I've seen a few iris roots like that, I don't remember what the circumstances were, though I'm guessing they were really crowded together when it happened. Have you thinned your irises every couple of years?

I just now ran a scientific search via Google (typed in "iris root rot") and got a hit: anoxic soil could be a problem. Does this sound useful? The discussion seems to be one about gardening for hummingbirds in the Southeast.

Another hit says manure on irises can lead to root rot, and here is an abstract from a paper that I would have to join the ScienceDirect folks to read, but maybe this is enough to make your eyes glaze over:

Disease suppression against Pythium root rot can be influenced by management practices applied in ornamental bulb culture. Different levels of suppression were established in sandy soil after several soil treatments. Percentage of infection in Iris caused by Pythium macrosporum was lowest in untreated soil and progressively increased in sterilized soil amended with 1% compost, fumigated soil (methylisothiocyanate) and flooded soil (fumigation and flooding corresponding to the same level of disease severity) and was highest in sterilized soil. The relationship between the level of disease incidence, Pythium growth rate through soil, and various microbial parameters was investigated. Soil microbial biomass and, after amendment of glucose as a non-selective carbon source, dehydrogenase activity, glucose uptake and respiration were determined. By using 14C-labelled glucose, measurement of uptake and assimilation of amended carbon could be distuingished from soil organic matter decomposition. All microbial parameters were negatively associated with Pythium growth rates, indicating that high microbial biomass and activity induced suppression of Pythium growth through soil. However, with neither of the quantitative microbial parameters could changes in disease incidence be fully explained. It is hypothesized that competition for carbon may not be the main mechanism in disease suppression.

SRS