Susceptibility of Hop Cultivars and Rootstock to Downy Mildew Caused by Pseudoperonospora humuli

Abstract

Annual downy mildew [Pseudoperonospora humuli (Miyabe & Takah.) G.W. Wilson, (1914)] epidemics threaten hop (Humulus lupulus L. var. lupulus) production throughout the eastern United States. Rootstock rot complicates foliar disease assessments because dormant buds may rot before producing a symptomatic basal shoot and noncolonized buds produce healthy shoots. We selected 12 cultivars to evaluate downy mildew susceptibility (2016 and 2017) and examined the rhizomes of a subset of six cultivars (2018) that showed clear differences in foliar disease ratings to determine rootstock rot susceptibility. Trials were conducted on nontrellised hop yards established at two research farms in 2016 and managed without fungicides. The relative area under the disease progress curve (rAUDPC) values for foliar disease severity and density ratings were consistently higher for ‘Cascade’, ‘Centennial’, and ‘Nugget’ than for ‘Newport’, ‘Tahoma’, and ‘Columbia’. Only ‘Centennial’ had a higher wet-rot cortex discoloration (CD) incidence (71.5%) and severity (31.9%) in its rhizomes than the other cultivars. Among cultivars with low foliar disease levels, Columbia was less vigorous (shoots/plant) than ‘Tahoma’, but had similar wet-rot CD (incidence, 46.4% to 47.2%; severity, 12.5% to 17.7%). The levels of wet-rot CD for ‘Tahoma’, ‘Newport’, and ‘Columbia’ were comparable to those of some cultivars with more severe foliar disease symptoms (‘Nugget’ and ‘Cascade’). Differences in foliar disease among cultivars with similar levels of rootstock rot suggest a resistance defense mechanism to P. humuli. Additional work is needed to clarify rootstock infection sites and characterize defense responses to aid future breeding efforts.