Characterization of Podosphaera macularis on the Most Prevalent Hop Genotype in the Pacific Northwest

Abstract

As of 2020, the hop cultivar ‘HBC 394’, trademarked name Citra brand, was the most widely planted hop cultivar in Washington, Oregon, and Idaho, with the acreage harvested in all three states nearly doubling from 2018 to 2020. This research characterized the pathogenic diversity of isolates of Podosphaera macularis on Citra brand and determined whether isolates with local or general adaption have emerged coincident with Citra brand becoming the most widely planted cultivar in the region. Among 27 isolates obtained from Citra brand, all could infect differential cultivars possessing the R-genes Rb, R3, and R5; nine isolates were also virulent on the differentials for R4 and R6. In a broader collection of 92 other isolates from Citra brand, 91 (98.9%) lacked R6 virulence. Inoculations of seven P. macularis isolates with complex virulence onto detached leaves of Citra brand all produced disease, indicating that a known major R-gene does not predict the powdery mildew phenotype in Citra brand. Citra brand-derived isolates and isolates representing the three predominant pathogenic races of P. macularis in the Pacific Northwest had no major difference in aggressiveness when inoculated on Citra brand leaves. Citra brand-derived isolates also caused disease levels on a range of hop cultivars similar to levels caused by isolates derived from other hop cultivars. These findings adduce that P. macularis isolates found on Citra brand may have varied phenotypes associated with known Pacific Northwest pathogen races. However, currently, isolates derived from Citra brand display neither local adaptation for increased virulence on Citra brand nor general adaptation for enhanced virulence on other cultivars. [Figure: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2023.

Keywords

• adaptation
• hop
• Humulus lupulus L.
• Podosphaera macularis