Fig 21-1. Root-knot disease is caused by the root nematode, which lives in the tissues and destroys much of the root system. Note the absence of root-growth beyond many of the swellings. The root-knot nematode causes far less serious injury than the meadow nematode which causes brown areas rather than knots. In Florida the sting nematode, which shortens the roots to about 2 inches, is so serious that fields are treated before planting.