This article presents the case of the discovery, made invisible by historiography, of the microorganism responsible for wheat diseases by the Italian physician and agronomist Giovanni Targioni-Tozzetti in Florence in 1767. To understand this invisibility, we first reconstructed the cognitive dimensions of the discovery and then examined the combination of contexts that made it possible. Thus, instead of endorsing the classical rhetoric of the historiography of science, which reduces discovery to experimental repetition alone, the article shows that it is necessary to repeat the context for experimental repetition to make sense. The context of Targioni-Tozzetti's discovery was constituted by the coordination of four traditions: microscopic research, botanical systematics, parasitology inherited from Francesco Redi, and the public health and agronomic tradition of Florence. At that time indeed, only Tuscany in Europe had the peculiarity of coordinating these four traditions, and since European scholars did not master this context, they were unable to repeat the experience properly, resulting in the invisibility of the discovery.