Louisiana Iris grow IN water for me here. No sign of a rusty fungus either, and we have every rust and fungus known to man floating around. So I'm thinking it's something to do with cool temperatures and a lot of rain maybe? I don't think the rotting fence itself could be the source of this, but maybe it is impeding the air flow around the plants?
to cut off the affected leaves so the fungus will not spread. Be diligent about the housekeeping (no dead leaves left around etc.) , and treat with something anti-fungal. Once the weather warms up, and the leaves will dry between waterings that stuff will go away.
You should also see if you can make sure there is good space for air circulation around the plant, so the leaves will be able to dry and always water in the mornings, not at night!
