This is the first year that I grow tomatoes. I purchased a small green house especially for tomatoes and cucumbers. I bought soil from the nearest nursery to fill in the green house.

My tomatoes (about 12 plants) are ready to blossom but yesterday I found one of them wilted with no visible reason. I found that the stem right above the roots (underground) looks dry, discolored and shrunken. The plant is still alive but no worms or pests are visible. I don't know how to treat this worm( if it is a worm), but I am afraid that it will eat my other plants. What to do? Help!