Nematodes

Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma
Oklahoma Cooperative Extension Service

Nematodes are microscopic, wormlike animals that live in soil. Nematodes are probably the most abundant form of animal life in the soil. Most species that occur in soil feed on fungi, bacteria, or small invertebrate animals; but many are parasites of higher plants, including turfgrass. All turfgrasses are susceptible to nematode damage.

In Oklahoma, nematodes can cause significant damage and need to be controlled in bentgrass golf greens and sod farms. They seldom, if ever, are controlled in home, recreational, or institutional lawns.

Plant parasitic nemadotes feed on turfgrass roots and other organs by puncturing the plant cells with a hollow, needle-like structure called a styles (Figure 31). Digestive enzymes are injected into the cells, and the nematode then sucks out the predigested cellular contents.

Nematodes cause direct impairment of root functions, and nematode weakened plants are susceptible to infection by various pathogenic fungi and bacteria that are attracted to the site of injury. These pathogenic agents become highly active in the presence of plant fluids leaking into the soil, and they can develop rapidly to cause much more severe diseases than those caused by nematodes acting alone.

Schematic drawing of a plant parasitic nematode (Courtesy of C.C. Russell).

Symptoms

Damage to roots is the most important effect of nematodes on turfgrass in Oklahoma. The roots may be killed or stunted resulting in poor and shallow root systems. The above ground symptoms of a nematode infestation include chlorosis (yellowing) of the leaves, slow growth, gradual thinning, poor response to adequate fertilization and irrigation, rapid wilting during dry weather and weed invasion. These symptoms are likely to be most evident in periods of hot weather, drought, low fertility and other stresses.

Because the visible symptoms of a nematode infestation are nonspecific and poorly defined, it is reasonable to suspect nematodes as a possible cause of yellow, nonvigorous turfgrass that does not respond to applications of fertilizer or water. The only way to determine if nematodes are involved is to have soil samples assayed for nematodes.

Life Cycle

Nematodes have distinctive life cycles very similar to those of insects. Females lay eggs, which hatch into larvae. The larvae mature through a series of four molts to become adults. Fertilization of the eggs by males is required for reproduction in many, but not all, species. Nematodes typically survive adverse conditions in the egg or larval stages and feed most actively when the turfgrass is also actively growing. They are mobile within the soil, but long distance spread usually requires their movement in surface water runoff, in soil on equipment, or in sod.

Factors Affecting Damage

Nematodes are most damaging in light, sandy soils which are low in nutrients and water-holding capacity. Increased fertilization and irrigation practices can often overcome the effects of some types of nematodes. Highly compacted and heavy-textured soils are less favorable to nematodes because they restrict nematode movement and prevent good turfgrass growth.

Nematodes are usually most active and most numerous on warm-season grasses during the summer and autumn, on cool-season grasses in mid to late spring, and again in autumn. These times correspond with and follow active growth of turfgrass roots. However, the result of nematode feeding generally becomes most apparent when conditions become unfavorable for the turfgrass.

Detecting Plant-Parasitic Nematodes

The only way to correctly diagnose nematode damage is by proper analysis of soil and root samples. Nematodes extracted from the samples must be identified by trained personnel and the numbers of each type of nematode determined before any control measures are started.

Soil samples can be collected at any time of year, but it's best to sample when the host plant is actively growing, because this is when the nematodes are most active and numerous. Sampling should be done in early morning or late afternoon to avoid overheating the samples in bright sunlight. A sample should consist of 10 to 20 soil cores, 1 inch to 2 inches in diameter taken to a depth of 6 inches to 8 inches, and taken from the margins of affected areas where nematodes are suspected. Include the turf roots with the soil. Collection of aerification cores (if done rapidly, preventing drying and heating) is a good way to sample greens and other turf areas. Place all soil cores in a strong, airtight bag to prevent drying. It helps to take similar samples from adjacent turf areas that appear normal, so that a comparison of nematodes found in the different areas can be made. Label the bags clearly so the identity of each is known, keep the samples cool, and don't allow them to dry out. If the sample must be stored temporarily, it should be in a refrigerator at above freezing temperatures. NEVER add water to the soil sample.

Nematode identification and analysis services are available in Oklahoma through the Plant Disease Diagnostic Laboratory in the Department of Plant Pathology at Oklahoma State University. There is a nominal charge for this service. Samples can be submitted for analysis through your local county Extension office.

Control

Selection of the most tolerant types of turfgrass and good management practices will help overcome the effects of many nematodes. These are the only practical means of control in low maintenance areas. Good sanitary practices that discourage the physical movement of nematode-infested soil and plants will also help.

For high maintenance areas, such as golf greens and sod farms, chemical control of nematodes may be necessary. Because of their distribution in the soil profile and their position on or in roots, complete chemical eradication of nematodes in established turfgrass is impossible, but reductions of large nematode populations to manageable levels can be achieved with chemical nematicides.

There are two types of nematicides: soil fumigants and contacts. Fumigants are generally most effective because of the rapid distribution of the gas in soil; however, they are highly toxic to turfgrass and are used primarily prior to planting. Contact nematicides are nonvolatile and must be washed into the root zone of established turf. Control is dependent on the nematicide coming into contact with the nematodes.

The preferred timing of chemical application is in spring or autumn when the soil temperature is above 60°F. Core cultivation prior to nematicide application may enhance nematode control. Most nematicides also control insects, and some additional benefit can be realized by this indirect control of soil-inhabiting insects as a result of nematode control efforts.

Once a nematicide has been used, annual treatments may be necessary as certain nematodes may increase rapidly after treatment because of decreased competition and a rapid rate of reproduction. Nematicides should, therefore, be applied only when high quality turfgrass cannot be maintained with good management practices and a nematode problem has been confirmed through soil sampling and analysis. Nematicides are highly toxic materials and should be used only by personnel who are certified to handle restricted-use pesticides. For specific chemical control recommendations for nematodes, contact your local county Extension office.