hello - this is my first year growing tomatoes in above ground planters. Things were developing well, until recently the leaves started to yellow and develop brown/grey areas, and die. I've been reading a number of articles about possible causes, but finding it difficult to figure out if this is a type of fungus like Septoria, and treatable - or something more serious like bacterial canker - in which case I need to get rid of the plants. The symptoms actually look like both - so it's confusing. Here are a few pics would love some feedback. thanks all!
While I can't definitively I.d. the problem, if I were you I would simply snip off & discard (bagged in the trash - NOT on the compost pile) any affected leaves/stems & then see how things progress. That's what I do when confronted with this type of thing - sometimes that's the end of it, sometimes not, but it's worth a try & less invasive than discarding all the plants.
... You didn't say what stage of growth your tomatoes are, but it's possible they are at the point when they -- all of a sudden -- start guzzling water. If you have been watering on schedule or giving the same amount out of habit, they may be needing more water at each watering.
As long as they are near the bottom of the plant no big deal. If they were yellowing near the top you've got issues. just pinch/cut them off and get rid of them. I'm in the same general area as you/planted around the same time and been getting this for the last week or 2 as well. It happens every year. Good luck.