Cleistothecia of Blumeria graminis are a means of survival under adverse conditions as well as a means of sexual reproduction, and are produced by the generative mycelium at the end of the vegetation period. Vital stains and 14C-labeled sucrose were transported from the host (Triticum aestivum) through the haustoria into the generative mycelium. Translocation was intense during the early developmental stage of the generative mycelium. Colonies of later stages with macroscopically visible developing cleistothecia showed reduced staining and labeling. This correlated with an increase in the number as well as the degree of haustorial encasement and papillae formation. Detached mycelia of later developmental stages produced ripe cleistothecia containing ascospores of high germination rates in vitro, but early stages with microscopically small primordia only developed dark fruit bodies that did not produce ascospores. The data indicate that nutrition supply by the host is essential at the early stages of the generative mildew mycelium. The resulting metabolites are mainly stored within the hyphae. At later developmental stages, the generative mycelium progressively becomes more independent of nutrition supply by the host, providing its own metabolites for the growing fruit bodies.