Southern Blight and Sweetpotato Plant Beds

Warm April weather in North Mississippi provides great growing conditions for sweetpotato plant beds. A few days near the 80s will have sweetpotato shoots pushing up the black plastic mulch that has been covering them since March. However, warm and humid conditions underneath plastic mulch can also provide an ideal environment for Southern blight, a disease of sweetpotato plant beds. On seed potatoes, the disease results in a soft rot. From the seed potato, the disease spreads to nearby shoots. Affected shoots wilt then die. This results in circular patches of dead plants in otherwise healthy plant beds. Under humid conditions, like those under plastic mulch and row covers, the pathogen forms a white mycelium. Within the mycelium you may find white, yellow, brown, and black mustard seed-like sclerotia. There are some varietal differences in response to Southern blight. ‘Evangeline’ is particularly known for its susceptibility. While ‘Beauregard’ is considered to be a tolerant variety, stressed Beauregard plants are susceptible.

The causal agent is a soil-borne fungus, Sclerotium rolfsii. The pathogen can survive in the soil for several years and has a very broad host range.

Tips for managing Southern blight in sweetpotato plant beds:

Use disease-free seed potatoes.

Apply a fungicide (like Botran) to seed potatoes at bedding. It is important to apply the fungicide with enough water to get adequate coverage on all seed potato surfaces, as it is thought that seed-applied fungicides provide protection at the point where sprouts emerge from the seed potatoes.

Choose well-drained sites for plant beds and sites with a lack of Southern blight incidence for the last three to four years. Soil fumigation can be used in fields with a history of disease, but for most growers this may not be practical.

As the weather warms, ensure plant beds receive adequate ventilation. For most growers, this will require additional slits be cut either in the top or side of the plastic mulch. For growers with yearly Southern blight injury, removing plastic mulch sooner after shoot emergence may be beneficial.

Cut slips at least 1″ above the soil surface.  Pulling slips may transfer the pathogen to the production field.

Sclerotium rolfsii mycelium in sweetpotato plant bed.

Mycelium and sclerotia.

Brown mustard seed-like sclerotia.

Affected plants wilt then die.

Southern blight often appears as dead circular patches in plant beds.