Fungi often cause yellow spots or blotches, but the bright yellow color could be from a virus. Some viral diseases cause a rather bright yellow color in the leaf. I don't know what kind of virus this might be in your iris, but here are some examples of virus pictures in other plants:
Cherry (scroll down to pictures 3 and 4)
https://www.canr.msu.edu/news/...
Rose mosaic:
https://ucanr.edu/blogs/blogco...
Has the symptom changed in the last few days?
