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Iris rhizome rot

A bacterial soft rot of bearded iris rhizomes, which turns firm tissue into a foul-smelling slime while the fan of leaves above it yellows and topples. The smell is the diagnosis — no fungal root rot in the catalogue produces it — and a rhizome can be saved if the rot is cut out before it reaches the growing point.

Caused by: Pectobacterium carotovorum · also called iris soft rot, bacterial soft rot of iris

What brings it on

Wet soil, deep planting and physical damage — slug wounds, careless forking, or a rhizome buried by mulch. Bearded iris rhizomes are meant to sit half exposed and bake; buried, they stay wet and the bacteria get in.

source

Sourced guidancehigh confidence

  • Royal Horticultural Society: Iris rhizome rot
  • Plant Finder — Missouri Botanical Garden: Iris bacterial soft rot

Checked 2026-08-13

Usually seen: May, June, July, August, September

Preventing it

Ordered by how much difference each measure actually makes, not by how often it is repeated.

  • Plant the rhizome with its top surface visible above the soil, on a free-draining site in full sun, and keep mulch away from it. This is the single condition the disease depends on.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Iris rhizome rot — prevention

    Checked 2026-08-13

Dealing with it once it is there

  • Lift the rhizome, cut every trace of soft tissue away with a clean knife until only firm white flesh remains, and replant the sound piece shallowly in a drier spot. Discard anything where the rot has reached the fan.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Iris rhizome rot — control

    Checked 2026-08-13

What it affects

On sprays and treatments

GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.