Not the original commenter, but I guess they were referring to the fact that closed (indoor) grow environments universally suffer from fungal (and algal) disease problems.

A lovely example of this is the germination rooms/tanks of malting plants where Barley gets germinated over a few days (2-3) before being dried/kilned to make brewing malt: they all have an on-going battle against fungi and algae, much of which battle takes the form of fungicides/algacides, chlorine washes, strong acids, etc. and they never "win" a permanent victory.

The answer, my friend, is blowin' in the wind...

"PM" (powdery mildew) is a huge problem in indoor grows[1]. It's especially bad in the Marijuana industry where it completely ruins the plants' suitability for sale (potentially lethal if inhaled/smoked), but it affects many other plants as well.

[1] https://en.m.wikipedia.org/wiki/Powdery_mildew