Answer: Iris are occasionally subject to bacterial soft rot and it smells terrible just as you described. It is likely to infect rhizomes that have been injured -- borer damage, for instance, is an invitation to rot. It can also affect rhizomes that are planted too deeply or are in a poorly drained soil. Some times, too, it "just happens". The best thing to do is to remove any infected rhizomes and then make sure the cultural conditions for the iris are as good as you can make them. It should not bother those other plants, although they too prefer a well drained soil and could possibly suffer some root problems if the area is poorly drained.
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