Fungicide and Nitrogen Effects on Winter Wheat under Low Foliar Disease Severity

Fungicide utilization is expected to increase on winter wheat (Triticum aestivumL.) in response to registration of fungicides that control powdery mildew (Erysiphe graminisDC) and Septoria nodorum blotch [Phaeosphaeria nodorum(Müller)]. Field experiments were conducted in New York in 1985 and 1986 on a Honeoye silt loam (fine‐loamy, mixed, mesic Glossoboric Hapludalf) to examine the influence of N [67 kg ha−1at Zadoks Growth Stage (GS) 25, and split‐application of 67 + 67 kg ha−1at GS 25 and GS 31], and timing of fungicide application (0.28 and 1.8 kg ha−1a.i. of triadimefon [1‐(4‐cholorphenoxy)‐3,3‐dimethyl‐1‐(1H‐1,2,4‐triazol‐1‐yl(‐2‐butanone] and mancozeb [zinc ion and manganese ethylene bisdithiocarbamate], respectively, at GS 31 or GS 55) on disease severity, CO2exchange rate (CER) of the flag leaf, yield components, and grain yield. Moderately dry and warm conditions resulted in low foliar disease severity from GS 31 to GS 55, thereby providing limited secondary inoculum to infect the flag leaf (<8% disease severity at GS 75) during excessively wet grain‐filling periods. Fungicides did not influence flag leaf CER and kernel weight, which was reflected in the limited grain yield responses to fungicide applications at GS 31 and GS 55 compared to no fungicide treatment (5.90, 6.07, and 5.81 Mg ha−1, respectively). The split‐N application increased grain yield compared to the single N treatment (5.58 to 6.27 Mg ha−1), especially in 1985 (5.47 to 6.40 Mg ha−1) when kernels per spike were increased (26.5 to 30.2). The data suggest that high rates of N, split‐applied, have potential to increase yield of winter wheat and foliar disease severity in the northeastern USA. Fungicides, however, should not be applied as a preventive measure for mildew and leaf spot control if severity is low from GS 31 to GS 60.