Southern Blight

What is Southern Blight?

Southern blight is a soilborne disease caused by the fungus Sclerotium rolfsii. This destructive pathogen can affect a wide range of crops and ornamental plants, leading to significant yield losses in severely infected fields.

Southern blight has a wide host range, infecting over 500 plant species. Some common host plants include:

Vegetables and fruits: Tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus, and parsley.

Ornamentals: Aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum, and viola.

Regions Impacted

This disease is predominantly found in tropical and subtropical regions, including the southern United States, Latin America, parts of Asia, and Africa. In the U.S., it’s especially problematic in southeastern states.

What are the Symptoms?

• Stem Lesions: The initial symptom is water-soaked lesions on the stem near the soil line.
• White Mycelial Mat: A white, web-like growth of the fungus appears on the soil around the base of the plant and on the stem.
• Sclerotia: These are small, round, tan to brown structures resembling mustard seeds. They form on the plant and in the surrounding soil.
• Plant Wilt: The affected stem portion becomes girdled, leading the plant to wilt and eventually die.

The first telltale signs of the disease are water-soaked lesions on the stem near the soil surface. As it progresses, a white, web-like growth (mycelium) of the fungus becomes evident on the soil surrounding the base of the plant and on the affected stem itself. This growth often engulfs the lower portions of the plant.

Fruit rots, crown rots, and root rots are also typical symptoms of the disease.

Integral to the disease’s life cycle are sclerotia, small mustard seed-like structures that are tan to brown. These structures form on and around the infected plant. As the fungus girdles the stem, it chokes off the plant’s nutrient and water supply, leading to wilting and, in severe cases, plant death.

The disease’s rapid spread, especially in conducive conditions, can lead to significant crop yield reductions. The presence of the distinctive white mycelial mat, coupled with the sclerotia, can help in diagnosing southern blight, though laboratory tests offer a definitive confirmation.

What Causes Southern Blight?

The disease is caused by the fungus Sclerotium rolfsii. The fungus survives in the soil as sclerotia, which can remain dormant but viable for several years. When conditions become favorable (warm and moist), these sclerotia germinate, producing mycelia that infect nearby plants.

How to Control and Prevent Southern Blight?

Controlling and preventing southern blight involves a multi-faceted approach, as the disease is resilient and can easily persist in soil and plant debris. Here are some measures to help manage southern blight:

• Crop Rotation: A rotating sequence of non-host crops can break the disease cycle and reduce sclerotia populations in the soil. Corn, sorghum, rice, or small grains (wheat, millet, or oats) are poor hosts of the pathogen.
• Soil Solarization: This method involves placing clear plastic over the soil during the hottest months, which increases the soil temperature, thereby killing the fungus. This is especially effective in regions with intense sunlight.
• Sanitation: Properly dispose of infected plants and debris by burning or bagging. Avoid composting, as the disease can survive the composting process.
• Deep Plowing: Turning the soil can bury the sclerotia, making it challenging for them to resurface and infect plants.
• Improving Drainage: Ensure that the planting area has good drainage to avoid prolonged soil moisture, which can facilitate the growth of the fungus.
• Resistant Varieties: Some plant varieties or cultivars have been shown to have a level of resistance to southern blight. Consult seed suppliers or local agricultural extension services for recommendations.
• Biological Control: Some beneficial microorganisms, like Trichoderma species, have been found effective in suppressing Sclerotium rolfsii in the soil.
• Mulching: Organic mulches can create a barrier between the soil and plant stems, making it more difficult for the fungus to reach the plant.
  • Pine Bark: This is a commonly used mulch, and its coarse nature allows for good aeration, which can make the environment less conducive to fungal growth. Additionally, pine bark may slightly acidify the soil, which could inhibit the growth of Sclerotium rolfsii, the fungus responsible for southern blight.
  • Straw: Straw can serve as a protective barrier, preventing soil from splashing onto plants, a common way for southern blight to spread.
  • Wood Chips: Like pine bark, wood chips allow for good soil aeration. Ensure they are from a disease-free source.
  • Composted Leaves: When fully decomposed, they can form a protective barrier. However, ensure they are well-composted to prevent them from being a source of infection themselves.
• Limit Irrigation: Overhead watering can splash sclerotia onto plants. Drip irrigation or soaker hoses, which directly water the soil, can reduce this risk.
• Monitor and Scout: Regularly check plants for signs of infection. Early detection can help in taking timely measures to manage the disease.
• Quarantine: If sourcing plants or soil externally, ensure they are free from the disease. Quarantining new plants for a short period can help in early detection before introducing them to the main garden or field.
• Chemical Control: Fungicides containing the active ingredient fluazinam have proven effective against southern blight. Apply as per manufacturer’s guidelines. However, the reliance on chemical methods should be minimal and used as a last resort.

Combining multiple strategies and ensuring regular monitoring can be effective in controlling and preventing southern blight. Moreover, staying informed about the disease’s occurrence in your region and networking with other gardeners or farmers can offer added insight into managing it.