Crown gall development on cannabis (Cannabis sativa L., marijuana) plants caused by Agrobacterium tumefaciens species-complex

Abstract Crown gall, caused by Agrobacterium tumefaciens (syn. Rhizobium radiobacter), is characterized by gall formation on crowns, stems or roots on many plant species. We describe the occurrence of this disease for the first time on cannabis (Cannabis sativa L. marijuana) plants in two licenced production facilities in British Columbia. Galls were observed on crowns, stems and roots of several genotypes, including ‘White Rhino’, ‘Pink Kush’ and ‘Sour Kush’. The affected plants displayed no other visible symptoms. The incidence of crown gall was extremely low, estimated at 0.01% of total plants. Isolations made from root and stem galls on MacConkey medium (selective for Gram -ve bacteria) and on D1 medium (selective for Agrobacterium spp.) yielded a range of bacterial species, which did not include Agrobacterium sp. However, the presence of Agrobacterium in gall tissues was confirmed following PCR amplification with primers for the indole-acetic acid (iaa) H gene on the T-DNA region of Agrobacterium spp. and showed > 99% similarity to A. tumefaciens. Artificial inoculations were conducted using an A. tumefaciens strain from ATCC (Rhizobium radiobacter, strain designation TT134 [6-1-2, CIP 104 336], cat. 15955) which resulted in gall formation on five cannabis genotypes tested. These galls contained the iaaH gene as determined by PCR and the pathogen was successfully re-isolated on MacConkey agar medium at 4 and 10 weeks post-inoculation. Our findings confirm the occurrence of A. tumefaciens on naturally infected and artificially infected cannabis plants grown under greenhouse conditions and demonstrate reproducible gall symptoms on several cannabis genotypes following A. tumefaciens inoculation.