I’m trying to find an easy way to asses leaf disease coverage. The disease is powdery mildew and the images of the leaves were taken on a light blue background. The tutorials that I have found suggest using the color threshold tool to select the total leaf area, then select measurement. The problem is that no matter how I adjust the thresholds, the leaf is never fully selected. This seems to be because powdery mildew is very light in color. Does anyone have a better method for accomplishing this task? I just need to know the percentage of the leaf covered by disease. I have a sample photo if anyone would like to try or can give pointers.