Sorghum downy mildew

Pathogen and disease type. The disease is caused by the oomycete Sclerospora sorghi, which induces a systemic infection in host plants. It is considered one of the most destructive diseases for certain cereal crops, as the pathogen colonizes the systemic tissues of the plant early in its growth stage, leading to persistent internal infection.

Host range and symptoms. The primary hosts for this pathogen are sorghum and maize (corn). Early symptoms include chlorotic stripes on the leaves, which gradually turn into necrotic tissue. In humid conditions, a characteristic white, downy growth of conidiophores and conidia appears on the underside of the leaves, which is a diagnostic hallmark of this downy mildew.

Conditions for development. Sclerospora sorghi thrives in warm, humid environments, with temperatures between 20 °C and 25 °C being optimal for conidial production. The pathogen survives in the soil as oospores, which can remain dormant for several years. High soil moisture and relative humidity are critical factors for both the primary infection from the soil and secondary spread by airborne conidia.

Economic impact. The pathogen causes significant physiological damage, resulting in stunting, malformation of ears or panicles, and overall reduced vigor. Systemically infected plants often fail to produce viable grain, leading to severe yield losses that can exceed 50 percent in susceptible varieties under favorable conditions for the pathogen's development.

Protection and prevention measures. Managing this disease requires a multifaceted approach to break the life cycle of the pathogen:

• Planting resistant or tolerant hybrids is the most effective management tool.
• Applying systemic fungicide seed treatments (such as metalaxyl-based products) to protect seedlings during the critical early growth stages.
• Rotating crops to non-host species for a minimum of 3 years to reduce the soil-borne oospore inoculum level.
• Implementing effective sanitation practices, including the removal of volunteer plants and weeds that may act as reservoirs for the pathogen.