Root knot of tomato is caused by a nematode. Nematodes are microscopic, worm-like organisms that can live in soil and feed on plant roots. Root knot nematodes are a type of nematode that can cause significant damage to tomato plants. They infect the roots of the plant and cause the formation of characteristic galls or knots on the roots. These galls can interfere with the plant's ability to take up water and nutrients, leading to stunted growth, reduced yields, and in severe cases, plant death. Controlling root knot nematodes can be challenging, but measures such as crop rotation, soil solarization, and the use of resistant tomato varieties can help manage the problem.