Additional information:

I'm growing Moneymaker which are Indeterminate. According to Google they grow up to 200cm (79") in height. Currently the top of the plant is 53cm (21") from the top of the soil.

The container is 12l (3.17 US Gallons) which may be too small?

I grew the plant in 100% compost (packaging says it's a reduced peat mix enriched with seaweed).

I use an inorganic liquid tomato fertilizer which has a mix of 4-5-9. I read somewhere to fertilize once every 2 weeks so have been doing that. I think I may have given 3 applications so far.

I normally water every 1-2 days, depending on the temperature.

Much too small a container for an indeterminate variety! I grow cherry types primarily - all indeterminate - and a 10G container is about the minimum size I consider. Mostly I use 15 or 20G pots. Too small a container restricts root development and that can affect healthy growth. It also increases the need for more frequent watering

While you may be able to get away with it, 100% compost is not a great container media. Better to select a potting soil with minimal peat or compost...something like the 5-1-1 mix. Fertilizing weekly is good :-)

Thanks for the suggestions. I'll head to the local garden center right away and get a larger pot.

Might be more than one thing, but it looks nutritional to me, which might be a result of the pot being too small.

Nutritional Deficiencies of Tomato

Looks nutritional to me too.

I water my 3 gallon containers at least twice a day.

Linda