Technical Abstract: Because of differences in life histories between Puccinia triticina, a wind-dispersed pathogen with the potential for many generations, and Pyrenophora tritici-repentis, a residue-borne pathogen with fewer generations, wheat mixtures are expected to be more effective at controlling leaf rust than tan spot. The objectives of this research were to determine the effect of wheat cultivar mixtures with differential resistance functions on the severity of tan spot and leaf rust, to evaluate yield of the mixtures in the presence or absence of disease, and to directly compare the relative effectiveness of cultivar mixing for tan spot vs. leaf rust. In a field experiment at two sites in Kansas over two growing seasons, winter wheat cultivars Jagger and 2145, which have differential resistance reactions to leaf rust and tan spot, were each planted in proportions of 0.25, 0.50, 0.75, and 1.00. Plots were inoculated with each pathogen alone, both pathogens, treated with a fungicide, or exposed to ambient conditions. For both diseases for all site-years, severity decreased substantially on the susceptible cultivar as the proportion of that cultivar decreased in mixture. Mixtures were significantly more effective at reducing leaf rust than tan spot in three out of four site-years. Mixtures generally yielded the same as the weighted mean of components in monoculture, though in two out of three site years at least one fungicide-treated and one diseased mixture each yielded higher than expected values. While this particular mixture produced only modest yield benefits, the potential for simultaneous reductions in tan spot and leaf rust was demonstrated.