Potato ( Solanum tuberosum L.) tuber yield and quality are impacted by irrigation and nitrogen (N) management. This study was conducted in the Pacific Northwest (PNW) region of United States to evaluate the effects of deficit irrigation (DI) and rates of pre-plant and in-season N applications on Ranger Russet and Umatilla Russet cultivars. In 2004, with Ranger Russet only, DI with 20% lower total irrigation for the entire growing period resulted in 28% tuber yield reduction compared to that of plants irrigated to replenishment full evapotranspiration (ET), i.e., full irrigation (FI). A subsequent study in 2006 and 2007 with DI (14% to 17% deficit) resulted in tuber yield reduction of 7% to 10% in both cultivars compared to full ET irrigation. Yield reduction in DI was generally attributed to reduction in large weight tubers, >0.227 kg/tuber, in both cultivars across three years. Petiole NO 3-N concentrations were greater in plants grown under DI as compared to those of plants in full ET irrigation across all years and cultivars, particularly during tuber maturation stage. This is an important consideration as increased N availability during the late growing season adversely affects tuber quality. Petiole NO 3-N concentrations increased with increased in-season N rates. In 2007, 112 kg.ha -1 in-season N resulted in petiole NO 3-N concentrations below desirable concentrations across most of the growing season in both cultivars. This, in turn, contributed to a significant reduction in tuber yield as compared to the 224 kg.ha -1 in-season N rate. Continuous DI with 14% to 20% reduction in water as compared to irrigation to replenish full ET, begun three to four weeks after seedling emergence, had significant negative effects on tuber yields of both cultivars in high-production irrigated growing conditions. Application of N up to 112 kg ha -1 as pre-plant soil applied plus 224 kg ha -1 of in-season fertigation in five applications at two-week intervals beginning four weeks after seedling emergence appears to be adequate to support high yields of high-quality tubers.
