Rhizome Rot is a bacterial disease of flag or German irises and arums. It is caused by the bacteria, Pectobacterium carotovorum. The first symptoms include yellowing and withering of the leaves and a soft, wet unpleasant smell as the rot spreads rapidly along the rhizome. Infection usually begins from injuries caused during cultivation or from attacks by slugs.
The disease is worse in wet conditions, this can be countered by planting in a well-drained soil. Plants that are affected should be lifted, the rotten areas cut out and wound should be treated with copper-lime dust before replanting in clean ground.