Following the announcement of the legalization of recreational cannabis in Canada, research into the country’s cultivation practices has experienced a resurgence. As the number of cultivation facilities in the country grows, so too does the likelihood of observing previously undescribed pathogens that can affect the cannabis crop. Pathogenicity studies can result in a better insight into the potential exposure risk involved with different growing methods, giving testing laboratories an edge in knowing what to screen for, and cultivators an increased awareness of how to protect their crops and improve their methods.
Common cannabis growth practices
Cannabis plants can be grown in a variety of environments: outdoors, in a field under natural light; indoors, suspending the plants in a hydroplaning nutrient solution; or (also indoors), but this time using soil or cocofibre as a growing medium in a greenhouse under controlled environmental conditions.
Pathogenicity studies have already been carried out on field-grown cannabis and on hydroponically-grown cannabis, revealing the prevalence of certain species of Pythium and Fusarium root rot pathogens in both growing methods. Until recently, similar research had not been conducted for cannabis crops grown indoors under a greenhouse. A recent publication, authored by leading plant pathology researcher, Dr. Zamir Punja of Simon Fraser University in British Columbia, offers the first pathogenicity data for this kind of indoor cannabis crop.
Studying fungal infection of indoor cannabis crops
Dr. Punja’s study gathered samples of cannabis plants over the period from May 2015 to May 2017, taken from four Health Canada licensed facilities spread across the provinces of British Columbia and Alberta. All four facilities grew their cannabis crops indoors, with three production facilities using exclusively greenhouse facilities using either cocofibre or soil, and one general indoor production facility opting to use primarily hydroponic methods.
Random samples of cannabis flower buds were taken pre-harvest, at around 8-13 weeks of production, with additional buds harvested for study after 12 or 14 weeks, depending on the strain of the cannabis plant. Some post-harvest flower buds that had been dried under commercial conditions, and leaves with visible mildew infection, were also sampled for the sake of completeness.
Cannabis plant material was placed inside plastic bags for 48 h at room temperature to allow any fungi present to develop, then small segments were plated on potato dextrose agar and incubated. The genus of any emergent colonies was identified visually, with representative isolates of each culture sent on to the University of Guelph Laboratory Services for species identification by polymerase chain reaction (PCR) analysis. Some cultures were also examined using scanning electron microscopy.
Pathogenicity tests were conducted by inoculating visually healthy cannabis plants with either mycelial plugs or atomized spore suspension, depending on which method produced the most consistent results. The extent of bud decay was analyzed using a 0-4 rating scale, where 0 indicated no visible mycelium, and 4 indicating a covering layer of mycelium and extensive bud rot. Intermediate numbers correlate to the extent of visible fungus presence and minor levels of bud rot.
Extent of infection and possible protective measures
Cultures of Botrytis, Penicillium, and Fusarium were isolated from select samples in the study. Further analysis identified the Botrytis species as Botrytis cinerea, two species of Penicillium — P. olsonii and P. copticola — and three Fusarium species — F. solani, F. oxysporum, and F. equiseti.
Of the 300 samples collected over the course of the study, 2% of samples were infected with B. cinerea, which causes noticeable brown rot, and 3.6% had been infected by F. solani, which results in visible internal decay. The remainder of samples appeared healthy by a visual inspection. Despite this, the plating and culturing of samples uncovered further instances of fungal infection. In order of decreasing frequency, these were: P. copticola (10.6% of samples), P. olsonii (7.4%), F. oxysporum (2.7%), and F. equiseti (0.9%). Traces of Aspergillus niger was also found on one sample.
Pathogenicity tests, where healthy plant samples were inoculated with the discovered fungal cultures and left to incubate, revealed the extent of bud decay each fungal infection was capable of causing. Ranked on the 0-4 point scale, the six species of identified fungal infections ranked as follows: B. cinerea (with an average of 4.0 ± 0.5), F. solani (3.5 ± 0.4), P. olsonii (2.9 ± 0.2), F. oxysporum (2.1 ± 0.3), F. equiseti (1.0 ± 0.1), and P. copticola (1.0 ± 0.1).
Outside of the effects these fungal infections can have on a healthy plant crop, some can also cause some adverse health effects on human workers. A study on the health effects of the Botrytis mold found that B. cinerea is a common allergen, and further research has identified F. solani and F. oxysporum as having particular risks to the health of immunocompromised individuals. As a result, managers of cultivation facilities that use these indoor methods have a strong obligation to ensure that all precautions are taken to avoid putting their crops and their workers at risk of these newly identified potential dangers.
Dr. Punja notes in his publication that different strains of cannabis plants are likely to differ in their susceptibility to flower-infecting pathogens, such as those found in this study. That, in conjunction with a lack of approved biological mold and fungal control products in the Canadian marketplace, puts cultivators at a certain disadvantage when it comes to tackling these infections and making sure that their crops can pass the necessary quality control assessments needed to make it to the market.
While further research and product development are urgently needed to develop full infection control strategies, cultivators can do their best to limit the spread of infection through thoroughly disinfecting all cultivation equipment. Some of the fungi found in this study, specifically P. olsonii and F. oxysporum, are also known to disseminate through the air, so it is also important to regularly clean and disinfect any ventilation or filtration system that might be in use in the greenhouse to combat the airborne risk.