“Pythium species cause damage to many greenhouse crops and is one of the most prominent diseases of cannabis. On cannabis, the fungal-like organisms cause crown rot and root rot, as well as pre-and post-emergence damping off on hemp.” Therefore, one section in the Handbook of Cannabis Production in Controlled Environments (in chapter 8, written by Cameron Scott and Zamir Punja, a book edited by Youbin Zheng) helps growers identify and manage this potential threat. “Once inside the growing environment, Phytium begins to spread and infect the roots of plants, particularly if roots are damaged during transplanting or by insect feeding. The root-infecting pathogens are favored by wet conditions and grow over a range of temperatures, with many favored by temperatures over 30°C in hydroponic cultivation systems.”
Symptoms
“Colonization of root tissues by mycelium causes browning, with Pythium spreading both between and inside root cells. As the destructive necrotrophic phase of infection continues, roots may appear stubby, with the absence of feeder roots. The outer region of the root, the epidermis, and the cortex, may slough off, leaving only the pith and vascular bundle. The crowns of infected plants may also appear sunken and dark, which can extend several centimeters up the stem. The Pythium disease cycle includes the formation of sporangia on the root surface, while oospores may be formed inside the root tissues,” Cameron and Zamir explained.
Foliar symptoms will become visible as the disease progresses, especially during warm weather, the researchers say. “Plants affected by Pythium appear stunted and may begin to wilt as their root systems are destroyed. Infected plants may also appear moderately chlorotic but, otherwise, look healthy. Initially, wilted plants may recover, but symptoms generally become more severe as the disease progresses. Wilting caused by Pythium occurs very rapidly, with plants drying out and dying over a few days.”
Management
Cameron and Zamir also give growers advice on how to manage Pythium. “Incoming plant material should be examined for symptoms and placed under quarantine and tested for the presence of the pathogen. To minimize the introduction or spread, shoe covers, gloves, hair, and beard nets, and footbaths with disinfectant should be placed at entrances in growing rooms. It is also important to sanitize equipment and tools regularly.”
It is also important to avoid damage to roots from transplanting. “Insect pests such as fungus gnats, shore flies, and root aphids may also wound roots or act as vectors of Pythium. Also, avoid overwatering, as this creates an anaerobic environment which is conducive to Phythium infection. It is also beneficial to apply biological control products such as Rootshield and Prestop when propagating plants.”
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.