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!