Occurrence, incidence, and severity of Alternaria leaf spot of cotton (Gossypium hirsutum L.) In major cotton-growing districts of the Lake Zone, Tanzania

Abstract

Alternaria leaf spot (ALS), caused by fungi in the genus Alternaria (family Pleosporaceae), is a significant fungal disease affecting cotton production in the Lake Zone of Tanzania, the country’s main cotton-producing region, where it reduces photosynthetic capacity, boll retention, and fiber quality. Despite its increasing prevalence, systematic baseline epidemiological data remain scarce. This study aimed to determine the occurrence, incidence, and severity of Alternaria leaf spot across the major cotton-growing districts of the Lake Zone, Tanzania. Field surveys were conducted during the 2024-2025 growing season in Bariadi, Bunda, Chato, and Magu districts; 20 representative cotton fields were systematically assessed using a diagonal sampling approach, and disease incidence, prevalence, and severity were recorded for 1,000 cotton plants. Fungal isolates recovered from symptomatic leaves were examined for morphological identification. Field prevalence was 100%, and overall mean incidence was 64.4%, with significant inter-district variation: Bariadi (77.6%), Bunda (68.8%), Chato (60.0%), and Magu (52.4%). Mean disease severity index (DSI) ranged from 44.0% (Magu) to 70.1% (Bariadi), indicating moderate-to-severe infection. Incidence and DSI were strongly correlated when uninfected plants were included (r = 0.942, P < 0.001), but this relationship weakened and became non-significant when restricted to infected plants only (r = 0.396, P = 0.084), suggesting that higher incidence reflected more plants becoming infected rather than greater severity among already-infected plants. Morphological characterization identified all 40 isolates as belonging to the genus Alternaria. These findings provide the first systematic epidemiological baseline for Alternaria leaf spot across the major cotton-growing districts of Tanzania’s Lake Zone. Disease incidence and severity varied significantly across districts, underscoring the need for tailored disease management strategies; this study recommends ongoing surveillance and integrated disease management to safeguard cotton productivity among smallholder farmers in the region.