Evaluation of the Performance of ImageJ, Leaf Doctor Applications, and Visual Assessments in Measuring Severity of Two Leaf Spot Diseases

Abstract

In this research, reliability for disease severity assessment was compared for two different digital imaging quantifying techniques (ImageJ and Leaf Doctor applications) and visual assessments included ImageJ (IJ), Leaf Doctor (LD) applications, and visual assessments (VA) for the downy mildew of lettuce and Cercospora leaf spot of chard. Samples of infected chard leaves with symptoms of Cercospora leaf spot disease and lettuce leaves with symptoms of downy mildew were randomly collected from a field and a grocery in Al Anbar Governorate, Iraq. Each leaf was shot with iPhone 8 plus, provided with Dual 12 MP Camera, f1. 8, 28mm (wide), and f 2.8 (telephoto). The percentage of the diseased tissue of each shot was subjected to visual analysis and the ImageJ (IJ), Leaf Doctor (LD) applications. The measurements for them were subject to statistical analyses. Generally the results revealed a compatibility in the performance of the evaluated methods for the both diseases. There was positive and significant correlation coefficients between the measurement results of the VA and the LD for the downy mildew of lettuce (r= 0.664 ) and the Cercospora leaf spot of chard (r = 0.8990 ). Regarding the correlation between the IJ measurements and that of either VA or the LD revealed to be weak for both diseases. There were also linear regressions obtained only between the VA and the LD measurements (R2 = 0.441 for downy mildew and R2 = 0. 809 for Cercospora leaf spot). The results of other experiment indicated positive correlation coefficients between the upper and lower disease severity measurements regardless the assessments methods for both diseases. The highest correlation coefficients and linear regression were found for the AV measurements (r =0. 936, R2 = 0.876 and r = 0.996, R2 = 0. 992 for lettuce and chard respectively) followed by that of the LD (r = 0.908, R2 = 0.824 and r = 0.835, R2 = 0.697 for lettuce and chard respectively). This research indicates that the LD application is easier, faster and more reliable and accurate for disease severity measurements regardless the disease and its severity.