Peronosclerospora Sorghi - Disease Cycle

Disease Cycle

Personosclerospora sorghi has a polycyclic disease cycle. It is capable of causing secondary infections of susceptible hosts throughout the growing season. Its resting structures, the structures that allow the pathogen to overwinter, are the oospores. These oospores are produced in the infected plants from the previous growing season. They are often disseminated by wind. The oospores can overwinter in the soil and in the debris on the surface of the soil. The oospores have very thick walls, which makes them capable of surviving in the soil for years under many different weather conditions.

It is also possible for oospores and mycelium to overwinter in the seeds of maize. The mycelium infects the scutellum of the seed. The oospores and mycelium that are present in the seed often lose their viability when the seeds are dried, but under the right circumstances, it is possible for these infected seeds to become a source of inoculum, infecting the maize plant as it grows. Infection of the seed itself often occurs with the plant that produced the seed had been infected later in development.

The oospores are the main source of the primary inoculum of this disease. They are present in the soil when the host seedlings are germinating. The oospores then infect the roots of the seedlings. This type of infection is a systemic infection of the plant. The pathogen grows throughout the plant, infecting the leaves as they grow, leading to chlorosis. The chlorotic leaves develop white streaks. These white streaks are the location of oospore production. This only occurs in plants that were systemically infected as a seedling. When the oospores become mature, the white streaks on the leaves turn brown and become necrotic. These necrotic areas become shredded over time, which is how the mature oospores are disseminated. The spores are carried by the wind, and they become the source of inoculum in subsequent generations.

As the pathogen continues to develop in the host plant, there may also be production of conidia on the leaf surface. It is the conidia and the conidiophores that cause the white, downy growth on the undersides of the leaves. Conidia develop quickly and are released from their conidiophores within five hours of maturation. After they are released, they are wind disseminated. If the conidia land on mature host plants, they infect and cause local lesions on the leaves. These lesions do not systemically infect the plants, and there is little overall damage from them. If the conidia land on a plant that is only a few weeks old or less, the plant is susceptible to systemic infection by the conidia. The conidia can be the main cause of the infection if they are being produced on a host plant that is in the same area as other susceptible host plants that are just emerging from the ground.