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Contributors: K.R. Barker, Plant Pathologist; D.M. Benson, Research Plant Pathologist; R.K. Jones, Extension Plant Pathologist

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NEMATODES

Nematodes are one of the most destructive groups of pests that cause decline of established woody ornamental plants in North Carolina. These microscopic roundworms feed on plant roots and cause various types of damage. For example, root-knot nematodes cause swelling, or galls, on roots of susceptible plants. Other types of nematodes cause plant roots to become stubby and to branch abnormally. Nematode-damaged roots often are destroyed further by fungi and bacteria. Several plant-parasitic nematodes such as root-knot, stunt, ring, sting, lance, lesion, stubby root, dagger and spiral have been associated with decline of woody ornamentals in North Carolina.

The root-rot nematodes (Meloidogyne sp.) are by far the most important pest. Their presence is easily recognized by galls on roots. Galls result from growth of plant tissues around the juvenile nematodes, which feed near the center of the root. Root-knot gall tissue is firm, without a hollow center, and is an integral part of the root. Removing a root-knot gall from a root tears the root tissue. Root-knot gall tissue could easily be confused with root nodules on leguminous plants formed by beneficial, nitrogen-fixing bacteria (Rhizobium sp.). Active Rhizobium nodules have a milky fluid in their centers. Other natural nodules or bumps are loosely attached to roots and usually have hollow centers.

Damage to plants from these root-feeding nematodes is progressive and often results in poor growth, low vigor, yellowing or bronzing of the foliage, loss of leaves, stem die-back, failure to respond to fertilizer because of root damage and, eventually, death.

Above-ground symptoms. Extensive root damage has often occurred before above-ground symptoms become obvious on established woody plants. Symptoms include root galls, stunting, root decay, and darkened roots. Above-ground symptoms are similar to those resulting from many kinds of root injury. Foliage loses its luster and wilts more readily than it should. Prolonged root stressed caused by nematodes may result in yellowing and eventual loss of foliage. New flushes of growth are weak, with fewer and smaller leaves than healthy plants. The damage is usually distributed irregularly, since nematodes are rarely distributed evenly in the soil.

Root symptoms vary widely. Root-knot and some foliar nematodes cause tissues on which they feed to grow strangely. Some nematodes stop the growth of the roots, while others kill the cells on which they feed, resulting in sporadic dead tissue. Fungi and bacteria often infect nematode-damaged roots more readily and more severely than roots that have not been injured. Death of nematode-damaged plants often occurs during or following drought or cold injury.

Managing Nematode Pests in the Landscape

The key to minimizing the effect of nematodes on landscape ornamentals are good horticultural care and preventive maintenance. A variety of solutions may help correct nematode problems on existing landscape plants. These solutions include mulching the plants, applying adequate water during dry periods, fertilizing and liming properly, and pruning out any dead branches. When half of the plant (or more) shows advanced stages of decline due to nematodes the plant should be replaced with less susceptible shrubs or turf.

Site Preparation. Plants should be given the best chance to become established rapidly. Soil should be well prepared, and old roots, debris, etc., should be removed from the site. Water and nutrient-holding capacity of the soil and activity of the natural enemies of nematodes are improved by incorporating organic amendments into the soil before planting.

If nematodes build up to high levels, or if plants that especially favor nematodes were recently removed from the site, soil solarization may give the new plants a better chance.

Soil solarization is a non-chemical method of reducing soil pest populations. This method is most effective on sandy soils in hot climates. Soil solarization is labor-intensive, and the treated area must be left bare for 6 to 8 weeks during the summer. Clear polyethylene is used to cover moist soil that is ready to be planted. Heat, generated by sunlight, will be trapped under the polyethylene. The temperature of the soil will increase and kill many nematodes, fungi, and weed seeds in the upper profile of the bed.

Remove contaminated soil. In some situations, the removal of contaminated soil or planting mix (from a raised bed, for example) may be a simple alternative. Nematodes may eventually invade the new medium, and plant roots can grow into infested native soil, but replacing the media with new, nematode-free soil will delay infection.

Nematode-free stock should be used. No matter how perfect and pest-free a planting site may be, an existing nematode infection in plant material can do substantial damage. Only top-quality plants should be purchased. Plant material that has clear evidence of nematodes or other hard-to-control pests should be rejected.

Well-adapted plants should be selected. The suitability of plants to a particular location is important at all levels: region (climate), soil type, shade, drainage, etc. Plants installed in improper environmental conditions are more likely than well-adapted ones to suffer stress at any given time and for longer periods of time. Plants that adapt well to an area also may have some degree of tolerance or resistance to common local pests.

Nematode-susceptible plants should be avoided. Plants that are susceptible to nematodes should not be used in planting sites. Plants vary widely in their susceptibility to different nematodes. Most references to nematode susceptibility in popular literature refer to one or more root-knot nematode species, unless other nematodes are specified. It is quite likely that any plant identified as susceptible should not be planted where root-knot nematodes are known to be serious.

Other pests should be kept under practical control. Stress from pests can set plants up for nematode infection just as readily as over- or under-supply of nutrients or water. Overuse of pesticides also can injure plants.

Table 1. Response of Several Ornamentals to Root Knot, Stunt, Lesion or Ring Nematode. Results of Three-year Field Studies

Maintenance. Plants should be fertilized as needed to maintain healthy growth. Excessive succulent growth invites attack by nematodes and other pests. Watering plants deeply will encourage the development of a deep root system. In turn, the deep root system can exploit a large volume of soil for water and nutrients. A large root system can withstand a small amount of nematode damage without major compromise to the overall health of the plant.

Sudden dry periods or pest outbreaks can quickly weaken plants. Even under normal conditions, erratic or inadequate watering can weaken plants, resulting in an inability to tolerate a modest nematode population that has been present for years.

Mulches. Mulches reduce stress on the plant as a whole and on the root system. Mulches specifically improve the plant's chances to do well despite nematode damage that may be present. Mulching a plant root zone keeps roots cool in hot weather, and it minimizes the evaporation of water from soil surfaces. Organic mulches contribute organic matter to the soil, thus enhancing the capacity of the soil to retain water and nutrients. Soil organic matter also stimulates the activity of natural enemies such as fungi and predatory nematodes which may help suppress the population of plant-damaging nematodes.

Currently, no effective nematicides are labeled for ornamentals already planted in landscapes. For new landscape plantings, or for replanting, nematode problems must be avoided; it is very difficult to correct these problems after planting. Where possible, it is best to avoid using highly susceptible plants on sites where damaging nematodes are known to occur. It is also very important to purchase plants that are free of damaging nematodes. See Table 1 for a list of susceptible landscape plants.

If damaging nematodes are known to exist in the planting site and highly susceptible plants must be used, the entire area may be treated with methyl bromide (restricted use pesticide), or a nematicide registered for landscape use before planting. These chemicals cannot be used between existing plants in a bed. To determine if damaging nematodes are present in the soil before planting, a representative soil sample (1 quart) should be collected and placed in a plastic bag from the area to be planted. The sample then should be taken to the county extension office . The county extension agent can provide more details on the procedures, forms, and fees for nematode identification.

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