That’s great! I’m glad it’s helping. You need to save the classifier (that saves your training) - then you can apply that same classifier to multiple images acquired in the same manner.
What is it that you want??
You can generate a probability map that you can use to auto-threshold and then analyze particles… this will set up your segmentation of your leaf and then mildew… to then measure their areas, etc.
There is a helpful workshop on Segmentation in Fiji (and corresponding slides) - that goes over the basics of this plugin, but also other ways to develop segmentation workflows in ImageJ/Fiji that will help you in your analysis.
I hope this helps! If you have more questions… feel free to ask.