I've tried spraying the iris with fungicide and found that the spray just rolls off the leaves and hasn't had much noticeable effect. Last year I also sprayed and the leaf spot wasn't as bad, but I'm thinking it might be because the rainfall wasn't nearly as bad last year as this past winter/spring. One iris farmer mentioned that a surfactant (ie. detergents, soaps) was needed to help allow the fungicide to adhere to the leaves. And as mentioned here already, although the leaves look unsightly, the plants typically suffer no significant harm.
