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First Report of Golovinomyces spadiceus Causing Powdery Mildew on Industrial Hemp (Cannabis sativa) in Ohio

    Affiliations
    Authors and Affiliations
    • C. Farinas
    • F. Peduto Hand
    1. Department of Plant Pathology, The Ohio State University, Columbus, OH 43210

    Industrial hemp cultivation is highly valued in North America for oilseed production as well as cannabidiol (CBD), which are considered as having many therapeutic qualities (Cherney and Small 2016). The 2018 Farm Bill’s decriminalization of hemp has expanded the ability of states to cultivate and process hemp and transfer hemp-derived products across state lines, paving the way to new business opportunities for U.S. farmers. At the same time, it has incentivized universities across the nation to engage in hemp research and provide extension services to farmers. In August 2019, patches of white powdery fungal growth typical of powdery mildew were observed covering the older leaves of 2-month-old plants of Cannabis sativa cv. Tangerine maintained in the research greenhouse of the Department of Plant Pathology at The Ohio State University in Columbus, OH. Approximately 80% of the plants within the greenhouse were affected. Total DNA was extracted from the fungal colonies of two infected plants using a Chelex extraction method (Walsh et al. 1991), and molecular identification of the pathogen was carried out by amplification of the 28S and internal transcribed spacer (ITS) regions of the rDNA using primers TW14/NL1 and nested primers ITS4/ITS5 and PM6/ITS5 (Matsuda and Takamatsu 2003). GenBank BLAST sequence analysis resulted in 100% identity to Golovinomyces spadiceus (Berk. & M.A. Curtis) U. Braun with 82% (28S) and 100% (ITS) coverage, respectively (AB077643.1, AB077644.1; Braun et al. 2019). Sequences from one representative isolate were deposited in GenBank under accession numbers MN970163 and MN970157. The specimens were also characterized morphologically to confirm identity. The mycelium was amphigenous and caulicolous, forming white patches. Conidiophores arising from the colonies were erect, the foot cells were cylindrical, measuring 55 to 75 × 5 to 10 µm (n = 15), followed by one to three shorter cells forming catenescent conidia. Conidia were ellipsoid to ovoid, measuring 25 to 36 × 13 to 20 µm (n = 50), with a length/width ratio of 1.7. Conidial germination was of the Euoidium type. The morphological description, especially the conidia germination type and narrow width (<20 µm), confirmed that the two isolates were G. spadiceus (Braun and Cook 2012). To fulfill Koch’s postulates, six 3-week-old cuttings of C. sativa cv. Tangerine were potted in 6-inch pots filled with Metro-Mix growing medium. Three plants were inoculated by tapping infected leaves on the surface of healthy leaves, and three plants remained noninoculated and served as negative controls. All plants were placed on a greenhouse bench with 18 h of daylight at 22°C and covered with a plastic dome to increase relative humidity. Control plants were placed in the same conditions but spatially separated from the inoculated plants within the same greenhouse. Four days postinoculation, powdery mildew colonies appeared on the surface of the leaves of inoculated plants, whereas control plants remained disease free. G. spadiceus has been previously reported on hemp in Kentucky (Szarka et al. 2019) and New York (Weldon et al. 2020). To our knowledge, this is the first report of powdery mildew caused by G. spadiceus on industrial hemp in Ohio. As the cannabis industry is developing in the state, it will be important to develop local management strategies for this important disease.

    The author(s) declare no conflict of interest.

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    The author(s) declare no conflict of interest.