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(04/25/24) BATON ROUGE, La. — Louisiana’s warm, humid weather provides a conducive environment for plant pathogens to quickly establish and spread. As a result, an important plant disease called southern blight has started to show up in Louisiana vegetable and ornamental production.
Southern blight is caused by the soilborne fungus Athelia rolfsii (Sclerotium rolfsii). The pathogen has a wide host range and is known to cause disease on various economically important vegetables (cucurbits, eggplants, peppers, potatoes, tomatoes) and ornamental plants.
The fungus attacks the lower stem of plants at or near the soil line during warm and wet conditions. Initial symptoms appear as wilting and yellowing of leaves. The lower stem becomes necrotic, and the whole plant eventually turns brown and dies.
Closer examination of the base of a diseased plant reveals a lesion that girdles the stem. As the disease progresses, white fungal growth (mycelium) and small, mustard seed-like, tan-colored sclerotia appear at the base of infected plants. Sclerotia turn reddish to dark brown as they age. The mycelium and sclerotia extend on the soil surface around the infected plant.
The fungus survives as mycelia or sclerotia on the plant as well as sclerotia in the soil, where they can persist for several years. The disease is favored by hot and humid weather, which is common in Louisiana. The pathogen may spread by several means including planting of diseased transplants, movement of infested soil, equipment, tools and plant debris. Running irrigation water may also aid in dispersal of sclerotia.
Management of southern blight starts with avoiding planting susceptible crops in areas known to be infested with the pathogen for two or more years. Turn the soil to bury the sclerotia as deeply as possible; 8 to 12 inches is recommended.
For small plantings, aluminum foil may be wrapped around the lower part of the stem from just below the soil line to approximately 2 inches above the soil. This provides a physical barrier that prevents the pathogen from reaching the plant. Remove infected plants and discard them properly. Do not compost the disease plants. Movement of infested soils should be minimized to prevent pathogen spread. Cleaning farm equipment to remove dirt is recommended.
For information on fungicides recommended to manage southern blight, consult the LSU AgCenter Plant Disease Management Guide, publication No. 1802, at https://tinyurl.com/24f9p4xv.
A green onion field scummed with southern blight caused by Athelia rolfsii. Photo by Raj Singh/LSU AgCenter
Presence of white mycelium and tan-to-brown sclerotia of southern blight fungus on lower parts of green onion plants. Photo by Raj Singh/LSU AgCenter
A bachelor’s button (Centaurea cyanus) exhibiting initial symptoms of wilting caused by southern blight. Photo provided by Carla Costello
