First Report of Powdery Mildew Caused by Golovinomyces ambrosiae on Cannabis sativa (marijuana) in Quebec, Canada

Marijuana (Cannabis sativa L.) is legal in Canada for medical and recreational purposes and is currently a multi-million-dollar industry. The province of Quebec follows British Columbia and Ontario in production acreage (Government of Canada 2018). During the growing season 2020-2021, five greenhouse growers throughout Quebec reported the presence of signs and symptoms reminiscent of powdery mildew including the presence of white powdery patches on the adaxial sides of leaves of several C. sativa cultivars. From one commercial facility, infected leaves of three cannabis cultivars (Sour Diesel, Orange Krush, and Lemon Sour) were photographed and the fungal mycelium was collected for identification in the laboratory. Fungal mycelium on leaf tissue was white and amphigenous and displayed unbranched hyaline conidiophores ranging from 130 to 275 μm in height (n = 50). Conidiophores arose from the upper surface of hyphal mother cells ranging from 35-70 × 8-13 μm in diameter (n = 25) and formed catenescent conidia. Conidia were broad ellipsoid-ovoid and measured, 24 to 35 × 12 to 19 μm (n = 50), and hyphae ranged from 3-8 μm in diameter (n = 30). Based on previous description (Qiu et al. 2020), the fungus was placed within the Golovinomyces genus. The species identification was confirmed through multi-locus phylogenetic using internal transcribed spacer (ITS), 28S large ribosomal subunit, and chitin synthase I (CHS1) genes amplified as recommended (Qiu et al. 2020), and directly sequenced with amplification primers (Centre Hospitalier de l'Université Laval de Quebec, CA). The three marker sequences shared 100% similarity for all the samples analyzed and were deposited in Genbank under accession numbers: OM131434 (28S), OM131448 (ITS), and OM141118 (CHS1). The phylogenetic analysis of the multi-locus sequences amplified grouped all three Quebec marijuana isolates in the G. ambrosiae accessions, confirming their identification. Pathogenicity was confirmed by transferring conidia onto detached healthy leaves of hop plants (Humulus lupulus) cultivar Northern Brewer kept under greenhouse conditions (28C, 50-60% relative humidity, and 14 h light) via paint brush inoculation. Hop leaves were used as surrogate due to the restricted availability of marijuana leaves. Inoculated leaves were placed in the growth chamber set at 20C, 50-60% relative humidity, and long days conditions as previously suggested (Weldon et al. 2020). The leaves developed powdery mildew colonies after 21 days, and the fungus was confirmed to be G. ambrosiae following morphological characterization and amplification of CHS1. Powdery mildew caused by G. ambrosiae (previous Golovinomyces cichoracearum) has been reported affecting hemp (Cannabis sativa) in New York and Oregon, United Sates (Weldon et al. 2020; Wiseman et al. 2021), and in British Columbia, Canada (Pépin et al. 2018; Punja et al. 2021), and this is the first report of G. ambrosiae causing powdery mildew on marijuana in Quebec. REFERENCES Government of Canada 2018. Online, retrieved January 7, 2021 https://www150.statcan.gc.ca/n1/daily-quotidien/180430/dq180430b-eng.htm Pépin N, Punja ZK, Joly DL. 2018. First report of powdery mildew caused by Golovinomyces cichoracearum sensu lato on Cannabis satia in Canada. Plant Disease. 102(12):2644. Doi: https://doi.org/10.1094/PDIS-04-18-0586-PDN Punja, Z. P. (2021). First report of the powdery mildew pathogen of hops, Podosphaeria macularis, naturally infecting cannabis (Cannabis sativa L., marijuana) plants under field conditions, Canadian Journal of Plant Pathology, Doi: https://doi.org/10.1080/07060661.2021.1960424. Qiu, P.-L., Liu, S.-Y., Bradshaw, M., Rooney-Latham, S., Takamatsu, S., Bulgakov, T. S., Tang, S.-R., Feng, J., Jin, D.-N., Aroge, T., Li, Y., Wang, L.-L., and Braun, U. 2020. Multi-locus phylogeny and taxonomy of an unresolved, heterogeneous species complex within the genus Golovinomyces (Ascomycota, Erysiphales), including G. ambrosiae, G. circumfusus and G. spadiceus. BMC Microbiology. 20:51. Doi : https://doi.org/10.1186/s12866-020-01731-9. Weldon WA, Ullrich MR, Smart LB, Smart CD, Gadoury DM. 2020. Cross-infectivity of powdery mildew isolates originating from hemp (Cannabis sativa) and Japanese hop (Humulus japonicus) in New York. Plant Health Progress. 21(1):47-53. Doi: https://doi.org/10.1094/PHP-09-19-0067-RS Wiseman, M. S., Bates, T. A., Garfinkel, A. R., Ocamb, C. M., and Gent, D. H. 2021. First Report of Powdery Mildew Caused by Golovinomyces ambrosiae on Cannabis sativa in Oregon. Plant Disease 105(9):2733. Doi: https://doi.org/10.1094/PDIS-11-20-2455-PDN.