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Occurrence of Powdery Mildew Caused by Golovinomyces cichoracearum sensu lato on Cannabis sativa in Canada

    Authors and Affiliations
    • N. Pépin , Department of Biology, Université de Moncton, Moncton, NB, Canada
    • Z. K. Punja , Department of Biological Sciences, Simon Fraser University. Burnaby, BC, Canada
    • D. L. Joly , Department of Biology, Université de Moncton, Moncton, NB, Canada.

      Cannabis sativa, a flowering plant belonging to the Cannabaceae family, increasingly is being grown in Canada for medical purposes. As of April 2018, there were 97 authorized licensed producers and more than 200,000 registered clients, making C. sativa an economically important crop in Canada (Government of Canada 2018). Symptoms of powdery mildew were observed on indoor-grown plants in production facilities in Atlantic Canada and British Columbia. Powdery mildew was present in all production sites sampled between 2013 and 2017 and ranged in incidence from 20 to 35%. Symptoms appeared as white circular patches consisting of epiphytic mycelia and conidia on the adaxial leaf surface. As the disease progressed, the mycelia, conidiophores, and conidia covered the entire surface of the leaves and spread to flower bracts and buds. The conidiophores were unbranched, cylindrical, measuring 140 to 260 µm in height, and erected singly from mycelium. Conidiophores produced between one and four conidia in chains. The conidia were hyaline, cylindrical to barrel shaped, and lacked fibrosin bodies, measuring 29 to 37 µm in length and 12 to 17 µm in width. Chasmothecia were not observed. Powdery mildew on hemp (C. sativa grown mainly for the production of seed or fiber and containing low amounts of tetrahydrocannabinol) has been previously reported to be caused by Podosphaera macularis (formerly Sphaerotheca macularis) and Leveillula taurica (McPartland 1996). To confirm the identity of the pathogen in Canada, the internal transcribed spacer (ITS) region of rDNA was amplified by polymerase chain reaction with ITS1-f and ITS4 primers and sequenced. Sequences obtained were deposited in GenBank (accession nos. MH161605 and MH161604). A GenBank BLAST search revealed 100% similarity to species belonging to Golovinomyces lineage III according to Takamatsu et al. (2013), formerly included in G. cichoracearum sensu lato. The overlapping species included G. spadiceus on Dahlia pinnata (KX821733) and G. ambrosiae on Helianthus annuus (KM657962). Voucher specimens from both species (Takamatsu et al. 2013) had 99% similarity with the cannabis isolates (G. spadiceus on Zinnia elegans [AB769425] and G. ambrosiae on Helianthus tuberosus [AB769420]). ITS sequencing is insufficient to separate G. ambrosiae and G. spadiceus (Takamatsu et al. 2013). However, G. spadiceus conidia are ≤20 µm wide, similar to our observations, whereas G. ambrosiae conidia can be wider (Braun and Cook 2012). Pathogenicity was confirmed through inoculation of four healthy C. sativa ‘Pineapple Express’ plants by pressing diseased leaves onto healthy leaves. Plants were maintained at 23 ± 2°C. Inoculated plants developed powdery mildew symptoms after 7 to 8 days, whereas uninoculated plants remained healthy. To our knowledge, this is the first report in Canada of Golovinomyces sp. on plants from the Cannabaceae family. Previous reports recognized P. macularis as causing powdery mildew on C. sativa (McPartland 1996). However, it is unclear whether this pathogen was misidentified or is a new occurrence. Recent microbiome analyses of cannabis buds revealed the presence of Golovinomyces sp. in 84% of tested samples (Thompson et al. 2017), highlighting its prevalence among licensed facilities, where powdery mildew can weaken cannabis plants, cause early leaf drop, and reduce flower bud quality.