Alternaria leaf blight

The disease is caused by the fungal pathogen Alternaria undulata, which belongs to the class of mitosporic fungi. This pathogen acts as a primary cause of necrotic leaf blight, invading plant tissues through natural openings or mechanical injuries. The fungus persists in the soil and on crop debris, serving as a recurring source of infection for subsequent planting seasons.

Alternaria undulata primarily targets members of the Amaryllidaceae family, with onions and garlic being the most susceptible economic crops. While the disease is often localized, severe outbreaks can occur in specific ecological niches where the environment favors fungal sporulation and dispersal, leading to significant yield losses in susceptible varieties.

Visual symptoms initially manifest as small, chlorotic or light-brown lesions on the leaves. As the fungus progresses, these spots expand, developing dark brown or black concentric zones, which are a diagnostic hallmark of the genus. In advanced stages, the foliage wilts, turns necrotic, and collapses, severely reducing the photosynthetic capacity of the plant.

Environmental conditions are critical for disease development. The pathogen thrives under conditions of high humidity (exceeding 75% relative humidity) and moderate temperatures, typically between 18°C and 25°C. Leaf wetness duration is the decisive factor for spore germination and infection, often following periods of heavy dew or rainfall, which facilitate the spread of spores via wind or water splashing.

Effective management requires an integrated approach to reduce the pathogen population. Essential strategies include:

• Implementing long-term crop rotation to break the infection cycle.
• Ensuring proper sanitation by removing and destroying crop residues.
• Improving field drainage to reduce local humidity around the canopy.
• Applying preventive or curative fungicides, such as those based on strobilurins or triazoles, according to regional guidelines.