Elaboration and validation of a digrammatic scale to evaluate soybean bacterial pustule

ABSTRACT Bacterial pustule, caused by the bacterium Xanthomonas axonopodis pv. glycines, is considered one of the most important bacterial diseases affecting soybean. The aim of this study was to develop and validate a diagrammatic scale for assessing the severity of soybean bacterial pustule. To elaborate the scale, soybean leaflets with different severity levels were collected to determine the maximum and minimum values of disease severity, while intermediate levels were established according to Weber-Fechner law of visual stimulus. The values proposed for the scale were 1.4%, 3%, 6%, 9%, 20% and 29.5%. To validate the scale, eight evaluators, with no experience in bacterial pustule evaluation, assessed 39 photos of leaflets with different severity levels, without and with the aid of the scale. The results of severity estimated by the evaluators were related to the actual severity values (determined with Assess 2.0 software from digital images of leaves with symptoms) and analyzed with simple linear regressions to assess the accuracy of evaluators. The precision of evaluators was determined based on the coefficients of determination and the variance in absolute errors. Estimates using the proposed diagrammatic scale indicated a significant increase in the coefficient of determination and lower variance in absolute errors, providing greater precision and accuracy of evaluations, which could be adopted to assess the severity of soybean bacterial pustule.摘要：由野油菜黄单胞菌大豆致病变种（Xanthomonas axonopodis pv. glycines）引起的大豆细菌性疱斑病（soybean bacterial pustule），是危害大豆的重要细菌性病害之一。本研究旨在开发并验证一套用于评估大豆细菌性疱斑病病情严重度的标准分级图。为构建该分级图，研究团队收集了不同病情级别的大豆小叶，以确定病害严重度的极值；同时依据韦伯-费希纳视觉刺激定律设定中间病情级别。本次拟定的分级图对应的病情级分别为1.4%、3%、6%、9%、20%及29.5%。为验证该分级图的有效性，8名无大豆细菌性疱斑病评估经验的评价者，分别在未借助分级图与借助分级图的前提下，对39张不同病情级别的大豆小叶数字照片进行病情评估。将评价者估算的病情值与实际病情值（通过Assess 2.0软件对带症状叶片的数字图像分析得到）进行关联，并通过简单线性回归分析评价者的评估准确性；评价者的评估精确性则通过决定系数与绝对误差方差进行评定。使用本研究拟定的分级图进行病情估算的结果显示，决定系数显著提升，绝对误差方差更低，评估的精确性与准确性均得到有效提升，因此该分级图可用于大豆细菌性疱斑病的病情严重度评估。