First report of powdery mildew caused by Podosphaera macularis on hemp in Oregon

Authors: BATES, TAYLOR - Oregon State University; BLOCK, MARY - Oregon State University; WISEMAN, MICHELE - Oregon State University; GARFINKEL, ANDREA - Oregon Cbd Seeds; Gent, David - Dave; OCAMB, CYNTHIA - Oregon State University

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2021
Publication Date: 8/31/2021
Citation: Bates, T.A., Block, M.H., Wiseman, M.S., Garfinkel, A.R., Gent, D.H., Ocamb, C.M. 2021. First report of powdery mildew caused by Podosphaera macularis on hemp in Oregon. Plant Health Progress. 22(4):567-569. https://doi.org/10.1094/PHP-04-21-0071-BR.
DOI: https://doi.org/10.1094/PHP-04-21-0071-BR

Interpretive Summary: Industrial hemp is a new and expanding crop in the Pacific Northwestern U.S. This region of the U.S. also is where the overwhelming majority of hops are produced. Hemp and hop belong to the same plant family and powdery mildew diseases are known to affect both plants. Two distinct powdery mildew fungi have been reported on hemp, a Golovinomyces species and Podosphaera macularis. The former is the most widely reported cause of powdery mildew on hemp, and does not appear to infect hop. The latter organism is the well-known cause of hop powdery mildew, but has been reported on hemp only occasionally in primary literature. In this study, we report on detection of a natural infection of hemp in the field in Oregon by Podosphaera macularis. We confirmed through measurement of morphological characters, genetic similarity, and controlled inoculations that the fungus is indeed the same organism that infects hop. This finding may have important implications for management and potentially quarantine policy, as the the departments of agriculture in Oregon, Washington, and Idaho have quarantines in place that restrict introduction of living hop plant materials to prevent the introduction of novel strains of the hop powdery mildew fungus. Such quarantines are not in place for hemp. Further research is needed to understand how often the hop powdery mildew fungus occurs on hemp and the disease risk consequences for both crops.

Technical Abstract: Industrial hemp is a new and expanding crop in Oregon and elsewhere. Two powdery mildew fungi are reported to affect hemp. Most often, a Golovinomyces species is reported to cause powdery mildew on this plant. Podosphaera macularis is reported as a pathogen of hemp in some secondary literature sources, or following artificial inoculation. However, reports of natural infection of hemp by P. macularis in the field are rare in primary literature. In September of 2020, we confirmed natural infection of hemp by P. macularis in a field in Linn County, Oregon. The isolate derived from hemp was able to infect hop and one of two hemp cultivars tested. The isolate was MAT1-1 mating type, the only mating type of P. macularis presently known to exist on hop in the Pacific Northwest, and lacked virulence on the hop powdery mildew resistance gene, R6. Given the overlap of hemp and hop production in western Oregon, it is plausible that P. macularis isolates found on hemp in the Pacific Northwest may have cross-infectivity capabilities on these two crops. The occurrence of the hop powdery mildew fungus on hemp has management implications for both hemp and hop. Studies are underway to assess the extent and diversity of P. macularis populations on hemp to inform quarantine policy and management recommendations for both hop and hemp.