Louisiana’s hot and humid weather and extended growing period provide a conducive environment for plant pathogens to quickly establish and spread. As a result, when industrial hemp was first introduced into Louisiana, the crop succumbed to several plant pathogens. These foliar and soilborne pathogens impeded industrial hemp cultivation in the state. This article highlights the top three high impact diseases of industrial hemp in Louisiana including Lasiodiplodia stem canker, Phytopythium root and crown rot and southern blight.

Stem canker

Lasiodiplodia stem canker of industrial hemp is a foliar fungal disease caused by Lasiodiplodia species. The pathogen enters the stem through an injury or wound. Once inside the host tissue, the pathogen colonizes the healthy tissue and causes dieback below and above the entry site, killing the entire stem in a short time (Figure 1). As the disease develops, the affected stems exhibit a dry rot, and the affected area turns tan colored. Pycnidia, which are tiny black fruiting bodies containing fungal spores, appear on the symptomatic stems (Figure 2). The pathogen then moves throughout the rest of the plant and may kill an entire mature plant within a few weeks. The fungal spores may spread from infected plants to healthy plants via splashed irrigation or rainwater.

Root and crown rot

The root and crown rot of industrial hemp is caused by an oomycete species known as Phytopythium, a soilborne fungal-like microorganism. Members of this group are known to flourish well in poorly drained, waterlogged, compacted soils that predispose the roots to pathogen infection. The pathogen spreads via irrigation water or splashed rain, planting of diseased seedlings or movement of contaminated soils. Symptoms start with yellowing of leaves, followed by wilting of the entire plant (Figure 3). Leaves turn brown and affected plants eventually die. Affected plants have a reduced root system (Figure 4) due to root rot and sloughing off. As the disease progresses, the pathogen invades the crown of the affected plants and causes further rot (Figure 5).

Southern blight

The southern blight of industrial hemp is caused by soilborne fungus Athelia rolfsii. Initial symptoms appear as yellowing and wilting of leaves (Figure 6). Lower stems appear necrotic near the soil line and the plants eventually turn brown and die. As the disease progresses, white fungal mycelia and small, tan colored sclerotia (resting spores of the fungus) appear at the base of infected plants (Figure 7). The sclerotia turn reddish to dark brown as they age. The mycelia and sclerotia extend both into the root zone and on the soil surface around the 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 and spreads by the planting of diseased transplants, movement of infested soil or plant debris, or use of contaminated equipment and tools. Running irrigation water may also aid in dispersal of sclerotia.

Disease management

Industrial hemp diseases are highly favored by poor agricultural practices that create conditions conducive for disease development such as deep planting, overcrowding of plants, over-fertilization, over-irrigation, planting in clay rich soils, soil compaction and poor drainage.

Successful disease management begins with accurate identification of the cause of the problem. Additionally, hemp growers must adopt integrated disease management strategies to effectively manage plant diseases to reduce yield losses. Growers must plant disease-free healthy transplants and avoid any physical injury to roots and lower stems while handling and transplanting the seedlings. Fields with a previous history of disease must be avoided. Hemp fields should be scouted on a regular basis for any disease symptom development. Rogue out symptomatic plants immediately. After removal, do not carry infected plants across the field or leave them in the row middles. Dig out each infected plant with a shovel and remove the entire plant with its root ball and soil around it. Place the plant in a trash bag to contain the infested soil and sclerotia and then remove it from the field. Movement of infested soils should be minimized to prevent pathogen spread. Cleaning farm equipment to remove soil is recommended.

Personnel working in fields infested with plant diseases are encouraged to clean their boots. Growers should work with disease-free fields first followed by infested fields. Keep weeds under check as some species may serve as alternative hosts for these plant pathogens. Destroy crop debris as soon as possible after harvesting. Currently, no conventional fungicides are available for industrial hemp growers to manage these three diseases; however, several biopesticides are listed for use on industrial hemp. Growers can find more information on registered biopesticides at https://www.epa.gov/pesticide-registration/pestici....

Raj Singh is the LSU AgCenter horticulture pathology extension specialist and director of the Plant Diagnostic Center, Department of Plant Pathology and Crop Physiology.

This article appears in the fall 2023 edition of Louisiana Agriculture.

Figure 1. A container-planted industrial hemp plant affected with stem dieback caused by Lasiodiplodia species. Photo by Raj Singh

Figure 2. An industrial hemp stem infected with stem dieback exhibiting tiny black fruiting bodies (yellow arrows) of Lasiodiplodia species. Photo by Raj Singh

Figure 3. An industrial hemp plant exhibiting yellowing and wilting of the entire plant caused by Phytopythium species.  Photo by Raj Singh

Figure 4. A Phytopythium-infected industrial hemp plant with reduced root system, left, compared to a healthy root system. Photo by Raj Singh

Figure 5. An industrial hemp plant shows crown rot (yellow arrow) and sloughing of the roots (red arrow) caused by a Phytopythium species. Photo by Raj Singh

Figure 6. Industrial hemp plants exhibiting initial yellowing and wilting (yellow arrows) caused by southern blight. Photo by Raj Singh

Figure 7. White fungal growth (mycelia, yellow arrow) and small, tan colored sclerotia (red arrow) present at the base of an infected plant.  Photo by Raj Singh