Peronosclerospora sorghi

Peronosclerospora sorghi is a plant pathogen. It is the causal agent of sorghum downy mildew. The pathogen is a fungal-like protist in the oomycota, or water mold, class. Peronosclerospora sorghi infects susceptible plants though sexual oospores, which survive in the soil, and asexual sporangia which are disseminated by wind. Symptoms of sorghum downy mildew include chlorosis, shredding of leaves, and death. Peronosclerospora sorghi infects maize and sorghum around the world, but causes the most severe yield reductions in Africa. The disease is controlled mainly through genetic resistance, chemical control, crop rotation, and strategic timing of planting. Peronosclerospora sorghi has a broad host range, particularly plants in the sorghum family. These host plants include Sorghum bicolor, or sorghum, Sorghum sudanense, or Sudan grass, and Sorghum palepense, or Johnsongrass. Its hosts also include Pinnisetum glaucum, or pearl millet, and Zea mays, or maize. When maize plants are infected as seedlings, the first symptom to appear is chlorosis of the leaves. As the plant grows, it often shows stunting, and necrotic lesions begin to form on the leaves. As the leaves continue to grow, a white, downy growth may develop in the necrotic lesions, especially on the underside of the leaves. This downy growth is the result of the production of conidia and conidiophores under the proper environmental conditions. Sorghum and maize express very similar symptoms. In sorghum, chlorosis of the seedlings is very common after infection. As the leaves get older, they can express white striping, which eventually leads to the necrosis of the white striped tissue. When the leaves die, they begin to become shredded in appearance, similar to hail damage. This symptom is associated with the production of oospores in the leaf tissue. If sorghum plants are young and produce conidia on their leaves, this type of infection often leads to the death of the plant. If the sorghum plants are older, the pathogen will often produce oospores in the leaves. Plants are usually capable of surviving this type of infection and will survive until maturity. The disease may cause the tassels and ears of maize plants to develop improperly or not form at all. This also occurs within the panicle of the sorghum plant. Key factors in diagnosing downy mildew on maize and sorghum caused by Peronosclerospora sorghi include chlorosis of the leaves, followed by white streaking of the leaves. The leaves could also have a white, downy growth on the underside of the leaf blade. The white streaked areas of leaves become necrotic over time, leading to a shredded leaf appearance. In the maturing plant, the reproductive structures do not form properly or do not form at all. This leads to a sterile plant. 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.