posted 5 years ago
Yes sunburn is also my best guess but may I give my 5 cents to it?
I would taking care when watering that you not water the stem.
Make a heap around the stem and then a ditch or just stick two cut plastic bottles beside in the ground about 6-8 inches away from the stem.
Tomatoes hate getting wet knees..
In modern times the only right way forward is to come back to nature.