Development of a charcoal rot rating index for multilocation trials of sorghum

Comparing the performance of sorghum genotypes for charcoal rot resistance in a multilocation trial becomes difficult due to non-uniform disease reactions over locations obtained by applying mean number of nodes cross (MNC) scale. This study analyzed the relationship between disease incidence and severity and developed a disease rating index for use in the multilocation trial. The incidence-severity relationship for the disease was studied based on multilocation and multiyear data sets obtained from the all India coordinated research programme on sorghum. The incidence, severity and relationship between them varied depending on the crop growing environments (year, locations) suggesting that it would be inappropriate to consider incidence in place of severity and vice versa for the measurement of the disease. The reasons for the inconsistent I-S relationship were attributed to the environmental and pathogenic variations across the locations and years. Five charcoal rot rating indices (CRI 1 to CRI 5) were developed combining incidence and severity in different proportions to average out these effects. Suitability of the indices for comparing the performance of sorghum genotypes in the multilocation trials was tested using genotype + genotype-environment (GGE) biplot analysis. CRI 2 that combined incidence and severity in the 40:60 proportions was found to be the most stable index in terms of performance across the locations. CRI 2 rated the charcoal rot reactions of the sorghum genotypes with greater accuracy and reliability (R2 = 0.341, p = 0.001) compared to the existing MNC scale (R2 = 0.001, p = 0.965) in replicated field trials. The index (CRI 2) will help identify disease reactions among sorghum genotypes with greater consistency in multilocation trials.