Nitrogen dynamics of the carrot crop and influences on yield and Alternaria and Cercospora leaf blights
The influence of nitrogen (N) management (N rates: 0, 50, 100, 150, 200% of recommended; foliar N), cultivar (‘Idaho’ and ‘Fontana’), soil type (organic and mineral), and year (2002, 2003, and 2004) on the seasonal patterns of N uptake, dry matter (DM)production, and DM and N partitioning were examined to understand carrot susceptibility to Alternaria (ALB) and Cercospora (CLB) leaf blights. Annual N application rate without rotation had minimal effect on carrot yield, quality, or storability, except in 2003 and 2004 on mineral soil when yield increased up to 200 and 91% of recommended N rates. Seedling death increased with increasing N rate in 2003 and 2004 on mineral soil. Increasing preplant N consistently decreased the severity of ALB and CLB on both soil types and for susceptible (‘Fontana’) and less susceptible cultivars (‘Idaho’). Carrots developed fibrous roots up to 150 cm depth (50-65% below 30 cm) and N uptake per root was equal at each depth. Nitrogen and DM in the storage root accumulated slowly up to 50-60 days after seeding (DAS) and accumulated rapidly thereafter until harvest, whereas N and DM in the tops accumulated prior to 50-60 DAS and generally levelled off or declined beyond 100-120 DAS. Differences in ALB or CLB susceptibility among cultivars, treatments, or soil types were unrelated to leaf N concentration, DM or N partitioning, or rooting depth and distribution. This work suggested that carrots can access deep soil N, and residual N from the preceding season had more effect on yield than current year application. The effects of high N rates on disease severity were partially explained by a delay in leaf senescence that delayed ALB development, and regeneration of leaves that increased tolerance to both diseases. Preplant N had more influence on yield and disease susceptibility than sidedressed or foliar applied N, a result attributed to deep N uptake and early season N partitioning to leaves. Carrots remove up to 280 kg.ha-1 N and can be used as an N catch crop to remove residual soil N. The results highlight the importance of integrating crop nutrition into disease management studies.