Species: Meloidogyne incognita (southern root-knot nematode), M. arenaria (peanut root-knot nematode), M. javanica (Javanese root-knot nematode), M. hapla (northern root-knot nematode; not found in corn)
Size: Adult females are up to 1/16 inch in diameter.
Color: Adult females are a translucent cream color.
Description: Adult females are pear shaped and sedentary.
Range: M. incognita, M. arenaria, and M. hapla are commonly found throughout Virginia. However, M. javanica is rare and found mostly in the warmer areas such as near the coast.
Habitat: Root-knot nematodes are obligate endoparasites that complete most of their life cycle inside the host plant. The only stages found outside the host plant are the egg, the motile portion of the second juvenile stage, and the adult vermiform male.
Hosts: Root-knot nematodes have a wide range of hosts. The more than 2,000 host plants include all major field crops, most vegetable crops, fruit trees, ornamental plants, and weeds. Some of the common host plants are bean, cabbage, cantaloupe, carrot, celery, chard, corn, cotton, cucumber, dandelion, eggplant, lettuce, mallow, okra, onion, peach, pepper, plantain, potato, pumpkin, purslane, radish, rhubarb, sorghum, soybean, spinach, squash, sugarbeet, sugarcane, sweet potato, tobacco, tomato, turnip, and watermelon.
Life Cycle: The short-lived adult males are vermiform and motile, and move into the surrounding soil to copulate with nearby females. However, males are not necessary since female root-knot nematodes are able to reproduce asexually. Adult females deposit eggs into a protective gelatinous matrix, near or just outside the root surface. A single female lays about 500 to 1,500 eggs during her life, which lasts about two to three months (Fig. 1). Eggs hatch only under favorable conditions, such as adequate moisture and warm temperatures. The life cycle takes anywhere from 17 to 57 days, depending on the host plant and environmental conditions. Typically, root-knot nematode development begins inside the egg. After the completion of embryogenesis, the first-stage juvenile remains inside the egg until it molts into the second-stage juvenile. Second-stage juveniles hatch from the egg and move freely in the soil in search of a suitable host plant (Figs. 2a, 2b). Root-knot nematode second-stage juveniles undergo an additional three molts before transforming into the adults. Root-knot nematodes have a low host plant penetration rate at temperatures below 50°F, and a reduced reproduction rate at temperatures below 58°F.
Type of Damage: The second-stage juvenile enters the plant root behind the root cap, in the zone of elongation where it then migrates toward the root tip. Once it reaches the root tip, it turns 180° and travels in the center of the root cylinder to the zone of differentiation. In the zone of differentiation, the second-stage juvenile injects esophageal gland secretions that cause proxylem root cells to divide, enlarge, and form multinucleate giant cells. These giant cells act as metabolic sinks that actively transfer nutrients from host plant to the developing nematode. Cells around the developing juvenile also become hypertrophic and more numerous, which ultimately results in the formation of the characteristic root galls, associated with Meloidogyne infections.
Although corn is not often damaged by root-knot nematodes, it can serve as a host to three of the four common species of Meloidogyne (M. incognita, M. arenaria, and M. javanica); therefore, root-knot nematode populations may be increased during corn production so that the crop following corn in a rotation scheme may be severely affected if it is sensitive to one of these nematodes.
Corn damaged by root-knot nematodes often is stunted and has the appearance of moisture and nutrient deficiencies. Severe infestations can result in the death of younger plants. Affected plants typically occur in patches in the field and symptoms are often attributed to other causes such as compacted or acidic soils, nutrient deficiencies, or soil insects, such as annual white grubs, 