This morning I was looking at my tomato plants and I noticed that a captain lucky plant had dry leaves, all the plants around it were fine, the plant was about 60 cm and already had small tomatoes forming, I discovered the soil a bit and observed that a few centimeters from the root it was like a bite, it was something similar to the mosquito fungus or damping, every year I have damping problems because I germinate very early in the greenhouse and it is cold and wet, but now I don't think that is the problem since it is a perfect time and nothing cold, also I think that only that type of fungus attacks younger plants, perhaps it is a slug that abounds in my garden? Thank you

So sorry!

Damping off can be in the soil and happen at any time.
From what I have heard.

It could be a nasty slug bite, and then looks like a fungus got into it... sad.
We have slugs here too, they are so destructive! I had them in the tomatoes one year, but I didn't see any stems bitten, only fruit.

One year I had a tomato with rot in part of the stem that had been buried after removing a low leaf at planting time. It grew very well for a while then collapsed. It was not so rotten as yours, Nico, and I could see where the rot had started where the leaf had been removed.
Now I take any lower leaves off a few days before planting so the wound can seal and heal over.

It looks like it's probably Pythium to me. That's one of the pathogens that causes damping off disease, as worth talked about (but it causes other issues instead, too, at times, like a moldy-looking stalk). Plants can live with a Pythium-infected stem at times.

I have a couple of small sites in my garden where Pythium tends to occur. When I notice a plant starts to wilt for no reason, I usually find the dried up shrunken area on the stem close to the ground. Often it's too late to replant for the season as the plants already are good sized with small fruit starting. I save it by spraying the stem liberally with rubbing alcohol or hydrogen peroxide. Then I get something like an ice cream pail or old large pot, cut out the bottom and make a slit up one side so you can fit it around the stem. Then fill it to the top with clean fresh soil and keep watered well. The plant will grow new roots above the old shrunken spot. Production won't be as good and a bit later than healthy plants, but if it's a one of a variety, at least you will be able to taste the tomatoes. At the end of the season, I dig out some of the original soil at the old base of the plant and discard it, to hopefully at least thin the Pythium population in that area.

Thank you all very much for the help, this is the first time this has happened to me, it has only been in one plant and I have started it quickly in case it is a fungus that does not multiply.
 Almost every year the problem that I have to deal with is the "mildew" fungus, a fungus that always appears in the hottest months here, in August, I treat my plants with copper in very low doses to prevent, but I have never had fungus problems so early.Every year when I germinate in very cold months under greenhouse and I have buffering problems, but once the plant grows and is big I no longer have buffering problems, some plants are more resistant than others, I put Many more plants than I need due to the losses I have due to the fungus, I quickly discard when I observe that the fungus attacks them so that it does not multiply, I think that next season I will use some method for the mosquito fungus not to develop.
Pythium did not know him.
Very interesting ddsack, if it happens again, I will do what you recommend.Thanks