nothing you can buy over the counter will be effective against fungus. Even stuff that is available to large commercial farms is not effective. Also, you absolutely don't want to spray that shit on anything with flower growing. None of the anecdotal remedies work... just a bunch of people lying to themselves and spreading it all around the plant wherever it drips, lol. Just a bunch of smeared fungus all over stems, leaves, buds etc... plus, cleaning the surface does not rid the plant of fungus. it grows beneath the surface, too.
any antifungal that is "systemic" is also dangerous to use in vegetative phase because it stays in the plant and can still poison your flowers later (see "eagle20" as an example of that context)
when you see fungus, you can try to amputate it and remove infection from area ASAP... might get lucky. If it's as developed as that leaf, it's most likely all over the place as that is several weeks of fungual growth, minimum. It's spreading spores after 3-5 days of getting a foothold -- when you can barely see a spot of it.
if it is miraculously isolated to that one or 2 leaves, you can try removing it and pray. if other plants are clean, you may just want to remove the entire plant that is infected. Up to you.
The leaf shown doesn't match the plants shown... it should be all over if it has had time to grow that much on that pictured leaf. either something is fishy with this grow question or ....???