Every year I have lots of people ask me about pruning tomato plants. The only hard and fast answer I give anyone is to make sure that no foliage touches the ground and any branches that have off colored leaves should be removed immediately to prevent/slow spread of disease. Other than that, you can prune severely and maintain one main stem or you can prune a little to maintain the shape of the plant, or you can let the plant do what it wants to do…grow!
If you stake your plants and only have a few of them, you might consider only allowing one or two main branches to grow. This makes the plant easier to tie to the stake and is a pretty way to display the fruit. Martha Stewart would likely approve of this method. Be careful though…if you can see the fruit easily, then you are exposing the fruit to the sun and can end up with sun scalded fruits.
If you want to prune a little to maintain the shape and size of the plant you can prune lateral branches “suckers” or prune stray side branches. This is a good idea if you planted your plants close together (less than 3 feet). Forget the myth that “suckers” take energy away from fruit production. Suckers have leaves…leaves have chlorophyll…chlorophyll is used in photosynthesis…photosynthesis makes sugar which is energy for the plant. I personally don’t prune suckers except when I want to make a clone of a plant. You can take the sucker, stick it in some damp potting soil and put it in the shade for a couple of weeks and you’ll end up with a plant you can put in the garden for a later harvest.
I prune my plants at least once a week. I start by making sure that any branch that touches the ground gets snipped off. I also remove any branches that have any leaves that are yellowing or show signs of disease. This may not stop the disease, but it will slow it down and may prevent it from spreading to other plants. I use scissors or a small pair of shears to remove the offending foliage and remove the snipped off branches and toss them in the compost pile.
There are a ton of videos out there to show you different methods of pruning. I made one this evening to show you what I do. I hope it helps to make your tomato plant healthier and more productive.