Nematodes: turf's hidden enemy

The decision to use a nematicide on established turf is based on the determination that the population levels of plant nematodes are high enough to reduce turf quality.

Plant-parasitic nematodes can cause yellow and thin turf by destroying the roots' ability to absorb water and nutrients. However, many other biological and environmental factors can cause the same symptoms. That's why diagnosing a turf problem, including nematodes, just by the appearance of the grass can be difficult and misleading.

The trial-and-error approach to detecting nematode problems typically proceeds as follows. Patches of turf become yellow and thin, so you increase irrigation. The turf is still yellowing and thinning, and the patches of damaged turf are growing — you try applying a fertilizer. The problem is persisting and worsening, so you try a fungicide. Still no improvement — how about an insecticide? No luck, so you finally take a soil sample and have it checked for nematodes. Aha!

This try-everything approach is a too-common occurrence for uncovering nematode problems in turf. In fact, some publications on nematode management state that, “a general yellowing and thinning of turf that is not remedied by other means,” is a primary symptom of nematode problems. While we don't expect you to become an expert nematologist by reading this article, we do hope that by raising your awareness of nematode problems in turf, you will be armed with the tools to optimize turf health when nematodes are present. And perhaps you can avoid the try-everything approach next time!

Numerically, nematodes are the most abundant animals on earth; four out of five animals are nematodes. They occur in soil, decaying organic matter, in fresh and salt water, and are parasitic on all forms of plant life, and most animals, including domesticated species and humans. Although many nematodes feed on plants or animals, others are beneficial as they contribute to nutrient cycling or serve as biological control agents of insect pests. Though nematodes are nearly ubiquitous, it is only a relatively small number of species that cause headaches for turf managers.

Plant-parasitic nematodes: the bad guys

Plant-parasitic nematodes are tiny roundworms, circular in cross-section (see illustration, above right) and so small that you need a microscope to observe them. All plant-parasitic nematodes are armed with a stylet (hollow needle) in the anterior (“head”) portion of their body, which they use to puncture plant root cells and suck fluids.

Plant-parasitic nematodes are obligate parasites, which means that they must have plant hosts to grow and reproduce. They impact agricultural production in a dramatic fashion. They are often referred to as the “hidden enemy,” because they occur belowground, feeding on root systems and causing non-specific symptoms such as reduction in vigor. However, their feeding suppresses plant growth and reduces crop yields.

Nematode feeding on plant roots diminishes their capacity to absorb water and nutrients. As a result, plants infested with nematodes become yellow, less productive and may eventually die. Nematode damage also provides portals of entry for other plant pathogens and decreases a plant's defenses against other organisms such as fungi and bacteria. In addition, some nematode species act as vectors of plant viruses. On a worldwide scale, annual losses to parasitic nematode of all kinds are estimated to be $78 billion for the major food and fiber crops; about 12 percent of that total, or $8 billion per year, occurs in the United States. When you consider the magnitude of these numbers, you might think turf got off easy. However, this is little consolation to turf managers who do battle with these pests — they do take their toll on grasses.

Life cycle and reproduction

Most plant-parasitic-nematode life cycles are similar and rather simple. There are six life stages: the egg, four juvenile stages, and the mature adult. The developing embryo becomes the first-stage juvenile inside the egg. For most species, this juvenile molts before hatching from the egg and becomes the second-stage juvenile. Upon hatching the second-stage juvenile begins feeding and undergoes three additional molts to become an adult.

In some species, both a male and female may be required for reproduction. However, females of many species do not need to mate to produce eggs. The time it takes for an egg to develop into an adult varies from a few days to over a year, but averages about 3 to 6 weeks for most important plant-parasitic nematodes. Given their high reproductive rate and relatively short life cycles, population densities of plant-parasitic nematodes can increase quickly during warm weather.

Where nematodes feed and how they affect roots

Plant-parasitic nematodes feed in different ways. Juveniles, adult males and adult females of the same species may differ in their feeding habits. They possess a hollow stylet that functions like a hypodermic needle. Most plant-parasitic nematodes inject substances through the stylet to modify the root-cell contents. All or a portion of the contents of the cell are then withdrawn by the nematode through the stylet and digested.

Nematodes can be classified in three main groups according to their feeding behavior on roots.

  • Ectoparasites feed externally and never enter the roots.

  • Endoparasites enter the root and feed from within the root tissue. Some of them even migrate inside root tissues and are called migratory endoparasites. Sedentary endoparasites, by contrast, remain at their original feeding site.

  • Semi-endoparasites feed with only the anterior end of the body embedded in root tissue.

Aboveground symptoms of nematode-infected plants, including stunting, unthrifty growth, yellowing, early decline and wilting, are direct results of the mechanical and physiological damage inflicted on roots by nematode feeding. In direct examination of roots, the following symptoms are typical: necrosis (dead tissue) due to cell destruction, suppression of root-tip growth resulting in excessive root branching and smaller root systems, and internal necrosis resulting in root rotting. Root-knot nematodes induce the formation of galls or knots.

Nematode damage to turf is common where sandy soils and long growing seasons favor population increases and create conditions, especially drought, in which grasses are especially sensitive to their effects. In effect, this means that nematodes are more commonly a problem in Southern turf.

Collecting and submitting nematode samples

Despite the diversity of parasitic habits among nematodes, most species spend at least part of their life cycle in the soil. Therefore, soil sampling is the most reliable way to identify nematode species affecting turfgrass. To diagnose nematode damage correctly, it is necessary to extract, observe, identify and count the nematodes associated with injured plants. We recommend that you consult a plant nematologist or pathologist at the nearest land-grant university or cooperative-extension office for sampling guidelines that work best for the soils in your region. However, there are some general rules to follow.

  • Begin the sampling process in early spring at the time the soil temperature at the 2-inch depth reaches 50°F and continue to sample at 3 to 4 week intervals throughout the growing season. Continuous sampling is important because nematode populations fluctuate widely throughout the growing season.

  • Soil samples for a nematode assay can be collected with shovels, trowels, soil augers or tubes. A typical cylindrical tube 1-inch in diameter is the most popular tool.

  • The depth of sampling should be where most of the root system is located. For example, in turf the depth of sampling should be 2 to 6 inches. For deep-rooted perennials, soil cores as deep as 24 inches may be appropriate.

  • In turf, collect 15 to 20 cores where the grass shows apparent symptoms of nematode damage and collect the same number of cores from an adjacent healthy-looking area for comparison of nematode species and numbers. You can combine and mix the soil cores from a homogenous area, but do not mix them from sites with differing symptoms. Collect at least 0.5 pint of soil for a composite sample in each location. At the time of collection, place the sample in an airtight container such as a plastic bag to prevent desiccation.

Soil samples for nematode analysis are handled differently from samples taken for fertilizer recommendations. Nematodes are living animals and require moisture and moderate temperature to survive. To be recovered and identified, they must reach a nematology laboratory alive. Take all precautions to prevent rough handling and exposure to temperatures below 40°F or above 80°F. Ship the soil samples in a strong insulated container to the extraction laboratory within 24 hours of collection.

You've got nematodes — now what?

Management practices to limit losses to plant-parasitic nematodes involve one or more tactics intended to reduce the rate of nematode population increase, or reduce initial population levels. The former would involve the use of resistant grasses. Other management practices include the prevention of spread from nematode infected turf on tools and other equipment, use of planting materials free of nematodes and the use of nematicides. Recent research has shown that entomopathogenic nematodes used to control insect pests in turf also may reduce plant-parasitic nematode numbers.

The decision to use a nematicide on established turf is based on the determination that the population levels of plant nematodes are high enough to reduce turf quality. With the loss of methyl bromide, control options are even more limited than before. Pesticide use restrictions vary among states and users should consult their state Cooperative-Extension Unit for current information use in their state.

The nematicide fenamiphos (Bayer's Nemacur) can control plant-parasitic nematodes in established turf, but it is only approved for golf course turf and (in some states) cemeteries and industrial turf. Check the label for specific instructions. Nemacur is applied to the turf in granular formulation and irrigation water or rain dissolves the active ingredient and incorporates it into the soil.

Dow Agrosciences manufactures Curfew nematicide (1,3-dichloropropene — the same active ingredient as in Telone II). Its use is limited to golf course fairways. Curfew currently is only available for use on golf courses in Florida under an experimental use permit.

Curfew application utilizes coulters and knife-shanks to open the turf, allowing injection at least 5 inches below the soil surface through a delivery tube located behind the knife-shank. The cut turf seam is then closed and sealed by a press-wheel and the material is watered in after application.

Dazomet (BASF's Basamid) also is labeled for nematode control. Dazomet is a fumigant with broad-spectrum activity. Thus, it is intended for use in complete renovations, not on existing turf. Dazomet is a granular product that, when incorporated into the soil via irrigation, releases gasses that are toxic to nematodes (plus most other pests).

If you undertake a nematicide program, sample periodically to determine if and when you should treat the affected areas again. Populations may eventually rebound.

Though nematodes are among the most important of the many pests and diseases that can damage turfgrasses, they are also among the least understood. Indeed, they are often ignored due to their small size, cryptic habits and hard-to-define damage. Managing nematode populations in turfgrass depends first upon recognizing their existence. As with most pest management, information is the most powerful tool in your arsenal against nematode pests.

Drs. Enrique Perez and Edwin Lewis are nematologists at Virginia Polytechnic Institute and State University (Blacksburg, Va.).

Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by Virginia Tech and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

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