Alternaria leaf spot of brassicas

Pathogen and disease type. Alternaria japonica is a destructive fungal pathogen that causes Alternaria leaf spot, a common disease affecting various cruciferous crops. It is characterized by the production of dark, necrotic lesions on foliage and other plant parts, significantly impacting the physiological integrity of the host plant.

Susceptible crops. The pathogen primarily targets members of the Brassicaceae family. It commonly affects vegetables such as broccoli, cauliflower, cabbage, bok choy, radish, and mustard greens. The fungus can persist in the soil for several years and may also be seed-borne, posing a continuous threat to susceptible commercial fields.

Symptoms and signs. The initial infection manifests as small, dark spots on the leaves. As the disease progresses, these lesions expand and develop concentric rings, creating a characteristic target-like appearance. In humid environments, a dense, dark olive-to-black velvety mold appears on the surface of the lesions, indicating active spore production.

Development and spread. The development of Alternaria japonica is highly dependent on environmental conditions. It thrives in warm, humid weather, with optimal infection occurring at temperatures between 20°C and 25°C. Splashing rain, high humidity, and heavy dew are crucial factors that facilitate the dissemination of conidia throughout the canopy.

Damage and mitigation. The disease causes reduced photosynthetic efficiency, yellowing of foliage, and reduced marketability of the produce. Post-harvest damage is also a concern during storage. Integrated management includes:

• Using certified pathogen-free seeds.
• Implementing a rigorous crop rotation cycle.
• Sanitizing fields by removing crop debris immediately after harvest.
• Applying preventive or systemic fungicides when environmental conditions favor disease outbreaks.