Sorghum downy mildew, or Sclerospora sorghi, is a devastating fungal disease that can cause significant losses worldwide. The disease is severe especially in areas with warm and wet environments, where the fungus is able to spread rapidly. Therefore, itrghum producers need to understand the disease cycle, its interaction with the environment and risk factors to prevent and manage this disease.
Cultural practices, such as crop rotation and the use of resistant varieties, must be practiced. This article will provide an overview of sorghum downy mildew disease, including its symptoms, biology, and control.
Downy Mildew Disease Management in Sorghum
The Causal Organism of Sorghum Downy Mildew Disease
- Peronosclerospora sorghi, the pathogen is white, filamentous, oomycete, systemic and obligate parasite, has a wide host range, attacking sorghum, maize, millet and wheat. It produces intercellular, aseptate mycelium capable of infecting all parts of the plant.
- Sporangiophores sprout out via the stomatal pores, either solitary or sometimes in bunches, robust and dichotomously divided.
- Spores are globular, uni-cellular, transparent, and thin-walled, and can remain viable in the soil for atleast 4 years. The dark brown, thick walled oospores are shaped like a sphere.
The Disease Cycle of Sorghum Downy Mildew Disease
- The disease cycle begins when the fungus infects the sorghum plant through meristematic tissue. Infection occurs when the spores land on the leaves of the plant and germinate.
- The germinated spores then produce hyphae, or filaments, which penetrate the leaf epidermis and cause infection. Tissue degradation results in long and narrow cuts on the plant. The favorable temperature for the formation of conidia in huge quantities is 20-23°C at night.
- The disease continues to persist on the collateral host, Heteropogen centortus.
Mode of Spread and Survival of The Pathogen in Sorghum Crop
- Control of the disease is difficult, as the fungus persists in the soil and plant residues as oospores, primary source of the systemic infection.
- Sporangia carried by wind currents are the secondary source of infection. The presence of mycelium in the seeds of infected plants is another source of infection.
Causes / Conditions Favorable for Sorghum Downy Mildew Disease Spread in The Field.
- High Relative Humidity – RH at 100% results in maximum spore formation.
- Temperature – Favorable temperature for spore formation is 21-23°C at night.
- Climate – Cooler climatic conditions along with slight drizzling favors disease spread.
Symptoms of Sorghum Downy Mildew Disease
The most common symptom is the presence of white, downy mildew on the leaves of infected plants. These patches of mildew are oval shaped, will often start at the base of the leaf and spread along the veins. As the disease progresses, these patches of mildew will darken, eventually turning gray or black. The lesions can coalesce and form large patches on the leaves. Small, black, round spores may also be visible on the affected leaves’ undersides.