I arrived late on this feed but I believe it needs to be mentioned that iris borers have a moth stage that lays the eggs in spring. There's several biological controls that can be used against them as mentioned in this article. The borers also leave a bacteria that causes the rot in rhizomes. It probably doesn't stay in the soil for long so it's the rotted rhizomes themselves that can spread it and which should be removed.
Here's the article on them:
https://www.gardeningknowhow.c...
