I found these spots on my tomato plants today.They are still very small and I'm quite uncertain: Is that some kind of disease, and if yes what is it?
Should I risk planting them with the rest of my plants that are healthy?
George, I think your tomato plants are fine. Those spots don't look like they're caused by any disease - I think they're just from sunscald or windburn. That can happen easily with tender young seedlings.
I was surprised to see the wild shamrock (oxalis) in the center-right of your first photo. Some varieties of that are cultivated as flowers, but it's mostly a weed for me. I know it grows in California and Missouri, but had no idea it's also in Greece.
Water splashing on leaves...I'll keep that in mind, but in general I try to avoid this, since it can lead to the development of diseases.
As for oxalis, it's a weed here as well. It's not very widespread, and has not caused me much trouble. I'm not sure if it's a native plant of Greece or not. In any case, it does not show invasive tendencies.