Yes, Monikka and Welcome here!

Those appear to be the same leaf miners from our American leaf miner genus here. They are the scourge of my newly hardening transplants too and can completely destroy the vigor of a plant here.

-Do not spray the plants with an insecticide - leaf miners usually multiply when you do because they are relatively protected and the bugs that eat them are killed.

-Use your thumb and forefinger on each side of the leaf to crush them in their mine. Apply pressure to as small of an area possible as you will also be killing a little of the leaf when you do that, but on balance you will save all the rest.

-Buy yellow trap cards (link) or make them (Link ones are crazy expensive price, try eBay, etc). Those leaf miners you have are attracted to yellow and the cards are buttered with a very persistent sticky substance. The adult flies when they come to lay eggs into the leaves go land on the yellow card first. If it is sticky, they get stuck like in amber.

-I thought it was too late to start tomatoes in Austria now and get ripe tomatoes before cold temperatures start, were did you get the transplants?

PS the dark brown may be a type of mold but it is soon to say. Seeing it on the stem is unnerving. Try to water roots only and not let the plant get water on the leaves or stem whenever possible. It could be something bad but time will tell and there isn't much else you can do. The spots on the leaf and browning on the stem could be bacterial speck or one of those like it, but that you will soon see as it develops more, if that happens. I hope all turns out ok!

Cheers

PC

Thank you so much for replying so fast! I will do everything you advised. I actually live in Florida too. "Austria" is a part of my name.

Upon further inspection, I saw some small yellow egg-like things on the underside of the leaves. I am not sure if this relates to the black spots on the plant but I will continue to monitor it.

Thank you again for your advice!

Your welcome, and I'm sorry for assuming Austria was the location :-), the way your leaf miners and timing is, you're probably my neighbor! Keep us posted here on how things are growing. If you decide to go for the sticky traps there are just 5 here (link) but it includes postage if the link goes through for that vendor, no minimum postage requirement for ordering. These particular ones are $6.38 and single sided, but designed to fold to make them double sided if you hang them that way. Sometimes tomato stems get black spots that don't amount to anything which I hope is the case. At this stage you ought to consider spraying them with either a copper or Daconil fungicide (both available at any garden center or Home Depot/Lowes place). I'm in northeast coastal FL and in September leaf mold and gray mold are a bad problem you can't get away from since conditions here promote mold and mildew for them at its favorite temps and humidity. Spraying in advance is the most effective preventative measure so the mold doesn't go nuts.

If you come back to the thread, please let us know what varieties you bought, that's always fun. I got 11 varieties hardening off today. Better Boy is my favorite of them, but there are a few "heirlooms" in there like Cherokee Purple and Delicious (it's in quotes because CP isn't really a heirloom).

Cheers

PC