Plant-parasitic nematodes feed on root tips and feeder roots and are capable of causing severe damage. Nematode-infected roots are less efficient at supplying a plant with water and nutrients and thus increase susceptibility to stress factors such as heat, drought, and nutritional deficiencies. Although nematodes may cause yield losses by themselves, they also combine with other soilborne agents, such as viruses, fungi, and bacteria, to cause complex disease situations. In pome and stone fruit, plant-parasitic nematodes reduce tree vigor, predispose trees to diseases, cause replant problems, and transmit viruses.
Plant-parasitic nematodes limit the availability of nutrients for tree growth and fruit production. Furthermore, trees must expend metabolic activity to repair damage caused by nematode feeding. This is a chronic problem that continues throughout the season. Without control measures, nematode populations often continue to increase. Ne-matodes become a problem when the population level surpasses the damage threshold; at this point a tree yields less than its potential. The threshold for damage will vary according to a number of factors, including tree variety, age, size, nutritional status, and moisture stress.
Research has shown that nematode feeding can produce physiological changes that predispose trees to other problems. For example, feeding by the ring nematode has been linked with susceptibility to bacterial canker and reduced winter hardiness. The basis for predisposition is likely the result of weakened natural defenses against disease and possibly a disruption of the normal hormonal balance in the root system.
Plant-parasitic nematodes have repeatedly been associated with the poor establishment of new trees on old orchard sites. Evidence suggests that these replant problems are caused by synergistic interactions between plant-parasitic nematodes and other soilborne microflora.
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