Genetic Variability for Resistance to Alternaria Leaf Spot in Pistillate Lines of Castor (Ricinus communis L.) Under Natural Epiphytotic Conditions

Abstract

Alternaria leaf spot caused by Alternaria ricini is a major constraint in castor (Ricinus communis L.) production, leading to significant yield losses under conducive environmental conditions. The present study evaluated genetic variability for disease resistance among thirty pistillate lines under natural epiphytotic conditions during Kharif 2025 at Sardarkrushinagar, Gujarat. Disease severity was assessed using a 0–9 scale and expressed as percent disease index (PDI), complemented by area under disease progress curve (AUDPC) to quantify disease progression. Substantial variability in disease response was observed, with PDI ranging from 10.42 to 81.06%. None of the genotypes exhibited complete resistance (≤10% PDI); however, thirteen genotypes were classified as moderately resistant (11–25%), while the remaining genotypes showed moderate to high susceptibility. Genotypes SKP 129, SKP 126, and SKP 130 consistently recorded lower disease severity and reduced AUDPC values, indicating slower disease development. Cluster analysis based on PDI and AUDPC effectively grouped genotypes into distinct disease response categories, highlighting clear separation between low and high disease severity groups. The findings demonstrate the predominance of quantitative resistance against A. ricini and identify promising genotypes for use in resistance breeding. Integration of temporal disease assessment and multivariate analysis provides a robust framework for selecting stable resistance under field conditions.