Phenotypic evaluation and genetic insights into brown spot disease resistance in diverse rice ( <i>Oryza sativa</i> L.) genotypes under field conditions

Brown spot disease, caused by Bipolaris oryzae, continues to pose a major constraint to rice production in Tanzania and globally. This study assessed the phenotypic performance of 100 rice genotypes under natural brown spot pressure across four hotspot locations during the 2023/2024 rainy season. Trials were implemented using a randomized complete block design with three replications, and data were collected on disease incidence, severity, percent disease index, grain yield, and essential agronomic traits following IRRI Standard Evaluation System protocols. Significant genotypic variation was detected for both resistance and agronomic attributes, including flowering time, tiller number, panicle length, plant height, and yield. Grain yield ranged from 0.65 to 1.12 t ha⁻¹, with a mean of 0.89 t ha⁻¹, highlighting substantial losses attributable to the disease. Only two genotypes, IR19A7037 and IR19A2096, exhibited consistent resistance, yielding 0.86 and 0.93 t ha⁻¹, respectively. Twenty-five genotypes were moderately resistant, while thirty-two and forty-one were moderately susceptible and highly susceptible. PCA biplot analysis grouped the genotypes into three clusters, with Cluster I associated with higher yield and resistance. PC1 (32.39%) and PC2 (14.48%) explained 46.87% of variation. Resistant genotypes represent valuable breeding resources for improving brown spot resistance in rice production systems across Tanzania today.