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.