Just as there is "good" bacteria, "good" fungi also exists. The entire fungi class mycorrhizae is one such type of fungi. Mycorrhiza (plural mycorrhizae or mycorrhizas) is a class of different types of fungi that symbiotically feed off of plants. This symbiosis provides a jointly beneficial relationship between the fungus colony and its host. Mycorrhiza fungi redirect plant carbohydrates (including the sugars glucose and sucrose) to the roots, where the fungus eats it, generating more of itself. The host plant's roots make use of the fungus, taking advantage of the fungus' surface area to absorb what it needs from the ground.
Through this symbiosis the host plant's mineral absorption is increased, chemically and physically. Also, pathogen resistance is better in colonized plants than in non-colonized plants, and drought resistance is better. Without a mycorrizal fungus colony, plants grown in artificial enviroments (such as a growth media) or in nutrient-deficient soil fair poorly.
What I end up with is a tomato plant that does better in droughts and that does better against insects pests and diseases. I have been using it in the desert southwest for 3 years now and I am sold. My plants have done great when it seems all my neighbors plants have died or just not produced well. Plants by themselves aren't always able to get what they need so try some mycorrhazae fungi and enjoy healthier plants next year.