Southern Blight - a disease becoming more prevalent in Missouri

Published: August 10, 2017

Southern blight, a disease caused by the soil-borne fungal pathogen Sclerotium rolfsii affects many vegetables, ornamentals, and agronomic crops. Some estimates suggest it can infect up to 100 plant families and 500 species. Southern blight causes damping off if the pathogen infects seedlings. Other synonyms of the disease include southern wilt, southern stem rot and Sclerotium stem rot. Because of the warmth-loving nature of S. rolfsii, the disease is most common in the southern climates (and hence named southern). However, it also affects a wider range of crops in other regions during seasons or months with higher-than-normal temperatures. In Missouri, the disease was seen in the Southwest region few years ago and it is anticipated to be frequent in the south eastern part of the state too where the average temperature is warmer. It usually occurs sporadically in other parts of the state. However, the recent unusual trends of warmth have been leading to frequent incidences of the disease, especially in protected production systems.

The typical diagnostic feature of the disease on susceptible hosts such as tomatoes and peppers is the wilt symptom. Brown-to-black lesions appear near the soil line or around the lower stem and crown areas. Under moist conditions, abundant white mycelium that often has a distinct fan-shaped pattern will develop on or around these lesions. If conditions continue to favor infection, a tan to reddish-brown 0.04-0.08 inch (1-2 mm) diameter, spherical sclerotium (plural sclerotia) forms on the mycelial mat. Mature sclerotia have hard rinds which protect the cortex and viable hyphae inside. Fruits with a sunken or ruptured epidermis coming into contact with the fungus, often develop a fruit rot.

Sclerotia can be disseminated by the movement of infested soil, contaminated tools and machinery, irrigation water, or plant material. Key environmental factors that favor the fungus and disease development are high temperature, aerobic, and moist conditions and acidic soil. Germination of sclerotia occurs in the range pH 2-5 but is inhibited at pH higher than 7 implicating southern blight less of a problem in calcareous soils with a high pH.

What can be done to control southern blight?

• Avoid fields with a southern blight history. Prevention is critically useful to avoid this disease. At the planning stage, select fields (or tunnels or beds) that are free of any southern blight inoculum (mycelium, sclerotia or any infected plant residue).
• Sanitation. Good sanitation involves early detection and removal of all diseased (symptomatic) plants and crop residues from the field or protected systems. Make sure no symptomatic or diseased transplants are introduced into the field or greenhouses/tunnels. Once the disease is seen in a tunnel or field, get protective gloves, boots, and disinfectants ready to avoid further spread of the disease.
• Crop rotation with non-host plants. Crop rotation is the next best management practice, also during the planning stage. Grasses including corn and small grains are non-susceptible and hence useful to reduce the inoculum level. However, it is worth noting that this pathogen has a wide host range. The other challenge with rotation is that the mustard-seed like brown-to-orange colored sclerotia, once they are formed, stay in the soil for long periods. Thus short ending rotations are ineffective. If rotation is an option, wait to come back with the same crop (family) until after 5-6 years have passed.
• Deep plowing. Plowing as deep as 8 inches or more and burying infected plant debrisgreatly helps reduce incidence of southern blight as the fungus causing southern blight (S. rolfsii) is highly aerobic.
• Best management practices. This includes procedures such as wider plant spacing, modifying the planting dates in a way to avoid wet and warmer temperatures (disease development is highly favored at temperatures 77 - 95F), avoiding injuring plants during cultivation, raising soil pH by liming, and use of physical barriers such as aluminum foil or plastic bags to protect the stem at the soil line (for home gardeners) or plastic mulch in commercial fields and tunnels. Addition of straw mulch to the soil has also been effective for reducing the disease incidence.
• Ammonium source of nitrogen. Using ammonium instead of nitrate has resulted in a reduced incidence of the disease.
• Biological control and solarization. Soil solarization followed by the introduction of Trichoderma harzianum has resulted in better control of the disease than either of them practiced alone. Other beneficial fungi such as Penicillium spp. and Gliocladium virens and the bacterium Bacillus subtilis have also showed good efficacy for controlling southern blight.
• Fungicides. The only chemistry mentioned for southern blight in the 2017 Midwest Vegetable Production Guide is Terraclor® whose active ingredient is pentachloronitrobenzene (PCNB). For crops like peppers and tomatoes, this fungicide can be applied as a transplant solution or applied as an in-furrow spray to the soil. Read and follow the labels for the rates, recommendations and safety precautions.

Before applying ANY product, read the label (1) to be sure that the product is labeled for the crop and the disease you intend to control in your state, (2) to take the necessary safety precautions and application restrictions, and (3) to make sure that the product is listed in your Organic System Plan and approved by your certifier (organic growers).

Trade names in this publication are used solely for the purpose of providing specific information. Such use herein is not a guarantee or warranty of the products named and does not signify that they are approved to the exclusion of others. Mention of a proprietary product does not constitute an endorsement nor does it imply lack of efficacy of similar products not mentioned.

Fig. 1. Results of in vitro studies at Lincoln University showing tan to brown colored sclerotia of Sclerotium rolfsii (left) and its mycelial growth on Potato Dextrose Agar (PDA) after three days of incubation at 17°C (62.6 °F) and 30 °C (86 °F).

Fig. 2. Typical signs of southern blight disease on tomatoes caused by Sclerotium rolfsii shown by the white mycelium and tan to brown colored sclerotia (red circled). Picture: courtesy of Patricia Hosack, University of Missouri Plant Diagnostic Clinic.

Fig. 3. Typical diagnostic feature, the fan shaped mycelia of southern blight on tomatoes caused by Sclerotium rolfsii. Picture: courtesy of Patricia Hosack, University of Missouri Plant Diagnostic Clinic.