Downy mildew of spinach, caused by the obligate pathogen Peronospora effusa, continues to be the most important constraint in the major spinach production areas in the US. Spinach downy mildew can potentially be initiated by asexual sporangia, sexually derived oospores, or dormant mycelium. However, the relative importance of the various types of primary inoculum is not well known. The ability of P. effusa sporangia to withstand abiotic stress, such as desiccation, and remain viable during short- and long-distance dispersal, and the ability of oospores to germinate and infect seedlings have not been established. Thus, the primary objectives of this research were to evaluate the impact of desiccation on sporangia survival and infection efficiency and evaluate the occurrence, production, and germination of oospores. The initial results indicate that desiccation significantly reduces sporangia viability and infection potential. Mating tests using isolate pairs on detached leaves, as well as intact plants, demonstrated that oospores can be produced in certain controlled crosses. Oospores were also observed in numerous commercial cultivars in both CA and AZ, although the frequency of occurrence varied between the two locations. Understanding the variables that effect sporangia viability and oospore production will help in improving the management of the spinach downy mildew pathogen.