Cause Sclerotium delphinii, a fungus. The sclerotia overwinter in soil or affected plant parts. Infection usually occurs on the stems near the soil surface. Disease development and spread can occur very rapidly under mild to warm, moist conditions in the field or in storage after digging. The disease is widespread and can be serious. In the Pacific Northwest, crown rot is more common on bulbous iris, tulips and lilies than daffodils.
Symptoms Rhizomatous iris-gradual dieback at tips of outer leaves caused by rotting at the leaves' base, near the soil line. The disease progresses toward the clump's center; inner leaves show dieback, and the plant dies. The fungus does not infect rhizomes. White, fan-like mats of fungus commonly are between the rotted leaf bases. Light brown sclerotia usually are on the rotted leaf bases.
Bulbous iris-affected plants are yellowed, stunted, and die prematurely, from rot either of the bulb or of the stem and leaves just above the bulb. Bulbs may appear soft and crumbly or fibrous. Outer leaves usually turn yellow before inner ones. Masses of soil usually cling to infected bulbs when dug. Sclerotia sometimes are on the bulb and in the soil.
Early infection of fleshy bulb scales appears as a soft, white, cheesy rot with a sharp margin but with no color change from a normal bulb. Sclerotia often are on or between the scales of dry bulbs. Bulbs may be rotted completely with nothing but the husk and sclerotia left. Infection usually occurs at the bulb tip but may be elsewhere. Under warm moist conditions, the fungus can spread to adjacent bulbs in the tray.
Cultural control
- Do not let iris become overcrowded.
- Discard all infected bulbs.
- Wash equipment and machinery between fields.
- Early digging before the warm temperatures of summer and rapid drying of harvested bulbs has been helpful.
Chemical control
- Heritage at 4 oz/100 gal water plus a non-silicone-based wetter sticker as an in-furrow spray over the top of bulbs just prior to hilling. Group 11 fungicide. 4-hr reentry.
- Moncut 70-DF at 1.63 to 6.53 lb/A as in in-furrow spray directed over the tops of planted bulbs prior to hilling. Do not use spray adjuvants. Washington only, SLN WA-040032B. Group 7 fungicide. 12-hr reentry.
- Terraclor 400 at 3 pints/3.2 gal water. Before planting, soak bulbs for 5 min. Add 1% sticker and maintain good agitation. Group 14 fungicide. 12-hr reentry.
- Soak clean bulbs for 2 to 3 hr in hot water plus formalin. (Soak 2 hours for most cultivars but 3 hr can be used for the cultivar White Wedgewood and its sports, 'Ideal' and 'Dominator'). Use Aldesan Microbiocide at 2 fl oz/gal of water and heat to 110°F to 111°F. Change dip tank water when it becomes discolored or when sediment is present. Dip dank wastewater may be applied to bulb fields when rain is not expected for 24 hours. Washington only. SLN WA-030004B. Formaldehyde Solution 37 is also registered with SLN WA-030004A.
- Preplant soil fumigation with metam-sodium.
Reference Chastagner, G.A. 2000. Potential alternatives to PCNB to control the development of crown rot and gray bulb rot on bulbous iris. In VIII International Symposium on Flower bulbs 570:301-306.