The Do’s and Don’ts of Disease Prevention in Greenhouse Hemp
Two University of Kentucky plant researchers weigh in on the best practices for hemp greenhouse sanitation to help minimize the source of infectious pathogens.
Hemp greenhouse. Pathogen propagules can spread through air currents, equipment and tools, potting media, water, and people. Photo of a commercial hemp greenhouse in Kentucky. Courtesy of Gauthier
Pathogens, such as fungi and bacteria, can cause severe losses in greenhouse and field hemp. Under greenhouse conditions, which are typically warm and humid, pathogens can infect and spread rapidly. Infective propagules (e.g. bacterial cells, fungal spores, survival structures) can stem from infected plant materials, infested potting media, irrigation water, and greenhouse equipment.
When producers neglect disease management, pathogen populations build up and increase the risk for yield loss. In addition, plant debris and soil particles can harbor overwintering propagules for months or years.
Hemp greenhouse with Fusarium wilt (left) and Pythium root rot (right). Photo left of an outdoor container hemp production nursery in Kentucky and photo right of a commercial transplant production high tunnel using a tobacco float system. Once a greenhouse is severely infested, eliminating pathogens may become more difficult, particularly as pathogen propagules spread through air currents (fans), equipment and tools (carts, pruners, pots), potting media (soil or soilless media), water (irrigation, condensation, runoff), and people (shoes and clothing). The limited availability of fungicide products for hemp makes pathogen elimination even more complicated.
Sanitation is an essential component of disease management. The following list of hemp greenhouse sanitation do’s and don’ts can help minimize the source of infectious pathogens.
Hemp greenhouse with powdery mildew. Hemp leaves with powdery mildew (inset). Photo of a commercial hemp greenhouse in Kentucky.
Do's:
1. Discard heavily infected plants, particularly those with untreatable diseases. Removing infected plants from the greenhouse can help minimize the source and spread of pathogen propagules. Diseases such as root rots and vascular wilts are difficult to treat, so it's best to destroy plants infected with them immediately.
2. Isolate infected plants and treat them with fungicide, if available. Producers can manage some diseases (e.g. fungal leaf spots) using fungicides. Fungicides will not bring dead tissue back to life, but they can help protect new growth. And in some situations, fungicides can help plants recover from disease outbreaks through healthy new growth. When possible, isolate infected plants until the disease is eliminated. If using fungicides, diseased plants should be removed prior to fungicide application.
3. Prune infected tissues and remove all clippings from the greenhouse. Similar to removing infected plants, pruning infected tissues (e.g., buds and leaves) can reduce sources of certain pathogens, such as Botrytis tip blight (Botrytis cinerea) and powdery mildew (Golovinomyces spadiceus) pathogens.
4. Remove weeds and volunteer plants to prevent the establishment of a “green bridge” between crops. Many plant species can establish a “green bridge,” meaning they become hosts for disease agents and allow pathogens to survive until a more suitable one becomes available. Remove entire plants, including roots of alternative hosts.
5. Clean and sanitize all surfaces to prevent carryover from one cropping season to the next. Sanitize pots, benches, floors, and tools. Use a commercial disinfectant such as quaternary ammonia compounds (Green-Shield, Quattro, Physan), 10% bleach, peroxide (Oxidate, Terraclean), or 10% Lysol concentrated disinfectant. (See chart below for more information.) Bleach and peroxide are corrosive, so growers should rinse tools after 5 to 10 minutes of exposure. Excess soil and plant debris on floors and benches should be removed prior to disinfestation.
6. Sanitize irrigation systems. Install a water-treatment system if using recycled water. Flush irrigation lines with disinfectants such as hydrogen peroxide and peroxyacetic acid, or quaternary ammonium compound between crop cycles to remove propagules that may have moved into water lines and emitters.
7. Clean, clean, clean. Use sanitizing footbath mats to prevent movement of propagules to clean areas. Work in clean zones before moving to infected zones or quarantined areas. Clean tools and equipment between zones or blocks.
Don'ts:
1. Do not leave cuttings and culled plants in the greenhouse. Pathogens may overwinter or continue to multiply in debris. Infected plant material should be buried, burned, or placed in cull piles that are located at least 100 yards from the greenhouse. All cull piles will eventually need to be burned. Do not compost diseased cuttings or soil because incomplete composting (temperatures below 160° F) may result in propagule survival.
2. Do not reuse soil or potting media, and do not bring outside soil into the greenhouse. Many soilborne pathogens can remain dormant in soil for months or years.Growers should consider pasteurization and/or sterilization if they want to recycle media or use field soil.
3. Do not move tools or equipment between zones without prior cleaning or disinfestation. Greenhouse tools and equipment can be a means for propagule dissemination. Set up a cleaning zone or washing station, if possible.
4. Do not drag hoses and other tools along floors. Infested soil and plant debris can stick to wet surfaces and be moved to clean areas.
Misbahkul Munir, Ph.D., is a postdoctoral scholar at the University of Kentucky. He earned his Ph.D. from Clemson University and has been working on hemp fungal pathogens since 2021. His current work includes infectivity and pathogenicity studies for Fusarium bud and flower blight of field hemp. He is also investigating the potential for mycotoxin production by Fusarium spp. in both grain and floral hemp.
Nicole Gauthier, Ph. D., is a professor of plant pathology at the University of Kentucky. She has been working with hemp diseases since 2014. Her work includes identification and speciation of hemp pathogens, overwintering potential of disease agents, and cross infection of fungal pathogens. Gauthier earned her BS in horticulture and her Ph.D. in plant pathology from Louisiana State University.
