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Brown spots only on underside of leaves

last year

Two leaves on a small tobacco plant yellowed very quickly, and have brown spots on the underside of leaves only. A fair amount of soil had gotten onto the leaves, so I assumed it stopped photosynthesizing because light was blocked, but I'm not sure. Plant is otherwise healthy. Two interesting things I noticed are that the brown spots are still slightly green on the top side - I double checked that the green spots are indeed above the brown. The larger leaf has some small brown spots near the midrib that go all the way through rather than only being on one side. I know that's a nutrient deficiency, but I've never seen anything like these brown spots on the underside before. Could it be related, or it just the soil on the leaves? Maybe something else entirely?