Diagrammatic scale for quantification of severity of white thread blight disease in yerba mate (Ilex paraguariensis Saint Hilaire)

Abstract The yerba-mate industry is one of the most important economic activities in Misiones, a province in the northeast of Argentina that is the world’s leading producer and exporter of this crop. White thread blight disease caused by Ceratobasidium niltonzousanum affects the cultivation reducing its quality and productivity. Due to the lack of a standardized visual method to quantify the severity of this disease in yerba mate, a diagrammatic scale was developed and validated. Yerba-mate branches were collected in a field in the north of Misiones province, and the actual severity was determined digitally. A six-level scale was developed using the DOSLOG software, based on the Weber-Fechner law. The validation was carried out by twenty raters. One evaluation without the diagrammatic scale and two evaluations with the scale were carried out in 14-day intervals. Accuracy, precision, and reproducibility of the scale were evaluated through linear regressions and correlation analysis, obtaining R2 values ranged between 0.70 and 0.94. Using the diagrammatic scale developed in this work, raters enhanced the accuracy and precision of the estimates, and the repeatability of the scale improved by 94.74%. The scale was appropriate to assess the damage of white thread blight in yerba mate