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a genus of roundworms, or nematodes, of the family Tylenchidae. The body is long, thin, and pointed. Adult worms are 0.8–1.5 mm long and 0.02–0.03 mm in diameter. The entire life cycle is passed in plant tissues. The most dangerous species are D. destructor, which infests potatoes, and D. dipsaci, which infests onions, garlic, parsnip, parsley, tomatoes, clover, and cereal grains. The nematodes attack bulbs, tubers, rhizomes, and stems. When the infected plant dies, the worms either drop onto the soil and search for a new host or remain in the tissues of the dead plant. The worms can be controlled by returning affected crops to their former position in the crop rotation no more often than once every three years.