Philippine downy mildew

Pathogen and disease type. The pathogen causing Philippine downy mildew is an oomycete known as Peronosclerospora sacchari. This organism acts as an obligate parasite, causing systemic infection within the host plant. Unlike true fungi, it is a water mold, which means its reproductive cycle is heavily dependent on surface moisture and humidity to produce and disperse its spores effectively.

Host crops. The primary host of Peronosclerospora sacchari is sugarcane, but it is a major concern for maize production in tropical and subtropical regions. The pathogen infects the crop at the seedling stage, invading the meristematic tissues and spreading throughout the plant as it grows, which severely compromises the physiological development of the maize plant.

Symptoms and signs. The characteristic symptoms begin as chlorotic streaks or stripes along the leaf veins, which may turn white over time. During periods of high humidity, a dense, whitish fungal-like growth appears on the underside of the leaves. Infected plants often exhibit stunted growth, excessive tillering, and distorted leaf structures, frequently failing to produce viable ears or seeds.

Development conditions and impact. The development of this disease is favored by temperatures between 20°C and 28°C and relative humidity exceeding 90%. Nighttime moisture is critical for the production of conidia, which are then disseminated by wind. The economic impact is devastating, as untreated fields can experience near-total yield loss, and the pathogen is strictly regulated as a quarantine pest in many countries.

Management and prevention. Controlling Peronosclerospora sacchari requires a combination of strict regulatory and agronomic practices. Management strategies include:

• Planting resistant or tolerant maize varieties specifically developed for affected regions.
• Enforcing strict quarantine regulations on the movement of sugarcane and maize germplasm.
• Implementing crop rotation to break the pathogen's life cycle.
• Sanitation practices, such as removing and destroying infected plants to prevent inoculum buildup.