(Some Benefits of Pruning)
What is the Three Finger Method?
Make a 
peace sign and extend your thumb at the same time. You should have your 
thumb, index finger and middle finger extended. Your ring finger and 
pinky should be closed. The three extend fingers represent a tomato 
plant. Your thumb is a leaf branch (the branch to keep), your index 
finger is the sucker branch (the branch to remove) and your middle 
finger is the tomato stem. I drew a picture to further explain the 
principle. When in doubt and in the garden, throw out those three 
fingers and compare it to the tomato section in question.
Keep in mind you typically only prune indeterminate tomatoes and not determinate tomatoes. You prune indeterminate tomatoes as to help keep and maintain the tomato vine that would naturally, if left alone, grow out of control.
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| Tomato Pruning - The Rusted Garden Blog | 
Identify the Sucker Branches and Remove Them
The
 branches aren't really sucker branches but giving them a name makes it 
easier to explain pruning. If they continued to grow they would grow 
into a vine branch - a branch from the main stem. It would flower and set tomatoes along with the main vine. 
A tomato plant is 
pruned to typically maintain one vine (sometimes two of more) and that will help
 to develop a strong healthy tomato plant. Sometimes I maintain two 
vines when I plant a new variety of tomato. That is just my preference. 
You may chose to let the main stem of your tomato branch into two or 
three vine branches.
Pruning tomatoes theoretically reduces the number 
of tomatoes a plant will generate or produce. However, pruning typically means the 
tomatoes that do grow - grow larger and it significantly helps 
to prevent diseases and mildews. If you don't prune there is a greater 
chance for diseases and you might lose the plant and all those extra 
theoretical tomatoes. You want to maintain good airflow through your plants and let the sun dry them quickly after rains. Pruning suckers helps with this.
If the sucker 
branches are left to grow it gives an illusion of a healthy green 
leafy tomato plant. In reality, come another month, a gardener ends up 
with a plant that is hard to tend and contain in the garden. An unmanaged
 tomato will have more leaves than needed and the plant will be at 
greater risk for disease as mentioned. They can grow and become unruly. It sounds counter intuitive but a little less (removing branches) is more 
when pruning and tending a tomato. I am pointing out two sucker branches
 in the picture below. You will notice a third sucker branch if you have
 a sharp eye.
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| Tomato Suckers - The Rusted Garden Blog | 
These
 sucker branches are large and that is what makes pruning difficult. 
It's not that they are hard to remove but that they look so healthy, 
many gardeners are tempted to let them grow. Don't. Prune them with a 
knife or snap them off if they are small enough. When my tomatoes are 
first growing, I am a little slower to prune them. I let some leaves, I 
would normally remove, hang around a bit in May. After that, they are 
gone. I do it to provide more photosynthesis to the newly planted 
tomatoes. It may or may not make a difference but it works for me. You want to slowly prune your tomato as to keep it managed but not remove so many leaves that you slow its growth or put it in a bit of shock. 
  
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| Pruned Tomato Sucker - The Rusted Garden Blog | 
The
 sucker branches have been removed. The tomato still looks fine. You 
will notice my fingers point to the empty spaces where the sucker 
branches were and you now only see two leaf branches. I also remove bottom branches when the tomato is large enough. This creates a splash barrier and helps prevent the spread of soil borne diseases.
Here
 is another one of my plants pictured below. It gives you an idea of 
plant size and the amount of space I keep between the leaves of a tomato
 plant and the ground. If you enlarge this picture you will also notice 
two branches coming off the main stem. This is one case where I am 
growing essential two vines or allowing the main stem to branch into 
two.  I will prune and maintain both of the main growing vines.
  
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| A Pruned Tomato - The Rusted Garden Blog | 
Why Prune Tomatoes?
Tomatoes
 don't have to be pruned but I recommend you do prune them. Tomatoes will still 
grow if the plant is left to sprawl across the ground without any care. 
That is what they are designed to do.  However, if  tomatoes are left to
 sprawl, they will be more susceptible to disease and mildews. More 
garden space is needed for a sprawling tomato plant and you probably 
won't get that many more tomatoes then a well pruned and tended tomato 
plant.
You prune a tomato plant to greatly 
reduce the risk of disease and mildews such as blights and powdery 
mildew. A pruned tomato plant creates a gap between the soil and leaves.
 It is harder for spores to splash to the leaves and take hold. A pruned
 plant has less leaves which allows air to circulate through the 
entire plant.
Better air circulation quickly drys leaves. Dry leaves are a 
good strategy in reducing the spread of disease and mildews. You
 prune a tomato plant because you will still get a large harvest of 
tomatoes without sacrificing space in your garden. A tomato that grows 
up a stake and has its growth managed, allows more room for more 
vegetable plants. In my book of a gardening the more space the better. 
In super summary, you prune to have healthier larger tomatoes and more 
garden space. It's that simple.
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