Before you decide that you have a tomato fungus, check how you’re watering the plants. Both over-watering and under-watering your tomato plants can result in the same type of symptoms as the Fusarium oxysporum based diseases. This fungus lives in the vascular system of the plant where the water is transported from roots to leaves. To see if your plants are infected, first check the roots. If they are discolored, this indicates root rot and you know that yes, your plants are infected. Also, cut a lower root and take a look at the tissue. If it’s dark brown, this is also an indication of an infected plant.