Pathogens and Molds Affecting Production and Quality of Cannabis sativa L.

Abstract

Plant pathogens infecting marijuana (Cannabis sativa L.) plants reduce growth of the crop by affecting the roots, crown, and foliage. In addition, fungi (molds) that colonize the inflorescences (buds) during development or after harvest, and which colonize internal tissues as endophytes, can reduce product quality. The pathogens and molds that affect C. sativa grown hydroponically indoors (in environmentally controlled growth rooms and greenhouses) and field-grown plants were studied over multiple years of sampling. A PCR-based assay using primers for the internal transcribed spacer region (ITS) of ribosomal DNA confirmed identity of the cultures. Root-infecting pathogens included Fusarium oxysporum, Fusarium solani, Fusarium brachygibbosum, Pythium dissotocum, Pythium myriotylum, and Pythium aphanidermatum, which caused root browning, discoloration of the crown and pith tissues, stunting and yellowing of plants, and in some instances, plant death. On the foliage, powdery mildew, caused by Golovinomyces cichoracearum, was the major pathogen observed. On inflorescences, Penicillium bud rot (caused by Penicillium olsonii and Penicillium copticola), Botrytis bud rot (Botrytis cinerea), and Fusarium bud rot (F. solani, F. oxysporum) were present to varying extents. Endophytic fungi present in crown, stem, and petiole tissues included soil-colonizing and cellulolytic fungi, such as species of Chaetomium, Trametes, Trichoderma, Penicillium, and Fusarium. Analysis of air samples in indoor growing environments revealed that species of Penicillium, Cladosporium, Aspergillus, Fusarium, Beauveria, and Trichoderma were present. The latter two species were the result of the application of biocontrol products for control of insects and diseases, respectively. Fungal communities present in unpasteurized coconut (coco) fiber growing medium are potential sources of mold contamination on cannabis plants. Swabs taken from greenhouse-grown and indoor buds pre- and post-harvest revealed the presence of Cladosporium and up to five species of Penicillium, as well as low levels of Alternaria species. Mechanical trimming of buds caused an increase in the frequency of Penicillium species, presumably by providing entry points through wounds or spreading endophytes from pith tissues. Aerial distribution of pathogen inoculum and mold spores and dissemination through vegetative propagation are important methods of spread, and entry through wound sites on roots, stems, and bud tissues facilitates pathogen establishment on cannabis plants.

Epidemiology of Fusarium oxysporum causing root and crown rot of cannabis (Cannabis sativa L., marijuana) plants in commercial greenhouse production

“…The distinguishable clades are represented by letters from A to C. the frequency of cuttings taken at this distance that showed recovery of F. oxysporum was about 0.5%. The pith tissues in cannabis plants could allow spread of potential pathogens such as F. oxysporum to occur, as demonstrated previously for endophytic fungi (Punja et al 2019a). The ability of F. oxysporum to colonize exposed pruning wounds from airborne inoculum and spread into the pith tissues could also provide opportunities for internal colonization of cannabis plants.…”

“…Among several challenges currently facing cannabis producers, management of diseases remains a high priority. Many of the recently reported diseases that affect cannabis production in Canada, such as root and crown rots, powdery mildew, and bud rots caused by Botrytis and Penicillium species, are appearing in both indoor controlled environment facilities and greenhouse production (Punja et al 2019a).…”

Scientific Prospects for Cannabis-Microbiome Research to Ensure Quality and Safety of Products

“…Medical, as well as recreational Cannabis grown in indoor facilities is exposed to a plethora of microbial contaminants occurring on pre-and post-harvest Cannabis inflorescence buds. The literature addresses an extensive list of fungal and bacterial contaminants, as well as of associated toxins, on Cannabis [20,[46][47][48]. Taken together, published results confirm that viable organisms, including those that produce mycotoxins and endotoxins, can be recovered from Cannabis, potentially posing a serious risk to immunosuppressed individuals [48].…”

“…Cannabis inflorescence buds are often contaminated with molds and mycotoxins [46], particularly if not stored properly [50]. Methods for controlling mycotoxins are mostly preventive during production, handling, transportation, storage, and processing [51,52].…”

Cannabis Contaminants Limit Pharmacological Use of Cannabidiol

“…The patient elected to discontinue treatment and passed two weeks after transitioning to hospice care (Dehal and Quimby, 2019). There are limited case studies demonstrating Cannabis causing Fusariosis; however, there are a plethora of studies that have found Fusarium to be in direct relationship with Cannabis (Mcpartland and Hillig, 2004;Punja et al, 2019).…”