First report of powdery mildew caused by Golovinomyces ambrosiae on Cannabis sativa in Oregon

Authors: WISEMAN, MICHELLE - Oregon State University; OCAMB, CYNTHIA - Oregon State University; BATES, TAYLOR - Oregon State University; GARFINKLE, ANDREA - Oregon Cbd Seeds; Gent, David - Dave

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2021
Publication Date: 2/2/2021
Citation: Wiseman, M., Ocamb, C., Bates, T., Garfinkle, A., Gent, D.H. 2021. First report of powdery mildew caused by Golovinomyces ambrosiae on Cannabis sativa in Oregon. Plant Disease. https://doi.org/10.1094/PDIS-11-20-2455-PDN.
DOI: https://doi.org/10.1094/PDIS-11-20-2455-PDN

Interpretive Summary: Oregon was the second largest producer of hemp in the United States in 2019, with 25,900 hec of hemp licensed to growers. Between 2018 and 2019, multiple independent greenhouse growers in western Oregon reported powdery mildew-like signs and symptoms on leaves and buds of several hemp cultivars, including ‘Cherry Wine’. Powdery mildew is a common disease of hemp, but has not been formally documented in Oregon. Microscopic examination of the fungus indicated that the morphological characters overlapped with several species of the fungal genera Golovinomyces. Several genetic loci were sequenced to conclusively identify the species of the powdery mildew fungus. The Oregon hemp isolates grouped genetically with Golovinomyces ambrosiae. Controlled inoculations with Golovinomyces ambrosiae on potted hemp plants conducted by Oregon State University personnel reproduced the diseased. Inoculation of detached leaves of the powdery-mildew-susceptible hop cultivar 'Symphony’ also produced powdery mildew. However, the disease developed slowly on hop and the fungus produced relatively few spores, consistent with other reports that this fungus is not an aggressive pathogen of hop. This is the first known report of Golovinomyces ambrosiae causing powdery mildew on industrial hemp in Oregon. While powdery mildew on hemp currently appears most severe in protected cultivation, rapid expansion of hemp cultivation and introduction of new varieties throughout Oregon could pose a risk to outdoor cultivation.

Technical Abstract: Oregon is the second largest producer of hemp in the United States with 25,900 hec of hemp licensed to growers in 2019, a nearly six-fold increase over the previous year. Between 2018 and 2019, multiple independent greenhouse growers in western Oregon reported powdery mildew-like signs and symptoms on leaves and buds of several Cannabis sativa cultivars, including ‘Cherry Wine’. Signs of the disease started as small, white, powdery patches, typically on the adaxial sides of leaves, and progressed to coalescent colonies on leaves, stems, and buds. Morphological characters overlapped with several Golovinomyces spp. including G. ambrosiae, G. cichoracearum, and G. spadiceus. Identification was confirmed by bidirectional sequencing and phylogenetic analysis of 1,386 nucleotides from the concatenated internal transcribed spacer (ITS), 28S large ribosomal subunit, and beta-tubulin (TUB2) regions of two isolates using primer pairs ITS1/ITS4 and NL1/LR5, and TubF1/TubR1, respectively. The Oregon hemp isolates grouped in a monophyletic clade with G. ambrosiae (recently emended to include G. spadiceus) accessions. Pathogenicity was confirmed in inoculations of potted plants of the same cultivar. The hemp isolates were also able to infect detached leaves of Humulus lupulus ‘Symphony’ via similar inoculations; however, colony development on ‘Symphony’ was slow and sporulation sparse. This is the first known report of Golovinomyces ambrosiae causing powdery mildew on industrial hemp in Oregon. While powdery mildew on hemp currently appears most severe in protected cultivation, rapid expansion of hemp cultivation and introduction of new CBD varieties throughout Oregon could pose a risk to outdoor cultivation.