Root-knot disease is caused by various species of Meloidogyne. It has long been considered "the nematode" disease by farmers and other plant growers because of the severe yield reduction and obvious root-galling symptoms that are caused by these pests. There are several species of root-knot nematodes found in Illinois, including M. hapla, M. incognita, M. arenaria, M. javanica, and M. naasi. Collectively, these species attack more than 2,000 different kinds of plants, including most vegetable, fruit, and field crops; many ornamentals; and numerous common weeds.