Yes it can spread, and it will spread. You need to get a handle on it one way or another. I am not a fan of beni's in hydro set ups, I think they should be left for soil grows. Possible sources are city water, spores in the air and even beneficial bacteria themselves, or bugs. Most likely fungus gnats. Fungus gnats and root rot go hand in hand, they either cause it or are attracted by it so keep an eye out for them.
Another possible cause is your system/environment. Are you water temps to high? Do your roots sit in water for to long? You might just need to adjust your flood times or something like that to keep the water on the roots fresh, or add a chiller.
Heres another thought, are you sure its root rot? You said the roots look ok, and your ppm of 1400 is pretty high. Try cutting that in half, maybe you just fried one of your plants. Also, look at some healthy root pictures on the internet then look at yours, do they still look ok? They should be pearly white, fuzzy and smell like cucumbers.
One telltale sign I get in my grow that indicates root root is a very slight curl around the edge of the leaves. Its only at the very edge and looks kind of like its outlining the leaf itself, other than that the plant may not look very bad for a few weeks and will either somewhat recover or end in a wilted plant/