If you suddenly find mushrooms popping up among your tomato plants, you might need to look to the health of your garden. The mushrooms themselves are simply the blossoms of a fungi. Fungi only blossom when conditions are warm and moist. In themselves, mushrooms are not a threat to your tomatoes. The conditions that created them, however, can be. If you find mushrooms among your tomato plants, your tomatoes are probably already showing symptoms of distress.
The mushrooms sprouting among your tomatoes are the first symptom of a problem. The leaves of the tomato plants may turn yellow or roll upward, giving the leaves a cupped appearance. This problem is caused, not by the mushrooms, but by excessive moisture in the soil.
Mushrooms come from spores that commonly blow through your tomato garden, especially if you set it up properly so you have plenty of breezeway. The spores lodge in warm, wet soil and mushrooms pop up. If you see mushrooms, look for wet, water-logged spots in your soil. If you added topsoil to raise your beds to to lift the soil around the roots of your plant, you may have some mushroom spores that came with the composted soil. Most commercial soils have mushroom spores in them that will sprout under the right conditions. It's the over-watered soil or rainy weather that causes mushroom growth that creates problems for your tomatoes.
Proper spacing of your plants allows sunlight and breezes to dry the plants and the ground around it. The plants should be two feet apart and the rows four to five feet apart in well-drained soil. Raise the soil around the roots of the plants a few inches above the surrounding rows. Watering should be done two to three times a week in dry weather using a soaker hose or other slow-watering style that does not wet the leaves and stems of the plant. Water deeply. Light watering creates a shallow root system. Deep watering encourages a deep healthy root system. The soil needs to be moist, but not wet or soggy. Mulch around the base of the plants to hold moisture without puddling.
If mushrooms or symptoms of over-watering appear, look for soggy spots and reduce watering accordingly. As the soil dries, the tomato plants will recover. If you see your plants drooping, give them a good soak, but not enough to create soggy spots. If you do see mushrooms, just pick them and throw them away.