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）引发的大豆细菌性斑疹病，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斑疹病严重度的病害分级标尺。为构建该分级标尺，研究人员采集了不同严重度等级的大豆小叶，以确定病害严重度的最大值与最小值，并依据韦伯-费希纳视觉刺激定律（Weber-Fechner law of visual stimulus）设定中间严重度等级。该标尺推荐的严重度参考值为1.4%、3%、6%、9%、20%及29.5%。为验证该分级标尺，8名无大豆细菌性斑疹病评估经验的评价者，分别在未借助标尺和借助标尺的情况下，对39张不同严重度等级的大豆小叶数字照片进行评估。评价者估算的病害严重度值将与实际严重度值（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评价者的评估准确性；评价者的评估精准度则通过决定系数与绝对误差方差进行评估。结果显示，使用本研究提出的病害分级标尺进行估算，可显著提升决定系数并降低绝对误差方差，使评估的精准度与准确性得到显著提升，该分级标尺可用于大豆细菌性斑疹病的严重度评估工作。