Effective transport and minimal invasion speed in a vector-host model of cassava mosaic disease with partial cross-protection

Cassava mosaic disease (CMD) can spread across agricultural landscapes through whitefly-mediated transmission, creating invasion fronts whose speed is governed by vector movement and survival. Motivated by mild-strain cross-protection as a management tool, we formulated a spatially explicit vector–host model that couples plant infection dynamics with vector advection–diffusion and includes a protected plant class representing partial cross-protection. We derived an explicit invasion threshold R 0 and showed that cross-protection enters through an effective susceptible fraction. For linearly determined (pulled) fronts, we characterized the minimal invasion speed and identified effective drift and diffusion weights that aggregate transport across the coupled host–vector system; near threshold, the speed scales with R 0 − 1 through the leading-edge growth rate. We computed monotone traveling waves numerically and found close agreement between observed wave speeds and the pulled-speed prediction in the parameter regime considered. The analysis highlights how increasing vector mortality, improving protection efficacy, and strengthening roguing can either prevent invasion ( R 0 1) or slow spatial spread. Here, we also discuss limitations relevant to field deployment, including human-mediated movement of infected cuttings, temporally varying wind conditions, and episodic planting and harvest.