Basil does not like the cool temperatures that you are exposing it to. Basil is a plant that thrives in heat of summer. The spotting could be a result of cold. It could also be too much fertilizer, or an accumulation of salts (more likely when watering from the bottom) or that the plant dried out too much between waterings. My guess is you are letting it dry out too much. Remember, the young plant's roots are mostly at the top of the pot. This is also where it dries out first and this could be stressing the seedlings too much. Ideally, the surface should just barely dry at this stage of growth to avoid any damping off disease. As the plant matures and the roots reach the bottom of the pot, you can let the top dry out a little more before watering.

If you have sprayed any insecticide on it, that could cause the damage you see as well. Overall the plants look pretty healthy though.

Your plants are crowding each other and will ultimately cause the leaves to turn yellow. However, your plants don't seem to be yellow in the picture. If you are talking about the seedling leaves (those first leaves closest to the soil), that is normal. They yellow and usually fall off as more true leaves develop.

Separate your large clump into smaller clumps of 3 to 4 plants and replant each clump into its own pot. Start with about a 3 or 4 inch pot. Once the plant is vigorously, you will need to step up to at least a 12 inch pot for best growth and harvest.

The bug is an aphid.

F. DeBaggio

Thank you so much Francesco. I haven't put any fertilizer on it, should I? Maybe at one time I did leave like 4-5 days without water, but I am taking good care now to bring it to normal again. I did not spray any insecticide on it.

I am planning on replanting them this weekend, I think that will help a lot, any special soil for the replanting? what should I do for the aphid then?? just monitor and kill them every now and then?

Thanks a lot!

I like to fertilize my seedlings at about 1/4 to 1/2 strength about once a week. Basil will respond well to fertilizer. Mature plants should be fertilized like veggies. Plants grown in a pot will need more than those grown in the ground.

Monitor the plants for aphids. There are lots of method people have put forth on various posts on this forum. I like to squeeze them by hand if the populations are low. I spray with combination of insecticidal soap and pyrethrum if populations get out of control. Neem oil is also effective. Black aphids are much harder to get rid of than the green aphids in my experience. I have not had good results trying to knock off aphids with a stream of water-they are usually back in a day or so.

I like using Pro-mix for all my plants. It is a brand name product that is peat moss based (70% or so) with the balance made up with perlite and vermiculite. You can skip the vermiculite if you make your own. It also has a wetting agent in it to make the peat moss soak up water right away. It is pathogen-free, drains well (good air space), and holds water. Don't use soil from your garden. Use the best potting mix you can buy, whatever that might be.

F. DeBaggio

Thank you so much Francesco. I haven't put any fertilizer on it, should I? Maybe at one time I did leave like 4-5 days without water, but I am taking good care now to bring it to normal again. I did not spray any insecticide on it.

I am planning on replanting them this weekend, I think that will help a lot, any special soil for the replanting? what should I do for the aphid then?? just monitor and kill them every now and then?

Thanks a lot!

Francesco, thanks again for the soon response. I kept looking today at the plant, and after moving the leaves a side and checking the soil on the pot and the basil stems, I found some more aphids on there... now Im really worried, any approach I should take before transplanting? I understand the water thing to spray for big plants, etc, but not for this baby pot basil. Thx again!

That is another disadvantage to having so many plants in one pot. It is very difficult to see what's going on and it is hard to spray if needed. Squish as many aphids as you can by hand. Once you have separated all those seedlings into smaller clumps and repotted each clump, you'll be able to get to the aphids more easily. Wait a week or so after transplanting before using an insecticide.

F. DeBaggio

Thank you so much Francesco, will update in case something else arises.