Foliar spray is a great idea for immediately providing a specific mineral that is capable of transitioning the stomata to an area of need - i.e. existing problem / symptoms etc.
foliar spray for other reasons is mostly masturbation.
preventative sprays are useful. context-specific reason i'm too unimaginative to think off off the top of my head is also a good reason. doing it just to do it is stupid. roots are more than capable of providing everything. when they are not the first thought should be to correcting the formular not a bandaid action like applying some mineral locally to a leaf.
unnecessary foliar sprays do not follow the "keep it simple, stupid" mantra.
So, don't be afraid to use them for a good reason. Spraying flower is even more restrictive, though.
btw, when you can see it, that means it's been there longer and already spreading. obviously the sooner you catch it and remove it the better off you are. inspect daily for new visible infections.