Southern Blight

Overview

Southern blight (Agroathelia rolfsii, formerly Sclerotium rolfsii), also known as crown rot, or southern stem rot/wilt, is a soil-borne fungus found across the world in tropical and temperate regions. The disease affects over 500 plant species, including herbaceous and woody ornamentals, vegetables, and fruit crops. Anemone, delphinium, hydrangea, impatiens, iris, narcissus, peony, phlox, pepper, rose, rudbeckia, tomato, tulip and more. Ornamental grasses and grain crops (corn, sorghum, wheat, etc.) are reportedly resistant to the disease. In the Midwest, the disease often affects hostas. Presence of southern blight generally requires the removal of the entire plant.

Symptoms and Diagnosis

One of the first symptoms of southern blight is a rapid wilting or flagging of leaves, often within days of infection. The disease causes deterioration and rotting of tissues at the crown of the plant, causing them to turn yellow, collapse and die. When the temperature exceeds 70°F, infected plants develop discolored, water-soaked stem lesions (cankers) near the soil line. Herbaceous/succulent plants are rapidly killed, but it may take a month or more before a small tree succumbs to the disease. In woody plants, the infection starts as a crown rot and will kill the bark within a few centimeters above and below the soil line. As the disease develops, a coarse, cottony webbing (mycelium) develops and fans out over the stem base and surrounding soil. Clusters of sclerotia, which resemble mustard seeds, form on the surface of the mycelium and are a key sign in diagnosing the disease. The sclerotia first appear white but eventually turn a light tan to reddish-brown color. Over time the mycelial mat will disappear, but the sclerotia will remain. 

Life Cycle

The fungus primarily survives and overwinters as sclerotia, which can persist in the soil and potting media for many years. Disease development is favored by temperatures between 86°F – 95°F and wet/humid weather. Both sclerotia and mycelia can be spread in landscapes via flowing water or transported through contaminated soil, infected plant material or tools.

Integrated Pest Management Strategies

1. Purchase disease-free material from reputable sources. To prevent introduction of the disease into your landscape, thoroughly inspect plant material, including the soil, for any signs or symptoms of the disease before planting. Resistant cultivars are not currently available, as the disease affects many species of plants.  

2. Remove infected plants and debris. The entire plant and leaf debris should be removed and discarded. Soil from at least 12 inches from the affected area should also be discarded. Do not place any material in a compost pile or yard waste container. Care should be taken not to spread any sclerotia present around the plant into other landscaped areas.   

3. Solarize the soil. Solarizing the soil using a clear, plastic sheet can help to reduce the viability of sclerotia. Summer is an ideal time for solarization, as temperatures must reach between 105°F to 120°F for 4 – 6 weeks. Studies have shown that solarization in conjunction with a fungicide application is more effective than solarization alone.  

4. Plant grasses in affected areas. Plant ornamental grasses or grain crops (corn, wheat, etc.) to reduce disease inoculum. Grasses are reportedly resistant to southern blight. 

5. Turn the soil over. Survivability of sclerotia declines with increasing soil depth. Any remaining soil in the affected area should be buried 8 – 12 inches.  

6. Improve air circulation and moisture levels. Avoid dense plantings and overwatering, as these conditions encourage disease development.  

7. Clean tools, gloves and shoes. Disinfect all tools used in an area with southern blight with a solution of 1-part bleach to 9-parts water. Be sure to rinse shoes and wash gloves to reduce the chance of spreading sclerotia into other garden areas.  

8. Apply Fungicides.  Fungicides with the active ingredient tebuconazole can be used to manage southern blight. However, it should be applied as a preventative, not a curative.  
 

Organic Strategies

Strategies 1, 2, & 4 - 7 are strictly organic approaches.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against crown rot. Not following the pesticide label before usage is a violation of federal law.