A Standard Area Diagram for Leaf Spots and Leaf Blights Disease Severity Assessments in Groundnuts

Leaf spots caused by <i>Nigrospora oryzae</i> and leaf blight caused by <i>N</i>. <i>sphaerica</i> are the major diseases limiting groundnut productivity in Zimbabwe. Severity assessments are key in disease management as they are directly linked to yield reduction. Although diseases can be visually quantified by humans, the approach is usually associated with errors, especially when conducted by inexperienced personnel. Standard area diagrams (SADs) are known to be efficient tools to quantify severity of diseases, but the approach is still poorly developed and is underutilized in management of groundnut phytopathogens in Zimbabwe. Here, a total of 302 symptomatic leaves were collected from farmers' fields, and leaf images were captured using a digital camera. The images were used to develop an SAD in the R Pliman package. The developed SAD contained 10 severity levels, ranging from 1 to 100%. Validation of the SAD was done using 12 inexperienced and 8 experienced raters with and without the use of the developed SAD. Lin's concordance correlation showed that the use of the SAD improved the accuracy of experienced (pc = 0.71 to 0.98) and inexperienced (pc = 0.47 to 0.98) raters. The SAD also showed to improve the interrater reliability (p) among inexperienced (<i>P</i> = 0.290 to 0.96) and experienced (<i>P</i> = 0.38 to 0.75) raters. The developed SAD demonstrated potential for application in disease epidemiology and monitoring studies, hence assisting in improving efficiencies in management of groundnut diseases.[Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.