Tomato suckers are side shoots that grow out of the place where a branch meets the stem. If left to grow, they will compete with the main plant for nutrients. You may get more fruit, but they will be smaller. It's recommended to prune them. They are really easy to remove; all you need are your fingers!
Tomato suckers have to be removed but all you need is :
. a pair of scisors to do neat cuttings
. alcool to clean the scisors when you go from one plant to another
. a very sunny day at midday with plenty of sun light on the plants to dry the cuts you make as quiclky as possible
Because you do not want to make it easier for the mildew to settled on your tomato plants. Take the same care when you cut the lower leaves or any other part of the plant because any cutting is an opened door for mildew.