Downy mildew of crucifers

The causal agent of this disease is the oomycete Peronospora parasitica, which is responsible for downy mildew. It is an obligate parasite that exclusively infects members of the Brassicaceae family, including cabbage, broccoli, cauliflower, radish, and oilseed rape.

Symptoms typically manifest on the leaves as irregular yellow or chlorotic spots. Under conditions of high humidity, a greyish-white fungal growth appears on the underside of the leaves. As the infection progresses, these lesions merge, leading to necrotic spots, leaf yellowing, and premature senescence of the foliage.

The development of the pathogen is favored by cool temperatures, ranging from 10°C to 20°C, and high relative humidity. The fungus produces sporangia that are dispersed by wind and water splashes. It survives between seasons as oospores in soil or on infected plant debris and volunteer cruciferous weeds.

The economic impact of downy mildew is significant, particularly in nursery stages where it can lead to high seedling mortality. In mature crops, the reduction in leaf surface area limits plant growth and leads to lower yield and reduced market quality. Furthermore, infected leaves are more susceptible to secondary soft rot pathogens during post-harvest storage.

Effective control strategies focus on integrated pest management practices. Key measures include crop rotation with non-host plants, the removal and destruction of crop residues, and rigorous weed control. Improving air circulation in greenhouses and avoiding overhead irrigation can significantly reduce infection pressure. Fungicide applications are recommended when environmental conditions are conducive to disease development.

• Utilizing disease-resistant cultivars.
• Ensuring proper spacing to improve ventilation.
• Treating seeds before planting to eliminate pathogens.
• Monitoring fields regularly for early symptoms.