Characterization of podosphaera macularis on the most prevalent hop genotype in the pacific northwest

Authors: Gent, David - Dave; BLOCK, MARY - Oregon State University; COOPERIDER, CARLY - Oregon State University; WISEMAN, MICHELE - Oregon State University

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2023
Publication Date: 3/22/2023
Citation: Gent, D.H., Block, M., Cooperider, C., Wiseman, M. 2023. Characterization of podosphaera macularis on the most prevalent hop genotype in the pacific northwest. PhytoFrontiers. https://doi.org/10.1094/PHYTOFR-02-23-0020-R.
DOI: https://doi.org/10.1094/PHYTOFR-02-23-0020-R

Interpretive Summary: Plant pathogens populations can adapt to overcome host resistance depending on the genetic nature of the resistance and how broadly plants with the said form of resistance are planted. At the time of this research, the hop cultivar HBC 394, trademarked as Citra®, was the most widely planted cultivar in the Pacific Northwest where most hops are produced in the U.S. In this research, we characterized various isolates of the hop powdery mildew fungus collected from the hop cultivar HBC 394, trademarked as Citra®, and other cultivars to understand if strains of the fungus have somehow adapted to become more aggressive on this cultivar. We found that multiple strains of the hop powdery mildew fungus may be found on this cultivar, but we did not detect any evidence that the fungus has yet to adapt to become more aggressive. This research can inform future management efforts by growers and breeding efforts when developing new hop cultivars.

Technical Abstract: The hop powdery mildew fungus, Podosphaera macularis, displays phenotypic variation and has developed virulence on cultivars with disease resistance based on both major R-gene and partial resistance when a given form of host resistance is broadly deployed in the landscape. As of 2020, the hop cultivar ‘HBC 394’, commonly referred to by the trademarked name Citra®, was the most widely planted hop cultivar in Washington, Oregon, and Idaho with acreage of Citra® harvested in all three states nearly doubling from 2018 to 2020. This research aimed to characterize the pathogenic diversity of isolates of P. macularis on Citra® and determine whether isolates with local or general adaption have emerged coincident with shifts of Citra® to be the most widely planted cultivar in the region. Among 27 isolates obtained from Citra®, all were able to infect differential cultivars possessing the R-genes Rb, R3, and R5; nine isolates were additionally virulent on the differentials for R4 and R6. A broader collection of 92 other isolates from Citra® found that 91 (98.9%) lacked R6-virulence. Inoculations of seven isolates of P. macularis with complex virulences onto detached leaves of Citra® all produced disease, indicating a known major R-gene does not predict the powdery mildew phenotype in Citra®. Citra®-derived isolates and isolates representing the three predominant pathogenic races of P. macularis found in the Pacific Northwest had no major difference in aggressiveness when inoculated on Citra® leaves. Citra®-derived isolates also caused similar levels of disease on a range of hop cultivars as isolates derived from hop cultivars. These findings adduce that the P. macularis isolates found on Citra® may have varied phenotypes associated known pathogen races found in the Pacific Northwest. However, at this time isolates derived from Citra® display neither a local adaptation for increased virulence on Citra® or a general adaptation for enhanced virulence on other cultivars.