Need at least a few facts, please.

Indoors or out?
Pots or the ground?
If pots, the growing mix/media?
If supplemental light, how much , how long, and how near the plants?
Fertilizer? If so, what?
Etc...?

Facts -

- outdoor (on a balcony facing west)
- in pots (quite big; I'm not very good with measurements)
- premium organic potting mix
- i provided some light seaweed fertilizer a fortnight ago; less than the recommended dose
- has just recovered from a bout of powdery mildew, sprayed an organic fungicide all over

Hi lily,

I think this is your key phrase, or chief complaint if you will: "the branch and stem are displaying dark patches, and - if left long enough, the whole branch becomes very weak; almost rotten. I can pull the branch from the stem with barely any resistence."

Unfortunately that sounds like blight, though I will admit I have not had to deal with it myself, so I could be very wrong.

There are some things I would suggest:

2. Take a peek at the pictures in the link below.

3. Check with your local extension office to see if any of the blights have been a problem in your area.

4. Ask if you can bring in a sample for diagnosis.

5. If you can, take some pictures with a little better light and closer to the problem areas, that way someone may be able to make a suggestion.

I'm sorry I don't have any better answers or suggestions for you.

Betsy

Here is a link that might be useful: Tomato Problem Solver (Photos for Diagnostics)

Doesn't sound like blight to me. Looking at the images, probably more like a root problem.

Why only fertilizing light a fortnight ago? Potted plants need regular doses of fertilizer.