Did you mulch ?

Did you fungicide ?

From the photos it appears to be a case of Botrytis Gray Mold as there are lesions on both the stems and the leaves. You can Google that disease for numerous photos to compare to your plants and read about the causes. Unfortunately there is no cure. It can be somewhat prevented by regular use of fungicides from the day of planting but once it develops it cannot be cured.

Recommendations for controlling it include removing all the infected foliage asap and spraying the remaining healthy parts of the plant with fungicides in the hope of slowing it down.

There is also a somewhat controversial, but often discussed and fairly well tested, recommendation for using Chlorox bleach spray on tomato plants with gray mold. However the strength of the bleach used, the ratio of bleach to water, the application times and rates, etc. are all quite detailed. So its best if you do the reading all about it yourself first to decide if it is for you. See the link below.

Chlorox Bleach Spray discussions

Dave

@daniel_nyc - No, I did not mulch but also I didn't treated the plants in any way.

@digdirt2 - Thanks. I'll follow your advice and I'll remove everything which seems remotely infected. Time to get some fungicides I guess...

Maja,
both mulching and fungicide are good ways to avoid problems.

Fungicide
is to PREVENT diseases, not to CURE them.

Imho, it’s
a little late for you to save those plants.

It’s
still early June, you can use new plants.

This time
do yourself a favor and mulch and fungicide.