Weather-based Epidemiological Analysis and Disease Progression Curve of Gray Mold (Botryotinia ricini) in Castor (Ricinus communis L.) under Different Sowing Dates

Abstract

A field investigation was conducted at the Tapioca and Castor Research Station, Yethapur, Salem District, Tamil Nadu, India, to examine the relationship between prevailing weather conditions and progression of gray mold in castor sown in June and July 2025. Disease severity on primary and secondary spikes was recorded weekly and expressed as per cent disease index (PDI), while maximum and minimum temperature, relative humidity, rainfall, and rainy days were monitored concurrently. Gray mold symptoms first appeared during the 38th standard meteorological week, when the maximum temperature was 32.6°C, the minimum temperature was 24.2°C, and relative humidity was 76.7%. Disease severity subsequently increased, reaching peak PDI values of 60.4% in the June-sown crop and 60.9% in the July-sown crop during the 48th week. Maximum temperature was significantly and negatively correlated with disease incidence in the June- and July-sown crops (r = -0.8478 and -0.8544, respectively), as was minimum temperature (r = -0.7433 and -0.7407). Relative humidity showed significant positive correlations (r = 0.4904 and 0.5036), whereas rainfall and rainy days were not significantly associated with disease development. The observed disease progression indicates that moderate temperatures combined with elevated relative humidity favoured gray mold development. These relationships may support weather-based forewarning and the timely scheduling of protective disease-management measures.