First Report of Powdery Mildew Caused by Golovinomyces spadiceus on Industrial Hemp (Cannabis sativa) in Kentucky

Abstract

Industrial hemp (Cannabis sativa) was reintroduced to the United States as a pilot research program under the 2014 Farm Bill. By 2017, there were over 25,000 acres of industrial hemp in the United States, with Kentucky having the second highest acreage in the United States (https://www.votehemp.com/resources/publications/). Hemp is processed for fiber (4%), grain/seed (18%), and cannabidiol (CBD, 62%), and grosses $7.5M for Kentucky growers. Between 2014 and 2018, powdery mildew was observed in numerous greenhouses, in multiple locations, and on several varieties including but not limited to Cherry Wine, Endurance, Otto, proprietary CBD varieties, and fiber and grain breeding lines. Symptoms initially appeared as small, inconspicuous white patches on the adaxial side of leaves. Colonies expanded in size, often coalescing and infecting entire leaves and succulent stems. The disease spread readily to asymptomatic plants. Pathogenicity of three isolates collected from hemp specimens was confirmed through touch inoculation where conidia from infected leaves were pressed onto asymptomatic leaves. Inoculated plants were moist chambered for 48 h and maintained in the greenhouse. Symptoms appeared within 6 to 7 days, and morphological features were identical to the original; noninoculated control plants did not develop symptoms. Mycelia were amphigenous and occasionally caulicolous; hyphae were septate with septations 5 to 6 µm apart. Foot cells were cylindrical, measuring (42 to) 57 to 107 (to 120) µm × 9 to 11 µm, followed by one to two shorter cells. Conidiophores were hyaline, singular, and erect, measuring (80 to) 115 to 187 (to 209) µm in length, followed by two to three immature conidia forming a crenate outline. Conidia were catenescent and ellipsoid to ovoid, measuring (29 to) 30 to 39 (to 41) µm × (13 to) 15 to 20 (to 22) µm. Chasmothecia readily formed during autumn, were round and dark brown at maturity, measured (96 to) 109 to 138 (to 159) µm in diameter, and displayed nondescript myceloid appendages. Mature chasmothecia contained five to 15 ovoid-saccate asci, most with short stalks. Asci measured (52 to) 56 to 75 (to 78) µm × (25 to) 29 to 43 (to 50) µm, and each ascus contained two ovoid ascospores measuring (15 to) 18 to 27 (to 32) × (9 to) 11 to 18 (to 19) µm. Morphological characteristics were consistent with descriptions of Golovinomyces spadiceus except that foot cells from these isolates had a wider range of lengths compared with previous reports, 30 to 80 × 9 to 15 µm versus 42 to 116 × 9 to 11 µm (Braun and Cook 2012). Conidial measurements were similar but not identical to G. ambrosiae, which is reported to have a longer conidial length/width ratio (2.0) than G. spadiceus (1.5 to 2.0); these isolates had conidial length/width ratios consistent with G. spadiceus (Braun and Cook 2012). Identification was confirmed by sequencing the 28S and internal transcribed spacer (ITS) regions with primers PM5G/NLP2 for the 3′ half of ITS and 28S and ITS5/PM6G for the 5′ half of ITS (Bradshaw et al. 2017). Sequence data were deposited into GenBank (MK305282). A GenBank BLAST search resulted in a 100% similarity to G. spadiceus (GenBank accession AB769427) and 97% similarity to G. asterum, G. orontii, and G. cichoracearum. The latter species were eliminated based on strong inconsistences in morphological comparisons. This species was determined to be G. spadiceus based on morphological features and sequence data. This determination is consistent with those of powdery mildew fungi collected from Cannabis in Canada (Pepin et al. 2018). All samples collected during this period were consistent; no other powdery mildew species was identified. This is the first documented report of G. spadiceus causing powdery mildew on hemp in the United States. With recent legalization of hemp in the United States, it is important to document the species and distribution of powdery mildew fungi affecting this crop.