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health What are these brown spots on a leaf?

My pepper plant, transferred recently from a nice and warm place under a grow light to a colder kitchen window sill near a winter sunny window, seams to be developing some brown spots on a leaf. Any idea what it is?

It looks like pepper bacterial leaf spot, but I don't think it is. Could it be nutrient burn? I mixed some coco coir with a little bit of liquid fertilizer, half what should be given. I'm not even sure is this plant is repotted with the coco coir or with some regular potting soil mixed with compost I had left. It doesn't really look like nutrient burn either.

It's the only plant in a group of eight that has these symptoms. It also (still?) seems to be on one leaf.



Other leaves look like this:


As far as I can see, there are no aphids. Am I overlooking something? Do I need to rip of the spotted leaf and just hope for the best?
Rust could be it. Or something else still. I've taken off the affected leaf and now I'll just wait I guess.

I'm not putting the plant in quarantine. Please do remind me of this later when some weird virus has spread to all the other plants.
Thanks for the link. Most results are not applicable, but on the third page I did find a thread with a very usefull post with some interesting info!

So I'm putting the plant in quarantaine after all. Apparently BLS can come from the seed. We'll see how it develops and if I need to throw it away. For now, the plant is moving to the living room window sill. (Sorry Mr. Ratat. 😄)
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That doesn't look like leaf spot to me. I suspect the plant is simply pulling mobile nutrients from a leaf that's no longer so much a part of its long term plans. I see this periodically on lower/older leaves that might not be getting as much light anymore so the plant doesn't maintain the leaf. Looks like the leaf next to it is similarly affected. Whether I remove such affected leaves or not is usually a balancing of how much I think the plant might still benefit from the leaf versus how ugly it looks.