Abstract / Summary
Abstract Cassava ( Manihot esculenta Crantz) root rots, caused by soil-borne fungi, constitute a major constraint in the West African humid tropics. However, their combined effects with topography, altitude and plant age remain insufficiently documented. This study evaluated these effects via the Disease Severity Index (DSI) across 192 plots and 1,920 plants aged 8–25 months, in three contrasting regions (Tonkpi, Guémon, Cavally) of the mountainous west of Côte d'Ivoire. An early-warning threshold (DSI > 0.5, i.e. 12.5% on the proportional scale), derived from the inflection of the age–severity curves, was retained as the operational benchmark marking entry into the active epidemic phase. The marketable-yield deficit was estimated on the basis of 10 t ha⁻¹. The Cavally lowlands (< 200 m) exhibited the highest disease levels (prevalence = 81.5%; DSI = 2.00 ± 1.33) and an estimated marketable deficit of 4.32 t ha⁻¹. These plots were dominated by Sclerotium rolfsii (98.2% of infection profiles), alone or in co-infection with Lasiodiplodia theobromae , with the early-warning threshold crossed as early as 8 months after planting. Conversely, the hydromorphic Tonkpi lowlands (> 300 m) exhibited reduced severity (DSI = 0.81 ± 1.19) and a dominance of Lasiodiplodia theobromae (94.3%). An estimated thermal difference of 1–2°C, derived from the adiabatic lapse rate (− 0.65°C per 100 m), reverses fungal dominance in the lowlands, illustrating a 'fungal niche inversion'. The Cavally summits (DSI = 0.46 ± 0.79) sustained higher pathogen pressure than the Tonkpi slopes (DSI = 0.23 ± 0.53), revealing a 'topographic risk inversion'. These two phenomena demonstrate that topography, altitude and physiological age cannot be disentangled in risk assessment. These results provide a spatial decision-making grid to guide phytosanitary surveillance and to adapt harvest calendars in the humid tropics.