During the Green Revolution, the disappearance of landraces with the spread of new genetically homogeneous varieties evolved into a global public problem. This “genetic erosion” endangered the future of plant breeding and, thus, of food security. The use and conservation of plant genetic resources are still considered a cornerstone by the FAO for global food security. The aim of this presentation is to discuss the foundation and evolution of scientific conceptions and practices of crop diversity conservation from the Green Revolution to date. First, I will show how experts in international arenas, especially the FAO, have negotiated the nature and framework of this problem. I will illustrate the construction of a sectorial management where the South serves as the genetic diversity provider and the North, with its techno-scientific power, functions as the administrator. In this geographical functional distribution, crop diversity scattered over an indefinite landscape is a raw resource that only scientific specialists can transform into something valuable as modern varieties. Second, I will give some examples from my ethnographic work and field research on practices related to farmer management of crop diversity. The aim is to participate to rethink boundaries between modern and old/local varieties, the impact of technoscientific power, and to put farmers' know-how back at the core of the analysis.