Iris: Crown Rot

Problem
The leaves of bearded and other irises grown from rhizomes (elongated underground stems) die, starting with the leaf tips and progressing downward. The leaf bases and possibly the rhizomes are dry, brown, and rotted. The leaves of Dutch and other irises grown from bulbs are stunted, turn yellow, and die prematurely. The leaves and stems at the soil level are rotted and the bulbs are soft and crumbly. White matted fungal strands cover the crown (where the stem meets the roots) and soil surrounding both rhizomatous and bulbous irises. Small, tan to reddish brown pellets the size of mustard seeds form on the infected plant tissue and soil.

Analysis
This disease is caused by a widespread fungus (Sclerotium rolfsii). It decays and kills the leaf and stem bases, bulbs, and often part or all of the rhizomes. Crown rot is spread by moving water, diseased transplants, infested soil, and contaminated tools. The fungal pellets can survive for many years in dry soil and extremes of temperature to reinfect healthy plants when conditions are suitable. Crown rot is most severe in overcrowded plantings, warm temperatures (70°F and up), and moist conditions.

Solution
Remove and destroy infected plants, bulbs, rhizomes, and the soil immediately surrounding them to 6 inches beyond the diseased area. Drench the area with a fungicide containing PCNB. Next year, repeat the soil drench at planting time and again when new growth is showing. Plant in well-drained soil with roots covered and rhizomes showing through top of soil. Thin out overcrowded plantings.