Fungus thrives in the damp, insulated conditions underneath your driveway's asphalt as long as the soil contains enough organic matter to feed it. Some types of fungus may damage your driveway and the surrounding lawn before their food is exhausted. To eradicate fungus, you must remove the infected soil by replacing your driveway and the soil beneath it.
When fungus grows underneath your driveway, you will notice fungal growths and mushrooms emerging on both sides of the asphalt. In some cases these growths and mushrooms may push through the asphalt and emerge from the surface. These signs indicate that an expanded webbing called mycelium has grown across your lawn and driveway, just below the surface of the ground.
Fungal infections are typically short-lived because they drain the areas that they occupy of the organic matter that helps them sustain life. By draining the soil, however, they negatively impact its fertility and damage the appearance of your lawn. Cracks in the driveway asphalt caused by growing fungus are unattractive and may lead to further splitting and crumbling.
The easiest way to combat underground fungal infections is simply to wait for the fungus to run out of food. Fungi feed on organic matter, and eventually the mushrooms around and underneath the driveway will use up the available matter and stop reproducing. If the damage to your driveway and lawn is great, you may wish to deal with the fungus more actively. Ripping up the driveway and removing the first foot of soil underneath it will eliminate most of the fungus. Replacing that soil with gravel before you reapply the asphalt will prevent fungus from gaining a foothold there again.
Even if you don't currently have a problem with driveway fungus, replacing the soil beneath the driveway with gravel prevents fungus from growing because mycelium thrives in soil immediately beneath the surface. Some mold growth is an unavoidable and natural response to moisture, shade and soil conditions, but you can prevent individual colonies of lawn-damaging mold from spreading by uprooting them with a shovel.