Crown rot, sometimes called southern blight or southern stem rot, is caused by several soil-borne fungi. It affects herbaceous plants and some woody plants but is most commonly found on ajuga, anemone, campanula, chrysanthemum, delphinium, hosta, hydrangea, iris, narcissus, phlox, rudbeckia, scabiosa, sedum, and tulip. The problem generally requires the removal of the diseased plant.

Symptoms and Diagnosis

Crown rot causes deterioration and rotting of the tissues at the crown of the plant causing the leaves to turn yellow, collapse, and die. When the temperature exceeds 70 degrees F, infected plants develop discolored, water-soaked stem lesions near the soil line. During periods of high humidity, coarse cottony webbing (mycelium) develops and fans out over the stem base and surrounding soil. Sclerotia, which resemble mustard seeds and vary from white to reddish tan to light brown in color, develop at the base of the plant. Enough sclerotia may form to create a crust on the soil.

Life Cycle

The fungi that cause crown rot (Pellicularia rolfsii, Sclerotium delphinii, and Sclerotium rolfsii) survive in the soil and are spread by flowing water, transported or contaminated soil, transplants, and tools. Conditions of 86–95oF for several days with intermittent rains are conducive for fungal development.

Integrated Pest Management Strategies

1. Remove diseased plants as soon as they are noticed. Plants can be buried, but do NOT place them in your compost pile.

2. Excavate the surrounding soil. Dig out and replace the soil to a depth of 8 inches and 6 inches beyond the diseased area.

3. Solarize the soil. If you do not remove the soil and the area receives at least two to three hours of direct sun, solarize it. Cover the area with clear plastic and leave it for two to three months in the heat of the summer.

4. Provide better drainage. Increasing the organic content of the soil and improving drainage will make the environment less desirable to the fungus.

5. Sterilize all tools. Clean all tools used in digging with a solution of 1–part bleach to 9–parts water to disinfect the tools and reduce the spreading of the disease to other locations in your garden.

6. Try fungicides. Pesticides registered for control of crown rot include mancozeb and thiophanate methyl (Cleary 3336).

Organic Strategies

Strategies 1, 2, 3, 4, and 5 are strictly organic approaches.