I agree it is just environmental, not a spreading disease. And just a guess.
Not clear at all from your first post that it was just a few out of 100.
I have close to 400, many more than i need, and just cull the weak. Or if a singleton that i would
hate to loose, i might segregate and watch for the next phase of health.
Without a 'home visit' and many questions and soil and temp and light and ...etc.
just a helpful guessing game like all diagnosis. Some more clear than others....
I'm outa here,