Given the look and how contagious, I would be concerned about Pythium fungus(black rot).
Routine fungicides do not kill this and the only product I am aware of that does is Supramaxx(should be Suprama$$)
Also consider Pseudomonas which is bacterial and treatable with Phyton 27. This is pretty easy to obtain. The yellowing of the leaf is a bit more characteristic of Pseudomonas imo.
Most insect infestations can be "seen" albeit with magnification and I don't "see" any bugs. With the number of plants infected so quickly, you would probably have seen if it was insects.
You should also increase circulation and decrease humidity a bit around your plants; furthermore, pull out some of those diseased plants from their pots and check their roots.
I hope you can get this under control as I know your frustration only too well