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First time growing tomato plants from seed this year, but the seedlings have brown marks on the cotyledons. I've put the affected seedlings aside through fear of blight, but honestly have very little idea what it could be.

From what I've read, I've been keeping the seedlings in a small (think 1.5m high, 1m across) plastic greenhouse to keep the temperature up. I keep the door open during the day for a breeze, but closed at night. It can get very humid in there during the day. I water the ground around the plant, but have noticed water droplets on the leaves from time to time, perhaps condensation...

Any ideas?

Tomato Seedling with Brown Marks

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Looks quite normal to me. I just checked my tomato plants and nearly all of them, while a bit more advanced than yours, have one or both of the cotyledon leaves showing tan patches. My suggestion would be not to worry, let the main leaves expand and if they are not fully green then is the time to start being concerned.

I think this could be because the cotyledon leaves are pre-formed in the seed while waiting for germination and expansion. Lots of things can happen as the seed matures, ripens and dries down, affecting the quality of the seedling leaves. The job of the cotyledons is to protect and make sure that the true growing point emerges intact so that the main leaves have the best chance of growing from just a few cells.

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It is difficult to see from the photo. I would water much less, just to keep just moist the dirt. Your pot has plastic on the side, and it is deep (and I think small). Additionally your greenhouse. So very probably there is some condensation, and so some water droplet on cotyledons, and so, I assume, there are some fungi.

In your case, I would try to keep on dry-side (you will see if it is too dry, and so you can water just a little more), to block the fungi.

Greenhouses are difficult to manage, and I find also difficult to find good rules: your local micro-climate is different to others, so you need to try many times, and correct, until you get the optimal way. But you can also growth the seedlings inside your house: more constant temperature and humidity. When they need sun (so with already real leaves, you could bring to green house, and then outside. By then, they should be stronger (either for watering, and for humidity).

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