on plants, showing an abrupt decline and death. Leaves wilt and die from the tips, moving toward the base. If you pull on the affected leaves, they are often rotted at the base and easily pull from the plant.

One possible cause of this decline is bacterial soft rot. The causal bacterium is Erwinia carotovora. This bacterium is commonly found in soil and plant debris in our landscapes, and it helps speed up plant decomposition. It needs a wound to enter a plant. Iris soft rot is often associated with the wounds provided by iris borers. For information on the iris borer, refer to issue no. 2, 2003, of this newsletter. Once the bacterium enters the leaves, it causes rotting of the leaf and the attached rhizome, leaving nothing but the outer shell of the rhizome intact. Soft rot bacteria have a distinct foul smell, something that is hard to miss.

Clean up your iris beds to get rid of the soft rot bacterium. In Illinois, it is best to do this in late July. For now, remove dead foliage and mark you calendar to finish the job in late July. In fact, most rhizomatous iris plants should be divided every 3 to 5 years to reduce the incidence of soft rot. The idea is to remove unhealthy plant material and thin out the planting to make room for new growth. Dig and lift the iris clumps from the bed and then, using a sharp knife, separate the rhizomes. To avoid spreading the soft rot bacterium, dip the knife in a bleach solution (1 part bleach in 10 parts water) before each new cut. Only replant firm, healthy rhizomes with roots and a fan of leaves. The rhizome should be just slightly exposed on the soil surface when planted correctly. Deep planting also causes rhizomes to rot.

Occasionally in Illinois, iris plants are affected by a fungal disease that causes similar symptoms. This disease is Sclerotium crown rot, caused by Sclerotium rolfsii. This fungus is more common in the South, but we see it where mulch has been used around plants over winter. Symptoms appear as they do with bacterial soft rot except rhizomes are brown, soft, and crumbly. A white mycelium of fungal strands is evident in the crown; and tan to reddish brown, mustardseed-sized fungal structures (sclerotia) can be found on the soil surface. These structures help the fungus survive adverse conditions. These structures can also be moved with the plant or soil. Beds infected with Sclerotium rolfsii require special help. Remove infected plants from the area. Also remove surrounding soil, putting it directly into a bag to get it out of the area. Be careful to avoid spreading sclerotia to other areas in the garden. Chemical options are available to be used as preventives but can not control this disease on their own. Consult the Home, Yard, and Garden Pest Guide or the 2005 Commercial Landscape & Turfgrass Pest Management Handbook for options.

Author: Nancy Pataky


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