Disease Cycle
Aphanomyces root rot is an example of a monocyclic disease, causing only one infection cycle per season. This is in contrast to polycyclic diseases, which can infect new hosts and produce multiple disease cycles within a single season. A. euteiches is not usually a saprophyte in nature, but can be grown in culture in the lab.
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.
When growing under optimal conditions, an infection of A. euteiches can result in symptoms within 10 days, and oospores can be formed between 7 and 14 days. Oospores become dormant after being formed, and can survive for more than 10 years.
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