Irrigation and Nitrogen Management Influenced Potato (Solanum tuberosum L.) productivity in the San Luis Valley Abdulssamad M.H. Barka1, Samuel Y.C. Essah2, Allan A. Andales3, Jessica G. Davis4 and David G. Holm2 1 Colorado State University, Dept. of Horticulture and Landscape Architecture, Fort Collins. 2 Colorado State University, Dept. of Horticulture and Landscape Architecture, San Luis Valley Research Center. 3 Colorado State University, Dept. of Soil and Crop Sciences, Fort Collins. 4 Colorado State University, Agricultural Experiment Station, Fort Collins. ABSTRACT Limited availability of agricultural irrigation water coupled with increasing cost of nitrogen (N) fertilizer requires the optimization of irrigation and nitrogen management. To address whether deficit irrigation combined with reduced N fertilizer rate can maintain yield of potato cultivar Canela Russet , field experiments were conducted in the San Luis Valley of Colorado. Experiments were performed over two field seasons and treatments included three levels of irrigation, one of them as control (100% ET replacement) and two as deficit irrigation treatments (90 and 65% ET replacement) and two levels of N fertilization (123 and 157 kg N ha-1). Source of N fertilizer was urea ammonium nitrate. Treatments were factorially arranged in a randomized complete block design, with four replications. The 90 and 100% ET replacement treatments at both N application rates produced higher and similar total and marketable tuber yields, compared to all other treatments in 2016. In 2017 the 90 and 100% ET replacement treatments with 157 kg N ha-1 (optimum) application rate produced higher and similar total and marketable tuber yields, compared to all other treatments. Early tuber bulking was observed in the 100% ET replacement treatment with low N rate in 2016 and with optimum N rate in 2017. Leaf area index (LAI) was at a maximum at 100% ET replacement for both N application rates in 2016. LAI was at a maximum with the 90 and 100% ET replacement treatments with optimum N application rate in 2017. During late tuber bulking, petiole N and K concentrations increased with the lowest irrigation water treatment (65% ET replacement). These results demonstrate that reducing irrigation water by 10% of ET combined with a lower N application rate of 123 kg ha-1 can produce maximum tuber yield with a high late tuber bulking in potato cultivar Canela Russet.