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

Abstract

Fusarium oxysporum causes root browning and crown infection on marijuana (Cannabis sativa L.) plants, resulting in stunted growth, yellowing of leaves, and plant death. Pathogen presence and diversity were assessed in samples of diseased crowns, stems, pith tissues and roots from five commercial production facilities in British Columbia and Ontario. PCR of the elongation factor (EF-1 α) region and sequence analysis confirmed the identity and phylogenetic relationships among 33 representative isolates from over 200 isolates collected. Stock (mother) plants of eight cannabis strains (genotypes) with symptoms of yellowing, crown rot and internal stem decay yielded F. oxysporum at a frequency of 70-100%. Cuttings obtained from asymptomatic plants showed pathogen recovery rates of 1-2% on potato dextrose agar at distances of 90-150 cm from the crowns. Symptoms of damping-off were observed on cuttings in a rooting and propagation facility. Pathogenicity tests confirmed the development of characteristic symptoms on cuttings and rooted plants inoculated with F. oxysporum. Phylogenetic analysis indicated that isolates of F. oxysporum from BC belonged to a separate clade from ON isolates and that clonal spread was occurring within licenced facilities. Air sampling conducted in a greenhouse facility revealed F. oxysporum was present in areas used for rooting, vegetative propagation, and where stock plants were grown. The pathogen was also recovered from inflorescences. These findings indicate that F. oxysporum is present in several commercial production facilities in Canada and reduces root development and establishment of rooted cuttings, and causes yellowing and stunting on flowering plants of many cannabis strains.

First report of Fusarium proliferatum causing crown and stem rot, and pith necrosis, in cannabis (Cannabis sativa L., marijuana) plants

“…The pathogen was found to be present in seven licenced production facilities in three provinces in Canada, and in one cannabis production site in northern California. The pathogen was observed to be more aggressive in pathogenicity tests compared to the recently described report of F. oxysporum (Punja 2020a), causing root and crown rot, and caused disease at all stages of cannabis production, from propagation to vegetative growth to flowering, as well as infecting the inflorescences. The progression of the pathogen into the pith tissues following inoculation is demonstrated through light and scanning electron microscopy.…”

“…Cannabis plants showing symptoms of leaf yellowing, stunted growth, and necrosis of leaves and browning of roots, were included in the study. The plants were sampled at various stages of development, ranging from early propagation (1-2 weeks old) to vegetative growth (3-4 weeks of age) to onset of full flowering period (6-12 weeks of age) (Punja 2020a). They were grown in greenhouses or in controlled environment rooms in Health Canada approved licenced facilities.…”

“…A mycelial plug (8-mm diameter) from 7-day old PDA cultures of either F. proliferatum or F. oxysporum was placed on top of the cotton plug, mycelial side facing up. The pathogenicity of an isolate of F. proliferatum obtained from diseased pith tissues (isolate BC-5, Table 1) was compared to isolate BC-4 of F. oxysporum from damped-off cuttings (Punja 2020a). A stem cutting (approx.…”

“…A few isolates were also recovered from air sampling conducted by exposing Petri dishes in the growing environment for 60 min as described by Punja (2020a). The phylogenetic analysis (Fig.…”

“…Pathogens which infect the foliage include the powdery mildew pathogens (Punja 2018(Punja , 2020c. Finally, the previously described root and crown-infecting pathogens include Fusarium oxysporum, F. solani, several Pythium species (Punja and Rodriguez 2018;Punja et al 2020;Punja 2020a), and Cylindrocarpon (Punja 2020e).…”