Pathogenicity of seedborne Alternaria and Stemphylium species and stem-infecting Neofusicoccum and Lasiodiplodia species to cannabis (Cannabis sativa L., marijuana) plants

Abstract

Cannabis (Cannabis sativa L., marijuana) plants grown at indoor and outdoor production sites in British Columbia (BC) and Ontario with stem canker symptoms were sampled and affected tissues were surface-sterilized and plated onto potato dextrose agar. Isolates were identified by colony and spore morphology as well as PCR amplification and sequencing of the ITS1-ITS2 rDNA, beta-tubulin, and actin gene regions. The most frequently isolated fungi were Alternaria alternata, Neofusicoccum parvum, and Lasiodiplodia theobromae. In addition, A. alternata was recovered from cannabis inflorescences in BC as well as surface-sterilized hemp seed. Stemphylium vesicarium was isolated from hemp seeds for the first time. The pathogenicity of all these fungi to cannabis plants was confirmed by inoculation of leaves, stems, buds, rooted cuttings and mature plants. Disease ratings varied depending on both tissue type and pathogen. Both L. theobromae and N. parvum produced extensive stem cankers on mature cannabis plants as well as extensive lesions on leaves. All pathogens showed optimal colony growth in culture at 25-30ºC. A comparison of the susceptibility of five cannabis genotypes to foliar infection by N. parvum indicated there were significant differences (p < 0.05) in lesion size, with some genotypes showing high susceptibility and others showing resistance. This is the first report of L. theobromae, N. parvum, A. alternata, and S. vesicarium causing stem canker and leaf spot symptoms on cannabis in Canada. The recovery of these fungi adds to the growing list of pathogens that cause infection and loss of quality in cannabis production.