Temporal and Spatial Dynamics of Rice Blast (Pyricularia oryzae) Epidemics under Irrigation in North West Nigeria

Rice blast caused by Pyricularia oryzae is the most damaging fungal disease of rice globally, and its epidemic dynamics in irrigated paddies of North West Nigeria, including Kebbi, Sokoto, and Zamfara States, remain poorly characterised at the population level. Understanding temporal and spatial aspects of blast epidemic development is essential for designing intervention windows and validating disease forecasting models. This study examined temporal disease progression patterns and spatial spread dynamics of rice blast across 45 irrigated rice fields in North West Nigeria over two growing seasons. Disease incidence and severity were assessed weekly from heading to ripening, with area under disease progress curve values computed for all sites. Spatial autocorrelation analysis using Moran's I statistics and semivariogram modelling was applied to geo-referenced disease data to characterise spatial structure. Temporal analyses revealed sigmoidal epidemic curves fitting a logistic growth model at 82 percent of sites, with epidemic onset occurring between days 14 and 21 post-heading. Spatial analysis detected significant positive autocorrelation at distances below 80 metres, consistent with local sporadic wind-driven dispersal as the primary spread mechanism. Hotspot analysis identified cluster centres located in paddies adjacent to major drainage canals, suggesting that canal-side humidity gradients create spatially structured epidemic foci. Meteorological correlation analyses confirmed temperature and humidity interactions as key determinants of epidemic onset date. Results provide empirical parameters for calibrating blast simulation models under North West Nigerian conditions. Keywords: rice blast, Pyricularia oryzae, epidemic dynamics, spatial analysis, North West Nigeria.