Nitrogen and Fungicide Effects on Yield Components and Disease Severity in Wheat

Spring N applications increase wheat ( Triticum aestivum L.) grain yields in the Mid‐South. The interactive effect of increased N rates and foliar fungicide applications on the common Mid‐South winter wheat foliar diseases—leaf blotch ( Septoria tritici Rob. in Desm.), glume blotch and leaf spot [ S. nodorum (Berk.) Berk.], and leaf rust ( Puccinia recondita Rob. ex Desm. f. sp. tritici )—is not clearly understood. Research was initiated in 1984 on a Collins silt loam soil (coarse‐silty, mixed, acid, thermic Aquic Udifluvents) and continued each year through 1987 to evaluate the effect of N rates and fungicide applications on disease severity in wheat. Spring N rates of 0, 30, 45, 60, 90, and 120 lb/acre were applied at Feekes' growth stage (GS) 3. Fungicides were applied twice during the spring at GS 8 to 9 and GS 10.1 to 10.5. The research was continued between 1988 and 1990 on a Loring silt loam soil (fine‐silty, mixed, thermic Typic Fragiudalfs), but with a reduced number of N treatments. Experimental sites were changed each year on these two soils. The effects of N rates and fungicide applications to wheat were evaluated through grain yields, kernel weights, test weights, and estimated percentage plant foliage and heads infected by diseases. The applied N rates did not have the same effect on increasing yields during the years of the experiment. Generally, yields were increased to a level and then decreased with higher N rates. Fungicide applications had a greater effect on yields produced at the higher N rates. Severity of diseases differed with year, N rate, and fungicides. The severity of the three diseases was increased with N rate, especially when fungicides were not applied, but fungicides reduced disease severity. Yield reductions could not be attributed to any single disease but to a combination of diseases, and reductions could not be ascertained from kernel weights or test weights. Research Question Throughout the Mid‐South, wheat yields may be reduced by one or more plant diseases that may incraese with incraesed N fertilization. This study evaluates spring N and foliar fungicide effects on grain components and disease severity of wheat. Literature Summary Diseases have had a negative but inconsistent effect on grain yields. Not only is severity inconsistent with years, but the predominate disease may differ. In this region, one or more diseases may affect wheat during the growing season. High N rates tend to increase infection by increasing plant susceptibility to these diseases. The disease severity increases with applied N when fungicides are not applied. Study Description Spring N rates and fungicides were applied over 7 yr, evaluating their effect on wheat. Experiment Design: Main‐plots, spring N rates of 0, 30, 45, 60, 90, and 120 lb/acre plus a split N rate (45 lb applied at Feekes’ GS 3 + 45 lb applied 3 to 4 wk later) were applied to a Collins silt loam soil for 1984 through 1987. Main plot N rates plus the split‐rate were applied to a Loring silt loam soil. Sub‐plots, fungicides, Bayleton plus Dithane M‐45 or Manzate 200 with Triton CS‐7 were applied between GS 8 and 9 and again between GS 10.1 and 10.5. Applied Question Were wheat yields increased with increased spring N rates and fungicide applications? Wheat yields were increased with spring applied N rates but the effect was inconsistent over the years of the study. Fungicide applications increased yields and decreased disease severity. Fungicide effectiveness for increasing yields was greater at the higher N rates. Disease severity increased with applied N rates but fungicides reduced severity, thus reducing the impact on yield. Reduced kernel and test weights and incraesed disease severity were associated with the higher N applications generally requiring fungicide applications. Trends in spring weather patterns were not observed that could serve as an indicator for fungicide application.