Downy mildew of brassicas

The disease is caused by the oomycete Hyaloperonospora brassicae, a specialized obligate pathogen targeting cruciferous plants. Historically classified as Peronospora parasitica, this organism is widely recognized as the primary agent of downy mildew in various brassica crops globally.

Susceptible hosts include all cultivated brassicas such as cabbage, broccoli, cauliflower, Brussels sprouts, kale, and oilseed rape. The pathogen serves as a significant limiting factor in intensive vegetable production, particularly in regions with temperate and humid climates where the pathogen can persist in plant debris and soil.

Symptoms initially appear as small, chlorotic, angular spots on the upper leaf surface, limited by leaf veins. Under humid conditions, a characteristic greyish-white downy fungal growth (conidia and conidiophores) develops on the underside of the infected leaves. As the disease progresses, these spots may turn brown or necrotic, leading to leaf drop.

The development of Hyaloperonospora is highly dependent on environmental factors, specifically high relative humidity (above 90%) and cool to moderate temperatures (10–20°C). Water films on leaf surfaces are essential for spore germination and infection, making rainy weather and heavy dew primary drivers for rapid outbreaks.

The impact of this disease is substantial, causing stunting, reduced biomass, and severe cosmetic damage that reduces marketability. In seedlings, it can cause damping-off or total plant death. In mature crops, the reduction in leaf area lowers yield potential and leaves tissues vulnerable to secondary infections, often resulting in significant post-harvest losses.

• Crop rotation with non-host species for at least 3 years.
• Strict control of cruciferous weeds that act as alternate hosts.
• Use of certified pathogen-free seeds.
• Improved air circulation and drainage in production fields.
• Timely application of fungicides belonging to groups effective against oomycetes.