Changing and more volatile climate conditions are leading to higher vulnerability and lower resilience for crop production. Recent studies indicate that crop diversity in agricultural fields may ensure pest control and yield stability in the face of environmental changes. However, few studies have evaluated crop diversity in the field, especially at the within-species level. Applying a new indicator, H-T*, which integrates both the spatial evenness of different varieties and molecular genetic data (within and between variety genetic diversity), we followed the evolution of bread wheat genetic diversity on French agricultural landscapes during the 20th century. To our knowledge, the monitoring of crop genetic diversity at such a large but detailed spatial and temporal scale has never before been conducted. In comparison to two frequently used but less integrative indicators (the number of varieties grown in the field and their allelic diversity as measured by the Nei index), the H-T* indicator revealed increasing genetic homogenization overall. This trend was due to the disappearance of diversity within varieties (initial replacement of landraces by more homogeneous old lines and later by modern pure lines), to the spatial homogenization occurring in the last period of the 20th century with the different 'departements' (French administrative territories) progressively cultivating the same varieties and to their increasing genetic similarities. This result calls into question the effects of plant breeding, seed system organization and seed regulation on wheat genetic diversity, especially in the context of current environmental changes.