From Nutritional Toxicology by By Frank N. Kotsonis, Maureen A. Mackey most bacteria and some fungi are reduce by 90% with radiation doses ranging from 0.1 to 1 kGy.
You kinda have to look in a book on food irradation or the text I mentioned, b/c it just depends on the microorganism. Quite a few fungi and spore forming bacteria can be quite radiation resistant…. requiring 3 or more kGy to kill.