EFFECT OF NITROGEN FERTILIZERS ON SEVERITY OF TAN SPOT OF WINTER-WHEAT

Field experiments were conducted over a 3-yr period in a split-plot design with the winter wheat (Triticum aestivum) cultivar TAM 105. Main-plot treatments were no fertilizer, NH4SO4, and CaNO3 (each at 112 kg/ha total N); and subplots were presence or absence of tan spot. Fertilizers were applied in the fall (one-third of total N applied), late March (one-third), and late April (one-third). Disease evaluation was at the soft dough stage. Application of N fertilizers significantly increased tissue N levels; however, no consistent differences were observed in either disease severity or yield loss from tan spot. In addition, three experiments in the greenhouse were conducted with the same six treatments. Fertilizer application in the greenhouse significantly reduced disease severity in the inoculated treatments. However, there was severe physiologic chlorosis and necrosis from N deficiency in the nonfertilized, noninoculated check which was indistinguishable from the disease. These symptoms were alleviated with both N forms in noninoculated treatments. Thus, the apparent reduction of disease from N fertilizers observed in the inoculated treatments was due to alleviating physiologic chlorosis and necrosis. These data suggest that NH4SO4 and CaNO3 fertilizers appear to reduce disease by delaying natural leaf senescence but do not have a direct effect on tan spot.