Development and validation of a diagrammatic scale for white mold incidence in tobacco leaf discs

Sclerotinia sclerotiorum is a geographically cosmopolitan species with a wide ecological distribution, although it is most common in temperate regions. More than 400 plant species are susceptible to this pathogen, including most vegetables (e.g., potato, tomato, lettuce, and crucifers), tobacco, and major crops. This study aimed to develop and validate a diagrammatic scale to aid in the visual assessment of white mold incidence in tobacco. The diagrammatic scale from 0 to 100% was validated for precision, accuracy, and reproducibility. Six evaluators visually estimated the disease severity of tobacco leaf disc images with varying levels of white mold infection, once without the scale and twice with the scale by considering six levels of disease severity using images of 50 leaf discs. Comparisons between individual evaluator visual assessments (using Pearson's correlation) indicated that the minimum r value increased from 0.38 without the use of the scale, to 0.84 and 0.74 for the first and second assessments using the scale, respectively. The accuracy of the evaluators (using Lin's Correlation Coefficient) also increased from 0.85 without the scale, to 0.95 and 0.93 for the first and second evaluations using the scale. Based on these findings, the diagrammatic scale developed increases the accuracy of visual assessments of white mold severity on tobacco and increases the consistency of assessments between evaluators .