A stage-weighted environmental risk and visual severity integration model for climate-smart management of cassava anthracnose in humid tropical systems

Abstract

Colletotrichum anthracnose is a persistent constraint on cassava root yield and planting material quality in humid tropical systems, where calendar-based management often leads to poorly timed or excessive fungicide use. This study developed and pilot-tested the Cassava Anthracnose Risk Index (CARI), a stage-sensitive, persistence-adjusted early-warning framework integrating low-cost environmental sensing with structured visual severity scoring for climate-smart, input-reduced disease management. Microclimate sensors were deployed at CSIR-CRI Fumesua, Ghana, across a 13-month rainfed crop cycle (March 2025 – April 2026) using the susceptible landrace Debo. A stage-weighted Environmental Risk Subscore (ERS; 0–600 points, ERS%) was constructed from six environmental components, moderated by a persistence factor requiring sustained infection-favorable conditions, and stratified across three growth stages. A Visual Symptom Subscore (VSS; 0–40) was derived from standardised weekly severity assessments of 15 sentinel plants, integrated into a composite CARI (0–100) with stage-specific alert thresholds. Across 73 valid Stage 3 monitoring days, CARI classified all observations as Low Risk and issued no Amber or Red alerts, including on 37 of 73 days with sustained nocturnal high-humidity conditions that would have triggered a precautionary alert under a simple threshold approach. In the single confirmed disease episode observed, an approximately one-day interval separated the highest environmental risk signal from confirmed symptom onset; this is not presented as a validated lag value, a preceding 25-day sensor outage means antecedent conditions cannot be characterised. CARI did not issue an intervention recommendation during this episode (11–24 February 2026) because concurrent environmental risk remained below the Amber threshold. Published decision support systems for Colletotrichum anthracnose in other pathosystems report fungicide reductions of 30–70% relative to calendar-based approaches; these figures are cited only for benchmarking and were not measured here, as no comparative fungicide trial was conducted. These results provide proof of concept for the technical operability of CARI and identify the dataset and analyses required for formal predictive evaluation in a planned multi-season study; they should not be interpreted as evidence of CARI’s predictive accuracy or input-reduction performance.