In‐Season Canopy Reflectance Can Aid Fungicide and Late‐Season Nitrogen Decisions on Winter Wheat

Core Ideas Wheat grain yield response to foliar fungicide is highly dependent on environment. Late‐season N can offset protein dilution from fungicide‐driven yield increase. Foliar N rates necessary to impact grain protein have potential for leaf burn. Controlled‐release N and urea ammonium nitrate both impacted wheat responses similarly. In‐season canopy reflectance can aid in fungicide decision. Winter wheat (Triticum aestivum L) response to foliar fungicide and late‐season N is inconsistent. We evaluated various late‐season N fertilizer‐by‐foliar fungicide combinations to test whether wheat anthesis normalized difference vegetative index (NDVI) could aid fungicide decisions. Six site‐years were conducted with N fertility >120 kg N ha−1 during 2012–2013 and 2013–2014 in Oklahoma, using a three‐way factorial arrangement of a randomized complete block design with four replications. Fungicide treatment (with or without), foliar N source (urea ammonium nitrate [UAN] or controlled‐release N), and N rate (0, 2.8, 5.6, or 28 kg N ha−1) were applied during anthesis. Statistical analyses were performed by site‐year, by yielding environment (low [LY] <3.2 Mg ha−1 and high [HY] >3.2 Mg ha−1), and pooled across site‐years. Grain yield was 0.21 Mg ha−1 greater due to fungicide in the pooled analysis, mostly led by the 0.59 Mg ha−1 yield advantage in HY as opposed to non‐significance in LY. Fungicide increased test weight in 6 to 7 kg m−3 across all analyses, and 28 kg N ha−1 increased grain protein by ∼10 g kg−1 but caused leaf burn. Anthesis NDVI <0.6 occurred when preanthesis abiotic stresses restricted yield potential, not warranting fungicide application. Fungicide can benefit grain yield and test weight and could be considered when anthesis NDVI >0.6, but yield response will depend on grain‐filling conditions. Foliar N rates needed to increase grain protein in a well‐fertilized crop have potential for leaf burn.