Alternaria leaf spot of cotton

Alternaria leaf spot is a destructive fungal disease of cotton caused by the pathogen Alternaria gossypina. This fungus belongs to the family Pleosporaceae and is known for causing significant foliage damage and yield reduction in cotton-producing regions worldwide. The pathogen targets the host plant by secreting enzymes that break down plant tissues, facilitating colonization and spread.

The primary host plant for this specific pathogen is Gossypium hirsutum (Upland cotton) and other cultivated cotton species. While the disease can manifest at various growth stages, it is particularly damaging during the flowering and boll development phases, as it directly competes with the plant for nutrients necessary for fiber maturation.

Symptoms typically begin as small, irregular or circular spots on the leaf surface, characterized by a brownish to dark brown necrotic center. As the fungus spreads, the lesions may enlarge and merge, often exhibiting concentric ring patterns. Infected leaves eventually turn chlorotic, wither, and undergo premature abscission. On the bolls, the fungus causes black or dark grey lesions that can lead to boll rot and internal contamination of the fibers.

The development and dissemination of Alternaria gossypina are heavily influenced by environmental factors. High relative humidity (frequently exceeding 85%) and warm temperatures are critical for spore germination and infection. The pathogen relies on wind and rain splashes for secondary inoculum spread. Dense plant canopies that lack proper ventilation significantly increase the severity of an outbreak.

• Implement strict crop rotation schedules to break the disease cycle.
• Ensure field sanitation by removing and destroying crop residues.
• Optimize plant spacing to improve airflow and reduce micro-humidity.
• Apply certified disease-free seeds to prevent early-season infection.
• Use systemic fungicides at the first sign of symptoms to minimize spread.

Economic damage is caused by reduced photosynthetic efficiency, leading to stunted plant growth and significant fiber quality loss. Effective management requires an integrated approach that combines cultural practices with chemical control. Regular scouting and monitoring of fields for early lesions are essential to determine the timing and frequency of protective fungicide applications to safeguard the crop investment.