“Even with modern diagnostic tools and management strategies, plant diseases still cause hardship and upwards of $200 billion USD in crop losses worldwide every year (FAO 2021). As the production of hemp and cannabis increases throughout the world, new diseases are emerging or less common diseases are spreading.” Therefore, one of the chapters in the Handbook of Cannabis Production in Controlled Environments (chapter 8 written by Cameron Scott and Zamir Punja, book edited by Youbin Zheng) helps growers identify and manage new potential threats. One such prevalent and potentially devastating pathogens is Fusarium. “On cannabis plants, Fusarium has been shown to be able to infect at all stages of growth, from propagation through to flowering. Affected plants develop crown and root rot symptoms, and damping off on cuttings may occur.”
Symptoms
“The roots of affected plants appear brown and necrotic (root rot). In cannabis, dark sunken lesions on the crown of the plant (crown rot) also may develop. Mycelium may be visible on the infected tissues, especially at the soil line or on areas affected by crown rot. When the stems of infected plants are cut open, the pith and xylem tissues appear brown or black. Once in the xylem vessels, the pathogen colonizes these tissues, which then collapse, reducing uptake of water and nutrients. Hence, symptoms of yellowing and wilting are observed. Sometimes, diseased plants may remain asymptomatic until plants are stressed,” explain Cameron and Zamir.
Fusarium can also cause damping off on cannabis cuttings, the researchers say. “These symptoms start as soft, dark waterlogged areas at the base of stems, which progress upward and eventually cause the cutting to collapse. The humid conditions present during propagation of cannabis increases development of this disease. As the infection spreads, the cutting will rot, and oftentimes the mycelium can be seen growing on the affected regions. Secondary organisms such as soft rot bacteria or Penicillium may also be present. Poor air circulation, as well as overly wet media, may increase the severity of damping off. Cuttings which originate from infected stock plants, even if they are free of symptoms associated with Fusarium, are at particular risk of developing damping off, as these cuttings may already have the pathogen present inside them. This is a common way by which Fusarium has the ability to spread.”
Preventing spread
Cameron and Zamir give growers some advice on how to prevent the spread of the pathogen. “Incoming plant material should be examined for symptoms (yellowing, wilting, root rot) and placed under quarantine and tested for the presence of the pathogen. Stock plants should be replaced approximately every six months to avoid a buildup of Fusarium internally. Also, minimize introduction or spread of the pathogen through contaminated soil, plant debris or potentially by spores on shoes or clothing of personnel and visitors. This requires that shoe covers, gloves and footbaths with disinfectant be placed at entrances to growing rooms. Filters and UV lights in air intakes may also reduce the number of spores being brought into the facility.”
In general, it is important to sanitize equipment and tools regularly. “Previously used trays, domes, pots, flood tables, humidifiers, dehumidifiers, and fans should be cleaned thoroughly and regularly with a detergent and water, as well as a disinfectant. Irrigation equipment such as drip lines or pipes may be cleaned by flushing them with hydrogen peroxide or other cleaning agents. For growers who recirculate nutrient solutions, filtration through various membrane systems or sand (slow filtration), heat treatment (pasteurization) or UV radiation may be required. Addition of chlorine-containing compounds, such as sodium hypochlorite or chlorine dioxide, to achieve levels up to 5 ppm chlorine, may reduce survival of Fusarium."
Click here for the Handbook of Cannabis Production in Controlled Environments, in which you can read the full chapter 8: Management of Diseases on Cannabis in Controlled Environmental Production.