There are certain infections going around at specific times of the year, especially one fungus that pops up every year in the autumn, it grows into the trachea of flies and can kill off entire fly populations. Your ants may have caught one of these seasonal infections (and it is not very odd that all colonies in a wide area are affected because these infections are indeed omnipresent in very large areas, even entire countries, especially those that are adapted to such common hosts as flies).
It in fact actually looks like something IS attacking the fungus. Not an insect, but it looks like there is a foreign mold growing on the main fungus. There are yellow pollen looking things on the main fungus. So, I think its being attacked by another mold.
Probably it is best then to dispose the entire fungus garden (since it is mostly dead anyway) and maybe even the ants tending it before the infection spreads to the other gardens.
Edited by Serafine, November 24 2016 - 1:28 PM.