I am not sure without seeing a photo but my best guess is the iris borer. This is a worm like insect that bores into the iris and leaves squiggle like lines. You can cut out the infected area to stop the borer from continuing it's course. It may have also tunneled into the root of the iris. If you notice the infected iris has loss of vigor or you see the borer has gone into the thick rhizome like root then the best thing to do is to uproot and destroy the entire plant so the borer does not get into any other iris plants. Keep growing, Marianne Binetti