The high heat and large amounts of rain you just got probably helped cause the rot. Spray all your remaining iris with fungicide as a precaution. If it was bacterial soft rot, but you caught and removed the rotted iris quickly, then you shouldn't have to do any heavy treatment of the soil. Add new soil in places you add replacement iris. If you are concerned you can spray the area the iris were located with a bleach solution, enough to saturate the soil, to kill the bacteria. 10 to 20% bleach to water ratio will not harm the iris still in the garden.
There is some argument about how long soft rot bacteria will stay in the soil. The older belief is that it sticks around, the newer thought is that the bacteria dies out after it's food source (the rotting rhizomes) is removed. Still a trial and error situation in my book.
