Four Root-Rot Pathogens Identified in Hydroponically Grown Cannabis
A three-year investigation traced root disease in hydroponically cultivated cannabis to four pathogens: two Pythium species and two Fusarium species. The findings, published in 2018 by Simon Fraser University researchers Zamir Punja and Gina Rodriguez in the Canadian Journal of Plant Pathology, provide a closer look at how root infections can develop and spread in recirculating growing systems.
From May 2014 to May 2017, the researchers examined approximately 50 symptomatic plants from a hydroponic production facility. Affected plants showed brown root lesions, root rot, stunted growth and mild leaf yellowing. Severe damage sometimes led to extensive root loss, although wilting was not observed in the facility samples. On average, about 1% of plants showed root-rot symptoms during each year of the study.
Laboratory testing identified Pythium dissotocum and Pythium myriotylum, along with Fusarium oxysporum and Fusarium solani. The researchers used PCR-based DNA analysis to confirm the identities of representative isolates. Strictly speaking, Pythium species are oomycetes—fungus-like organisms rather than true fungi—while Fusarium species are fungi.
Pathogenicity tests strengthened the connection between these organisms and the observed disease. Cannabis cuttings inoculated with either Pythium species developed brown, rotting roots, reduced root mass and stunted growth. F. oxysporum caused root and crown infection, along with discoloration of internal tissues and occasional wilting. F. solani also produced root and crown disease. The pathogens were subsequently recovered from inoculated plants, satisfying an important step in demonstrating that they were capable of causing the symptoms.
The study also found F. oxysporum in recirculating nutrient solution, indicating that a pathogen could move beyond an individual plant through the hydroponic system. In addition, some rooted cuttings obtained from commercial propagators carried Fusarium infections. When those cuttings were grown under hydroponic conditions, some developed severe stunting, yellowing, root necrosis and, in a few cases, plant death.
Genetic comparisons suggested that the Fusarium isolates were closely related to strains known to infect other crops, including tomato and cumin. That result indicates that the pathogens were not necessarily specialized to cannabis. The study also emphasized that both Pythium species have broad host ranges, making contamination from other crops or production areas a potential concern.
Implications for hydroponic growers
Recirculating hydroponic systems can spread water-associated pathogens quickly because the same nutrient solution reaches multiple plants. The researchers therefore identified sanitation, pathogen-free propagation material and early disease monitoring as important priorities. The British Columbia government’s cannabis disease guide likewise notes that Fusarium can move through contaminated water, tools, equipment and cuttings, while several Pythium species are associated with cannabis root and crown rot.
These findings do not establish that the four organisms are the only causes of root disease in cannabis, nor do they show that every hydroponic facility faces the same level of risk. Instead, the research demonstrates that routine inspection of roots, careful handling of cuttings and thorough cleaning of shared equipment can help growers detect and contain infections before they spread through an entire production system. Broader surveys have since documented additional root-infecting pathogens in cannabis, as summarized in the researchers’ review of pathogens and molds affecting cannabis production and quality.
