This appears to be abiotic damage (meaning, caused by non-living factors like environmental conditions) and not a pest or disease. Has anything changed in the past few weeks regarding the conditions of this part of the bioretention area? Was a nearby tree that may have shaded this spot removed or trimmed? Was the soil getting fairly dry between rainstorms? Was anything sprayed on the nearby grass, or onto the Irises for any reason?

Miri