Phytophthora Sojae - Disease Cycle

Disease Cycle

Phytophthora sojae overwinters in plant debris and soil as oospores. Oospores are made after the male gamete, antheridium, and female gamete, oogonium, undergo fertilization and then sexual recombination (meiosis). They possess thick cell walls with cellulose that enables them to survive harsh conditions in the soil without germinating for several years. They begin to germinate once the environmental condition is favorable during spring (see environment section for more info) and produce sporangia. They can either germinate directly or indirectly. In direct germination, sporangia directly penetrate the host cells at the plant’s root tips (if it’s within reach). Indirect germination involves sporangia releasing zoospores (if the root is at a farther distance from the sporangia) which encyst on the host plant cells and germinate. Zoospores are biflagellate asexual motile spores. They are dispersed by water flow in the soil and are able to inoculate the roots of plants or seeds. Chemicals such as deadzeins and genistein are released at the tip of the plant roots which attract the liberated zoospores.

Once zoospores have made contact with the host root, they encyst on the surface, break down the plant cell wall with proteolytic enzymes and begin to germinate. Their hyphae will begin to grow through the intercellular space of the plant cells. After establishing its haustoria for nutrients, more oospores will begin to form in the cortical cells of the root. The host plant will begin to exhibit secondary symptoms such as stem canker, wilting, and chlorosis as Phytophthora sojae continue to reproduce. This continuous reproduction renders the plant dead at the end of the season. The oospores are then left to overwinter in the dead plant’s debris and the soil. The cycle is repeated once again in the spring when environmental conditions are favorable (see environment section for more info). The disease is mostly localized where zoospores initially infected the host plant.

Phytophthora sojae is considered to be a monocyclic pathogen and has one effective infection in its cycle. This is because the oospores don’t germinate together at the same time; rather they each have their own distinct favorable condition in which they’ll initiate their germination.