can you post more pics of the plant, and also check under the leaves.

your leaves show purpling veins which could be a nut deficiency, but the white spot could be something else. Are you using any fert's and what type of soil?

Here's the underside of that leaf. I'm feeding it seaweed fertilizer, 1-0-4. The mix consists of vermiculite and peat moss.

Peat has next to no food value, vermiculite has no food value, you are feeding it with almost no food value. You are starving this poor plant.

Peat has next to no food value, vermiculite has no food value, you are feeding it with almost no food value. You are starving this poor plant.

Is that the problem w this plant? People were actually telling me to lay off the fertilizer.

With that potting mix, your seedlings need a balanced fertilizer with a little more N and a little P. If you want to stay organic, many people like fish emulsion for seedlings. Half strength would be enough.

The other possibility is that your mix is water saturated. The standard seed starting mix is equal parts peat, vermiculite and perlite, which holds less water.

The round white spot reminds me of the burn caused by sun on a water drop (the drop acts like a lens, concentrating the rays and burning the leaf tissue underneath).

You say "kept inside under lights," yet I see the plant in a window, with sunlight shining directly on the leaves.

I'm not quite sure about the discolored area adjacent to the round white spot, but it looks like it might be the path of run-off of some of that water, with the edge of the area retaining moisture a bit longer, and therefore burning a bit more.

Do you water from above or from below? [And IIRC, you mentioned in another thread that the planting cups do have drain holes? I always worry about this when I see cups without any visible saucer or tray underneath.]

There are a lot of planting-mix-type little woody chips and shreds in that red cup, so I'm sure it's not only peat moss and vermiculite.

So you don't think it's a fungal attack? I removed the plant from its location and placed it by the window to take a better picture. Also, it does have drainage holes and is watered from the top. I might start adding bat guano in the fertilizer mix.

So in other words, the plant doesn't sit on the windowsill and wouldn't have had any direct sunlight? That would mean the white spot can't be a burn from sunlight reflected through a drop of water, and there is still no explanation for it.
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I don't know of any fungus that would look like a white spot, but of course I-am-not-an-expert.
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One of the advantages of watering from below is that the leaves can't be burned in this way; that's why tomatoes outdoors are never supposed to be sprayed with a sprinkler, and may exhibit white spots when there is sun after rain.

The easiest way to water from below is to have a flat with a solid bottom, and inside that a mesh tray, and then atop that, whatever inserts or containers you're using to start the seeds and grow the seedlings.

When the seedlings need water, you lift up the tray (and with it, all the plants at once, even if they're in separate 16 oz. cups) and pour some water in the solid bottom. Replace the tray (with plants) in the water, wait 10-20 minutes, then remove the tray and pour out the water. Put the tray (with plants) back in the now-waterless solid bottom, and you're done until the plants dry out in a few days.

No, the plant is never exposed to the sun. It sits in a spot completely isolated from the sun under my grow lights. Lol I too am perplexed at this, as I've been unable to find anything online regarding this spot issue. Thanks for your suggestion on watering from the bottom though!

IMHO I wouldn't overthink this. The plant looks very healthy and with the seaweed fertilizer, I think you are fine. Unless the whole plant starts getting white spots, leave the matter alone. Gardening is supposed to be fun and stress isn't fun. Soon you will be planting it outdoors and then it is up to nature to care for it --- with a little help from you.

Thanks for the reminder, ryse! I tend to over think things, so I needed that reminder. The leaf got worse, so I just pulled it off the plant and it looks fine.

If you are still concerned, you could just pinch off that little damaged leaf. The plant would be fine. I do however recommend that you begin to slowly introduce your plants to natural sunlight, but be very carful. Plants grown under grow lights will need to be hardened off by introducing sunlight a little at a time, slowly increasing the duration. You will probably experience at least some sun scald. But stick with it your plants will make it fine.