Development and validation of a standard area diagram set to aid estimation of bacterial spot severity on tomato leaves

Abstract

A standard area diagram (SAD) set to aid assessment of bacterial spot (caused by Xanthomonas spp.) severity on tomato (Solanum lycopersicum L.) leaves was prepared and evaluated for improving accuracy, precision and reliability of visual estimates. The proposed SAD set contains illustrations of leaves with 12 distinct disease severity values from 0.5 to 90 %, and was validated by 12 raters without experience of plant disease evaluation. The severity of bacterial spot on 150 images of diseased tomato leaves was first estimated without the SADs, and then assessed using the SADs by the same group of raters. Regression analysis and Lin’s concordance correlation (ρ c ) analysis of estimated versus true disease severity (based on image analysis) showed precision and accuracy improved for all raters using the SADs. The SAD set improved accuracy (correction factor, Cb = 0.91 and 1.00, without and with the SADs, respectively) and agreement (ρ c = 0.83 and 0.96, without and with the SADs, respectively) of the estimates of severity. Severity estimates were more reliable using the SADs (coefficient of determination, R 2 = 0.74–0.94 without the SADs, and R 2 = 0.82–0.95 with the SADs, and intra-class correlation ρ = 0.85 without the SADs and ρ = 0.91 using the SADs). The SADs proposed in this study will improve the accuracy and reliability of estimates of bacterial spot severity on tomato leaves for inexperienced raters, and could be used for assessing other bacterial disease severity on tomato leaves.