Alternaria leaf spot of sesame

Alternaria leaf spot, caused by the fungal pathogen Alternaria sesami, is a significant constraint to sesame production worldwide. This disease is particularly prevalent in warm, humid regions where it can cause extensive defoliation, leading to reduced photosynthetic activity and, consequently, lower yields of both oil and seeds.

The host range of Alternaria sesami is primarily restricted to sesame (Sesamum indicum). The fungus attacks all aerial parts of the plant, including stems, leaves, flowers, and capsules. When the infection reaches the seeds, it significantly impairs their quality and viability, making it a critical issue for seed production farms.

Symptoms initially appear as small, circular or irregular dark brown spots on the leaves. As the disease progresses, these spots coalesce, forming larger necrotic patches with distinct concentric rings. Under humid conditions, a dense, dark-colored mass of spores develops in the center of these lesions, giving them a velvety appearance.

Environmental conditions play a crucial role in disease dissemination. The fungus thrives in temperatures between 20°C and 30°C coupled with high relative humidity. Spores are primarily dispersed by wind and splashing rainwater. The primary inoculum sources include infected crop debris remaining in the field from the previous season and contaminated seed lots.

Effective management and control of the disease involve several integrated practices:

• Selection and planting of high-quality, disease-free, and treated seeds.
• Implementation of crop rotation cycles of at least three years with non-host crops.
• Deep plowing to bury infected plant residues, which reduces the primary inoculum levels.
• Timely application of appropriate foliar fungicides when initial symptoms are observed.
• Maintenance of balanced soil fertility to ensure strong and healthy plant growth.