2018 ASHS Annual Conference

Root nitrate concentration is an increasingly important quality concern for a large segment of the processing carrot industry, most notably where carrots are grown for baby food. N fertilizer topdress applications are often made late in the season in part to promote adequate and healthy carrot shoot growth, a key requirement for effective mechanical harvest; however, this may also increase risk of high nitrate content at harvest. Late-season foliar disease severity has the potential to further exacerbate nitrate challenges by reducing shoot N demand. The objective of this research was to investigate how late-season N management strategies and foliar disease influence dynamics of carrot root nitrate accumulation and production tradeoffs. Two on farm experiments were conducted in 2017 on sandy soils in Hart, Michigan. Carrots (cv. Cupar) were planted in mid-April and harvested in mid-October. Experiment 1 evaluated the effects of late-season (September) N fertilizer rate (0, 34, 67, or 101 kg N ha^-1 as urea) and source (urea, slow-release, or foliar applied N) on carrot yield and quality, root nitrate content, and shoot growth. Carrot roots were subsampled every 2 wk leading up to harvest to evaluate temporal dynamics in root nitrate concentration. In Experiment 2, a subset of three treatments from a larger fungicide spray trial were selected to represent a gradient of foliar disease severity (low, medium, and high), and carrot subsamples were collected at harvest to evaluate impacts on root nitrate. Carrot yield was not affected by late-season N fertilizer rate or source treatments. Similarly, no significant differences in shoot biomass were observed, despite a modest trend toward greater biomass with higher N rates. All September N applications increased root nitrate content relative to the control. For N rates greater than 34 kg N ha^-1, nitrate concentrations increased linearly between fertilizer application and harvest. Slow release and foliar applied N both maintained lower root nitrate levels than urea over the course of the experiment. Foliar fungal disease severity was positively correlated with root nitrate at harvest, with concentrations over 7 times greater in high disease treatments (50-75% leaf area affected, Horsfall-Barratt scale) than in low disease treatments (3-6% leaf area affected). Our results suggest that late-season N applications in excess of recommended total season rates may have limited production benefits for carrots harvested in October, while also increasing risks of excessive nitrates, particularly if conditions are favorable for foliar disease.