Root-knot disease of tomato is caused by a type of parasitic nematode. Nematodes are microscopic, worm-like organisms that can infect the roots of plants. In the case of root-knot disease of tomato, the nematodes feed on the roots of the plant, causing knots or galls to form. This can result in reduced growth, reduced yields, and in severe cases, death of the plant. The nematodes can survive in the soil for long periods of time and they can infect a wide range of plants, including tomatoes, peppers, melons, and many others. To control root-knot disease, it is important to practice good crop rotation, avoid planting susceptible crops in infested soil, and use nematicides if necessary.