Iris: Bacterial Soft Rot

Problem
Leaves turn yellow, wilt, and eventually die. Dieback often starts at the leaf tips and progresses downward. The entire leaf cluster (fan) may be found lying on the ground. If pulled gently, the leaf fan sometimes lifts off the rhizome (the elongated, jointed, underground stem). Leaf bases and rhizomes are often rotted and foul smelling.

Analysis
This plant disease is caused by a bacterium (Erwinia carotovora). It is a serious and common disease of bearded and other rhizomatous irises. The bacteria enter the plant through wounds in the leaves and rhizomes, which are frequently made by iris borers. As the infection develops, the plant tissue decays into a soft, foul-smelling mass. Finally, the plant dies and the inner rhizome tissue disintegrates. Infection and rapid decay are favored by moist, dark conditions. These bacteria live in the soil and in plant debris. They are spread by contaminated plants and rhizomes, soil, insects, and tools.

Solution
Remove and destroy all diseased plants; they cannot be cured. Discard diseased rhizomes before planting. If only a small portion of the rhizome is infected, it may possibly be saved by cutting off the diseased portion. Avoid wounding the rhizomes when digging them up. After dividing rhizomes, let the wounds heal for a few days before replanting. Plant irises in a sunny, well-drained location. Plant the rhizome shallowly, so the upper portion is exposed. Clean up plant debris in the fall. Control iris borers.