Although "A. euteiches" can potentially infect hosts at any point in the growing season, infection usually begins during seedling emergence. The primary source of inoculum is oospores, which can be found either in the soil or in infected plant debris from previously infected host plants. Oospores germinate as a response to chemical signals detected from the roots of new hosts plants. Germination can either be direct or indirect. Either way, infecting begins at the cell in the root tips of the host plant. In direct germination, the oospore produces hyphae which directly penetrate host cells at the plant's root tips. In indirect germination, the oospore produces sporangia which release zoospores. The zoospores then encyst on the host plant cells, and germinate. After direct or indirect germination, coenocytic hyphae of "A. euteiches" colonize host tissue through inter- and intra-cellular growth. For sexual reproduction, hyphae develop male and female gametangia called, respectively, antheridia and oogonia. Because "Aphanomyces euteiches" is homothallic the antheridium and oogonium arise from the same hypha and are self-compatible, meaning separate mating types are not needed for sexual reproduction. The antheridium fertilizes an oogonium, which then develops into a single oospore, which is 20-35 micrometers in diameter.