So my tomato plant is around 4 months old and its stem is starting to get a bit light brown colored. This is my first time growing tomatoes so I was wondering if this is normal or if my tomato has a disease?

The plant that was growing next to it died (I know I shouldn't have kept it there but I didn't want to cut it back then) so I was a bit worried that my tomato plant could also die.

Thank you very much.