Members of the Fusarium oxysporum complex causing wilt symptoms in medical cannabis in Israel, Italy, and North America comprise a polyphyletic assemblage

Authors: JERUSHALMI, SHACHAR - Agricultural Research Organization - Volcani Center; MAYMON, MARCEL - Agricultural Research Organization - Volcani Center; O Donnell, Kerry; FREEMAN, STANLEY - Agricultural Research Organization - Volcani Center

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2022
Publication Date: 8/26/2022
Citation: Jerushalmi, S., Maymon, M., O'Donnell, K., Freeman, S. 2022. Members of the Fusarium oxysporum complex causing wilt symptoms in medical cannabis in Israel, Italy, and North America comprise a polyphyletic assemblage. Plant Disease. 106(10):2656-2662. https://doi.org/10.1094/PDIS-01-22-0155-RE.
DOI: https://doi.org/10.1094/PDIS-01-22-0155-RE

Interpretive Summary: Israel, Canada, and the United States are global leaders in medical cannabis production and research. Cannabis production of hemp and medical marijuana are rapidly growing industries with over 500,000 acres currently in production within the United States. The present study was conducted to identify the pathogens responsible for a vascular wilt disease discovered on six commercial cannabis farms in Israel. Filamentous molds were isolated from infected root and stem tissues and grown on a suitable microbiological medium in the laboratory. The fungal isolates were identified to species by collecting and analyzing DNA sequence data from two informative genes. Analysis of the DNA data collected from 24 strains revealed they comprised 12 genetically different groups within the Fusarium oxysporum complex. Two of the strains in Israel appeared to be genetically identical to ones causing disease on cannabis in California and British Columbia. The fact that 12 genetically different strains can cause wilt disease suggests that pathogenicity to Cannabis sativa may have evolved independently within the F. oxysporum complex. Pathogenicity tests of the Israeli strains on cannabis seedlings confirmed they could induce wilt disease. The current research study also revealed that the pathogens can be spread easily via propagation material. Results of this study will be of interest to the hemp and medicinal cannabis industries, including plant breeders, plant disease specialists and quarantine officials focused on developing disease resistant cultivars and control measures to prevent further spread of these economically important pathogens.

Technical Abstract: Members of the Fusarium oxysporum complex are ubiquitous soilborne fungal pathogens causing wilt diseases in various plant hosts. Fusarium oxysporum (Fo) f. sp. cannabis was first reported causing wilt disease in hemp in Italy in 1962. To date, Fusarium wilt continues to cause concern in industrial and medicinal cannabis cultivation worldwide. During a 3-year period (2018 to 2021), Fo strains were isolated from medical cannabis plants (Cannabis sativa) exhibiting wilt symptoms that were cultivated in numerous commercial farms in Israel. A diverse set of these strains was subjected to molecular phylogenetic analyses to assess their genetic diversity and to compare them with other f. sp. cannabis isolates included in prior studies. Maximum likelihood bootstrap analysis of a partial translation elongation factor (TEF1) dataset, which included 24 f. sp. cannabis sequences, revealed that the 11 strains from Israel comprised five TEF1 haplotypes. Two of the haplotypes from Israel were identical to isolates previously reported from British Columbia and California and British Columbia and Ontario. Overall, the 24 f. sp. cannabis sequences included 12 unique TEF1 haplotypes. These were phylogenetically diverse, suggesting that pathogenicity to C. sativa may have evolved independently within the F. oxysporum complex. Pathogenicity tests of the Israeli strains were confirmed by Koch’s postulates assays. Strains of the five different f. sp. cannabis TEF1 haplotypes all caused wilt in cannabis seedlings but with varying levels of aggressiveness. The same isolates that originated from asymptomatic infected mother plants were found in wilted cuttings indicating that the pathogen can be spread via propagation material.