(07/24/20) BATON ROUGE, La. — Louisiana’s first industrial hemp crop is getting hammered by a fungal disease called southern blight.
LSU AgCenter plant doctor Raj Singh says the disease is caused by a soilborne fungus called Sclerotium rolfsii.
“The pathogen has a wide host range and is known to cause disease on more than 500 plant species in 100 plant families,” Singh said. “Some of the economically important vegetables include tomatoes, potatoes, peppers, eggplants and cucurbits.”
The fungus also affects a wide variety of woody ornamentals, annual and perennial herbaceous, and bedding plants.
Hemp plants infected with southern blight start to wilt initially and later on turn brown and eventually die, Singh said.
On closer inspection of wilted plants, dead, water-soaked lesions can be observed along with white fungal growth (mycelium) and sclerotia on the base of the plant at the soil line.
“Sclerotia are fungal structures that help the pathogen survive adverse environmental conditions,” he said. “They are tiny, mustard seed-like, tan-colored when young and turn reddish to dark brown as they age.”
They can persist in infested soils for several years in the absence of a host.
The pathogen survives as mycelium or sclerotia on plant debris or in the form of sclerotia on topsoil around infected plants. These sclerotia are formed from mycelium extending from the base of infected plants.
Southern blight is favored by hot and humid weather, which is common in Louisiana. The pathogen has the ability to infect industrial hemp plants at any stage of their growth development.
The pathogen may spread by a number of 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 warrants an integrated disease management approach,” Singh said. “Growers must plant disease-free, healthy transplants and avoid physical injury to roots and lower stems while handling and transplanting the seedlings.”
Fields with a history of southern blight must be avoided. And growers should scout hemp fields regularly for symptom development.
Farmers should destroy symptomatic plants immediately and not leave removed infected plants in the row middles or carry them across the field. They should dig out infected plant with a shovel and remove the entire plant with the root ball and soil around it, place it in a trash bag to contain the infested soil and sclerotia, then remove it from the field.
Movement of infested soils should be minimized to prevent pathogen spread, Singh said. He recommends cleaning farm equipment to remove soil and encourages personnel working in fields infested with southern blight to clean their boots.
Growers should work with disease-free fields first followed by fields infested with southern blight.
“Keep weeds in check, as some species may serve as alternate hosts of the pathogen,” Singh said. “Destroy crop debris as soon as possible after harvesting.”
He recommends deep plowing to bury sclerotia to reduce pathogen inoculum.
Currently, no fungicides are available for industrial hemp growers to manage southern blight.
An industrial hemp plant exhibits wilting caused by southern blight. Photo by Raj Singh/LSU AgCenter
White fungal (mycelium) growth and small, mustard seed-like, tan-colored sclerotia surround the base of an infected industrial hemp plant. Photo by Raj Singh/LSU AgCenter