Development and validation of a diagrammatic scale for the assessment of the severity of bacterial leaf streak of corn

Abstract

The recent occurrence of bacterial leaf streak of corn caused by Xanthomonas vasicola pv. vasculorum represents a potential threat to the maize crop in Brazil. This study aimed to develop and validate a diagrammatic scale for the assessment of leaf streak severity in corn leaves. Linear regression and Lin’s coefficient approaches were used to explore the intra- and inter-rater reliability and accuracy of experienced and inexperienced evaluators. In cornfields in Cafelândia, Parana state, 450 leaves that were naturally infected by the bacterium were collected from plants in stages V12 to R2. APS Assess software was used to measure the disease severity. The proposed scale consisted of eight levels: 0.5, 1, 3, 6, 11, 18, 37 and 67%. Validation was performed by five experienced and five inexperienced evaluators. The use of the scale improved the precision of inexperienced (R2= 0.66 and 0.88, with and without the scale) and experienced (R2= 0.81 and 0.97, without and with the scale) evaluators. In general, inexperienced evaluators overestimated (β0 = 5.7) disease severity without the scale, and experienced evaluators underestimated (β0 = − 2.1) disease severity without and with the scale. The agreement (pc = 0.83 and 0.94, without and with the scale, respectively) and accuracy (Cb = 0.93 and 0.98, without and with the scale, respectively) improved with the use of the scale. The overall concordance correlation coefficient (OCCC) showed the improvement of the concordance between experienced evaluators. The use of the proposed diagrammatic scale proved to be adequate for evaluating the severity of the bacterial leaf streak and improved the inter- and intrarater reliability and accuracy of experienced and inexperienced evaluators.