Stability of hop yield and quality factors in response to delivery of irrigation and harvest timing

Authors: Gonzalez Tapia, Francisco; Gent, David; HARPER, SCOTT - Washington State University; COWELL, SARAH - Washington State University; JOBE, CADE - Oregon State University; SHELLHAMMER, THOMAS - Oregon State University; OCAMB, CYNTHIA - Oregon State University; RICHARDSON, BRIANA - Oregon State University; MASSIE, STEPHEN - Washington Hop Commission

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2026
Publication Date: 5/13/2026
Citation: Gonzalez Tapia, F., Gent, D.H., Harper, S.J., Cowell, S.J., Jobe, C.A., Shellhammer, T.H., Ocamb, C.M., Richardson, B.J., Massie, S.T. 2026. Stability of hop yield and quality factors in response to delivery of irrigation and harvest timing. HortScience. 61(6):1273-1280. https://doi.org/10.21273/HORTSCI19367-26.
DOI: https://doi.org/10.21273/HORTSCI19367-26

Interpretive Summary: Hop yards are expensive to plant and are expected to remain productive for many years. However, growers often worry that yields may decline over time, especially as plant viruses accumulate or as management practices affect long-term plant health. We conducted a five-year study in a commercial Citra® hop yard in Washington State to better understand how viruses, irrigation setup, and harvest timing influence yield and cone quality after planting clean, disease-free material. Over the course of the study, several viruses and a viroid gradually increased in the field. By the fifth year, most plants were infected with at least one virus. Despite this, we did not observe a steady decline in yield. Overall production remained stable from the second through the fifth year after planting. This suggests that, at least during the early life of a yard, virus buildup does not automatically lead to reduced yield in Citra® hops. In contrast, the irrigation configuration became more important as the yard matured. During the first two production years, yield was similar whether rows had one or two drip lines. But in years four and five, plants irrigated with two drip lines produced about 23–25% more cones than those with a single line. This finding suggests that as hop root systems expand, the way water is delivered becomes increasingly important. Harvest timing mainly affected cone quality rather than yield. Later harvest increased alpha acids (which contribute bitterness in beer) and often increased aroma compounds important for brewing. Importantly, harvesting later within the normal window did not reduce yield. Together, these results show that in young Citra® hop yards established with clean plants, early yield decline is not inevitable. Instead, irrigation design and harvest decisions may play a larger role in long-term productivity and quality.

Technical Abstract: Progressive yield decline in hop (Humulus lupulus L.) is often attributed to virus and viroid accumulation and to cultural practices that may influence perennial vigor. A five-year field study (2019–2023) was conducted in a commercial planting of ‘HBC 394’ (Citra®) established from virus- and Hop stunt viroid-free material to evaluate the effects of pathogen accumulation, irrigation configuration, and harvest timing on yield and cone quality. A split-plot design with repeated measures was used, with irrigation configuration (single vs. double drip line) as the whole-plot factor and harvest timing (early vs. late) as the subplot factor. Annual incidence of apple mosaic virus, hop mosaic virus, hop latent virus, American hop latent virus, and Hop stunt viroid was quantified by RT-qPCR and included as covariates in linear mixed models. The incidence of all pathogens increased significantly over time; by 2023, hop latent virus and hop mosaic virus reached 100% and 81%, respectively. Despite pathogen accumulation, there was no evidence of an overall decline in yield from the second through fifth year of production. A composite covariate representing mean virus–viroid incidence was not associated with yield, although individual pathogen effects were difficult to interpret because of multicollinearity. Irrigation configuration significantly affected yield in a year-dependent manner. No irrigation effect was detected in 2020 or 2021; however, in 2022 and 2023, double-line irrigation increased yield by 25% and 23%, respectively, relative to single-line irrigation. Harvest timing did not affect yield but consistently increased alpha-acid concentration and altered volatile terpenoid composition, whereas irrigation configuration had no meaningful effect on terpenoid profile. A principal component analysis indicated that volatile composition was influenced primarily by harvest timing rather than irrigation treatment. These results indicate that early yield decline in Citra® hop was not attributable to virus and viroid accumulation within five years of establishment, whereas irrigation system design became increasingly important as the planting matured.