Affected plants are severely stunted. In advanced stages of the disease, lower portions of stems become discolored similar to summer black stem. Occasionally, alfalfa stems of infested plants lose all green color. All stages of the nematode except the first, which molts inside the egg, can attack alfalfa. The nematodes enter bud tissue and migrate into developing buds.
Infected stems become swollen and distorted.
D. dipsaci can also infect the alfalfa seed if the flowers are infected. Roots are occasionally infected, which results in internal cavities. The stem nematode spreads by infested soil or infested alfalfa tissue. The nematode can also be spread by machinery during harvest and by rain and irrigation water from infected alfalfa debris left in the field. Use of resistant varieties is the most effective method of controlling the stem nematode.