Sclerotium rolfsii is a soil-borne fungus, a pathogen affecting numerous plant species worldwide. This fungus is commonly known by several names, including Southern blight, white mold, and stem rot, reflecting its characteristic symptoms. It causes severe damage to agricultural and ornamental crops, leading to crop losses.
Recognizing Symptoms and Affected Plants
Plants infected with Sclerotium rolfsii display visual cues, beginning with wilting and yellowing of the leaves. Symptoms include a brown to black rot that develops on the stem at or near the soil line. As the disease progresses, a coarse, white, fan-like fungal growth, known as mycelium, can be observed covering affected plant parts and the surrounding soil.
The pathogen infects a broad spectrum of plants, including common vegetable crops like beans, carrots, cucumbers, potatoes, and tomatoes. Field crops such as maize and small grains, along with ornamental plants like hostas and delphiniums, are also susceptible.
How it Survives and Spreads
The primary survival structure of Sclerotium rolfsii is the sclerotium, which allows the fungus to persist in the soil for extended periods, even in the absence of host plants. These hardy structures can remain viable for years within the soil or in infected plant debris. Sclerotia are the main source of inoculum and serve as the starting point for new infections.
The pathogen spreads through various means, including the movement of contaminated soil, infected plant material, and agricultural equipment. Water can also play a role in dispersing sclerotia across fields. Warm temperatures (typically between 25 to 35°C / 77 to 95°F), high humidity, and moist soil conditions are favorable for sclerotia germination and disease development.
Confirming the Presence of the Pathogen
Confirming a Sclerotium rolfsii infection relies on identifying specific visual signs. The presence of the coarse, white, fan-like mycelial mat at the soil line or on affected plant tissues is a key indicator.
The appearance of sclerotia further confirms the pathogen’s presence. These small, round structures, initially white, mature into light to dark brown bodies resembling mustard seeds in size and shape, typically ranging from 0.5 to 2 mm in diameter. Observing these structures on infected plant material or in the surrounding soil is a reliable identification method. For precise identification, laboratory diagnosis, such as culturing the fungus from infected tissue, may be necessary.
Control and Prevention Methods
Managing Sclerotium rolfsii involves an integrated approach combining several strategies to reduce disease severity and prevent its spread. Cultural practices include deep plowing to bury sclerotia at least 10-12 inches deep, which reduces their ability to germinate. Rotating crops with non-susceptible plants, such as maize or small grains, helps lower inoculum levels in the soil over time.
Improving soil drainage creates less favorable conditions for fungal growth, as the pathogen thrives in moist environments. Strict sanitation of tools and equipment, along with careful inspection of new plant material, prevents the introduction and spread of sclerotia to uninfected areas. Adjusting planting times to avoid periods of high temperature and humidity minimizes disease development.
Biological control agents, such as certain beneficial microbes, can suppress the pathogen’s growth in the soil. Chemical control options, including drenching transplants with appropriate fungicides, can be used as a preventative measure, particularly for high-value crops or in areas with a history of disease.