Can I use magnesium sulfate foliarly?

I don't think this warrants a foliar spray. I believe Mg is one of the ones that can traverse the stomata, fwiw. 100% magnesium sulftate or epsom salt with no odors or additives is totally fine to use as a readily avaialble source of Mg and S. Many packages will even say garden safe or something to that effect (walmart equate brand does). This could be an Mg issue.. interveinal chlorosis with spots.. It takes 30-35+ days from start of the problem inside the plant before you see visible symptoms. Your plant may not quite be old enough to show such advanced symptoms of Mg. would need some sort of expectation of your unique soil's contents and ratio of what you've fed so far to help deduce what is wrong. Too many unknowns and not enough info. Could be start of a k-def, but usualy see more damage around the tips in that case. Some trace elements are possible, but that woudl more likely be nute-lockout or pH issues. in a soilless/hydro context you maintain about 75-80 ppm of Mg in the substrate .. hard to translate to a soil context, though. Strongly suggest you learn how to feed through the roots rather than relying on a foliar spray.. just gunks things up and increases risk with standing water on leaves etc (not only that, but nutrient infused water)... also, stomata are for transpiration and that's how it gets CO2. Slowing that down at all is a big negative. If something extreme is happening and you need it ASAP, that's the only time i'd even consider it. So far, never had to apply a foliar spray for nute-related issues in (knocks on wood)