Hi, annabelleducasse. Can you tell us what variety the stumpy tomato is? Are the others the same variety?

Where in Australia are you, and how hot has it been? Planting less than 2 months ago seems late in the season to me, but it would depend on your climate.

If you're not familiar with your zone, here is a map of the Australia plant zones:
http://www.anbg.gov.au/gardens/research/hort.research/zones.html

Are you growing in the ground or in containers? If the latter, what sort of planting medium are you using?

Here is a tomato problem diagnostic which you might find helpful (click on the labels on the plant drawing):
http://vegetablemdonline.ppath.cornell.edu/DiagnosticKeys/TomKey.html

In addition to all the great questions for you already posted, three more:

(1) is there any chance they have been exposed to any herbicides, even any drift from herbicide spraying?

(2) What and how often have they been fed?

(3) Is it possible they have been OVER-watered? That is a very common problem for many inexperienced growers and quite a serious problems for tomato plants so exactly how much and and how often have you been watering these plants?

Dave

Thanks for your quick responses.

I live in Sydney and the weather here has been very temperamental as always. The temperature fluctuates from 25 degrees Celsius to 45 degrees Celsius (though the average has been around 30).

The tomatoes are of the truss variety and are being grown in individual pots. They get approx 7 hours of direct sunlight per day and I water them either everyday or once every 2 days.

I recently purchased some "tomato food" from the local nursery (sorry I cant remember the name of it) which is a soluble powder, and have added that to the water yesterday.

I water all the plants the same so I don't think it has been over watered. And it's unlikely the plant has been exposed to herbicides.

I'll upload some photos. It seems to be getting worse, should I cut off the dead leaves??

As you can see, the outer two plants are growing nicely, whereas Stumpy in the middle has just given up on life.

Stumpy close-up

The temperature fluctuates from 25 degrees Celsius to 45 degrees Celsius (though the average has been around 30).

For Americans, this translates to: The temperature fluctuates from 77 Fahrenheit to 113 degrees Fahrenheit (though the average has been around 86).

For Americans: "truss variety" translates to "cluster tomato."
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annabelleducasse, with 40 C. (113 F.), I think you're lucky the plants are still alive at all. But I don't have to deal with those conditions, so we need an opinion from someone who has that sort of weather.

Do you know the name of the variety? That would give us some idea how large the plant wants to get.

Do you know how large the pots are? And -- since I've never seen pots like that and digdirt suggested too much water, I'll just ask: are there drain holes in the pots? [The last time I asked that, the pot had no holes.]

Assuming the pots turn out to be on the small side for the variety (which is usually the case here), I'd suggest adding more potting mix; tomatoes will grow roots from their stems, and don't mind more potting mix being added. In any case, to help keep the potting mix and the plants' roots cooler, and to keep the soil moisture more consistent, I'd add several centimeters of mulch atop that (mulch can be anything from straw to bark to shredded paper).

I don't think it looks like an herbicide problem, but a close-up of a few of the damaged leaves (really zoomed-in!) would help. [We do see a lot of herbicide damage here; sprays can blow on the wind from great distances, broadleaf weed-killers used by the neighbor's lawn-care crew are a big problem, and certain herbicides can lurk in manure and straw for a year or more.]

annabelleducasse - to follow up on missingtheobvious post, while I am not certain it would be a direct reason for the yellowing/ lack of growth, the only issue that jumps out to me is the size of the pots.

Tomatoes can easily become root bound, and judging from the size of all the plants (they seem small for 2 months old unless they were started direct from seed) you could have a problem there.

The smallest pot I use is 7 gallons (27 litres), and is roughly 1/3 meter in height & width. Its hard to tell from that picture since there is no scale, but I am guessing the pots are much smaller than this?

Assuming thats the case, at minimum I would do what missingtheobvious said, and fill up the pots as much as you can (roots will grow off the stem). Also, if possible, you may want to consider putting them in a much larger container if you have one.

Either way, good luck!

Thanks for your responses and help.

The weather has definitely been extreme! We've had the regular heat waves which caused many bush fires. Check out http://www.bom.gov.au/climate/current/month/nsw/sydney.shtml .

The temperature was over 40 C for about 5 days so maybe Stumpy just couldn't handle the heat!

Yes I did start the plants from seeds. I got the kit as an early birthday present on Dec 23rd. The pots are 25L, and are the same width as yours (the picture makes them look smaller), and yes they have holes in the bottom :)

I'm going to mulch the plants as suggested, and give the Stumpy one a bit more TLC.