The irises have become the victim of bacterial soft rot. Remove the affected leaves and survey the extent of the problem. Scoop out all of the softened tissue, then cut into the good tissue with a clean knife. Leave the cut open to air and sun. Sprinkling with agricultural streptomycin or even Comet might help too.
Prevention is much easier than dealing with problems in progress.
Plant bearded iris rhizomes so that their top is at or just under the soil level, even showing above it if your soil tends toward clay. Insure good drainage. Do not mulch them or fertilize them heavily. This is iris leaf spot. It is a fungus that in small amounts will not do significant harm to the plant.