Cross-Infectivity of Powdery Mildew Isolates Originating from Hemp (Cannabis sativa) and Japanese Hop (Humulus japonicus) in New York

Abstract

In the recent decade, agricultural production of both hemp (Cannabis sativa) and hop (Humulus lupulus) has expanded throughout the Pacific Northwest, Midwest, and Eastern United States to support the growing industries for which these plants are key components. The significant and rapidly expanding overlap of production regions of these two Cannabaceae plant family members creates a potential dispersal route for organisms that are pathogenic to both hosts. Powdery mildew is a disease of high economic impact in both hemp and hop production systems, yet it was largely unknown whether the powdery mildew fungi commonly associated with hemp could also be pathogenic on hop, and vice versa. We isolated Golovinomyces spadiceus growing upon hemp in New York production greenhouses and Podosphaera macularis from feral hop (H. japonicus) plantings also in New York. Herein, we report the pathogenicity of P. macularis associated with hop to C. sativa cultivars ‘Anka’ and ‘Wild Horse’ and pathogenicity of G. spadiceus toward hop. The potential for P. macularis to establish, produce viable, infectious conidia, and undergo sexual recombination on hemp could complicate efforts to exclude the MAT1-2 mating type of P. macularis from western North America and could facilitate the spread of races pathogenic toward ‘Cascade’ hop, and hop cultivars with R6-based resistance to P. macularis, including ‘Nugget’. Further assessment of the pathogenicity of diverse P. macularis isolates, in both geographic origin and the range of hop species, is necessary to better understand the dispersal risk of P. macularis on hemp.