Abstract:Breeding for adult plant resistance (APR) is currently impeded by the low frequency of annual field?based testing and variable environmental conditions. We developed and implemented a greenhouse?based methodology for the rapid phenotyping of APR to leaf rust in barley to improve the efficacy of gene discovery and cloning. We assessed the effects of temperature (18 and 23°C) and growth stage (1?5 weeks) on the expression of APR in the greenhouse using 28 barley genotypes with both known and uncharacterized APR. All lines were susceptible in week 1, while lines carrying Rph20 and several with uncharacterized resistance expressed resistance as early as week 2. In contrast, lines lacking Rph20 and carrying either Rph23 and/or Rph24 expressed resistance from week 4. Resistant phenotypes were clearest at 18°C. A subset of 16 of the 28 lines were assessed for leaf rust across multiple national and international field sites. The greenhouse screening data reported in this study were highly correlated to most of the field sites, indicating that they provide comparable data on APR phenotypes for screening purposes.