A three factor experiment with winter wheat (Triticum aestivum L.) was carried out during two years
on a coarse sandy soil in Denmark. The factors comprised four irrigation strategies including no
irrigation, three nitrogen levels providing 67, 83 or 100% of the recommended nitrogen rate, and two
strategies for control of leaf diseases (with and without fungicides). Different varieties were used in
the two years, Pepital in the first year and Hussar in the second year. Mildew dominated the trial in
the first year, but was almost absent in the second year. Septoria occurred in both years, but most
severely in the second year. Irrigation increased grain yield, but there were no significant differences
between the three strategies, where irrigation was applied. The effect of irrigation on yield was almost
solely via an effect of increased transpiration, whereas water use efficiency and harvest index was
unaffected. There was a significant interaction for grain yield between irrigation and nitrogen
strategies with higher irrigation effects at higher nitrogen rates. This interaction was absent at high
disease levels. Increasing nitrogen rate increased grain yield in the second year, but not in the first
year. This was attributed to an increasing mildew incidence with increasing nitrogen rate. Irrigation
also increased mildew incidence. This caused significant interactions for grain yield between fungicide
application and nitrogen rate and between fungicide application and irrigation strategy. Septoria was
also significantly affected by both nitrogen and irrigation strategies, but to a lesser degree and not in
a consistent manner. The main effect of disease on grain yield was through a reduction in harvest
index and a reduction in grain weight. Increasing nitrogen rates slightly reduced harvest index both
on a dry matter and on a nitrogen basis. The interaction effects were smaller than the effects of the
main factors, and the effect of changes in irrigation or nitrogen strategy on disease incidence could
not substitute for the effective disease control obtained by fungicides. The results do, however,
indicate possibilities of improving the efficiency of current decision support systems for both fungicide
control by better timing of application and irrigation scheduling by reducing water application.
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