Wheat cultivar mixtures can provide control against numerous diseases such as Septoria tritici blotch (STB) caused by Zymoseptoria tritici. One important criterion for mixture design is the contrast in resistance levels between the mixed cultivars, but these resistance levels can vary with local conditions (eg. composition of pathogen populations, climate). The objective of this work was to assess disease control provided by a cultivar mixture in different locations in Europe, and its relation to factors including the local resistance level of cultivars in pure stand and climatic conditions. A field experiment was carried out in seven sites in Europe (in France, Belgium, Denmark and Ireland) during three successive cropping seasons. In each site, disease severity was measured on each cultivar (Apache and Cellule) in pure stands and in a 50:50 mixture. The resistance level of each cultivar was variable between site x year. Mixture effect, computed as the difference of disease severity of the cultivar in mixture compared to pure stand, ranged from +3% (more disease in mixture) to -3% (less disease in mixture) on Apache and from +1% to -12% on Cellule. The largest disease reductions in mixtures were observed for each cultivar in sites x years where it appeared as susceptible in pure stand. These results show that the efficiency of a particular mixture can vary between sites and years and open perspective to better adapt mixture design to local conditions.