Root-knot disease in tomatoes is caused by a type of microscopic roundworms called nematodes. These nematodes infect the roots of tomato plants and cause the formation of knots or galls, which can limit the plant's ability to take up nutrients and water from the soil.
The nematodes that cause root-knot disease in tomatoes are usually Meloidogyne species. They are soil-borne and can persist in the soil for several years, even in the absence of tomato plants.
Root-knot disease in tomatoes can cause a range of symptoms, including stunted growth, yellowing of leaves, wilting, and reduced yield. It is a significant threat to tomato production worldwide and can be managed through a combination of cultural, biological, and chemical control measures.
Therefore, the correct answer is "nematode". While viruses, bacteria, and fungi can also cause diseases in tomato plants, root-knot disease is specifically caused by nematodes.