Introduction
Nematodes are small invertebrate animals that are especially common in soils and aquatic environments. They generally range in size from 0.5 mm to 10 mm, but most are around 1 mm length. They are about half the diameter of a human hair and mostly transparent, so they are difficult to see without magnification. Nematodes are aquatic animals and need moisture for mobility.
Most nematode species are characterized as “free-living,” meaning that they do not parasitize plants. Free-living nematodes forage for microorganisms in the soil and water, and some attack other nematodes. Approximately 10% of the described nematode species are parasites of animals including birds, fish, mammals (including humans), insects, and worms. About 15% are plant parasites. It is likely that all plants with root systems are parasitized by plant parasitic nematodes; however, significant damage to plants generally only occurs in agricultural environments and only when nematode populations are relatively high.
Plant parasitic nematodes have a protractible hollow spear (stylet) in their heads which they use to puncture plant cells and withdraw the contents. Plant parasitic nematodes can be grouped by their mode of feeding. Those that remain outside of the plant and puncture the root cells with their stylets are called ectoparasites. Some ectoparasites have short stylets and only puncture the epidermal (surface) cells, while those with long stylets penetrate more deeply into the root. Endoparasites penetrate and live within plant tissue. Some endoparasites remain in one place and feed while others migrate through the tissues. Endoparasites do more damage to plant tissues than ectoparasites. The lance nematode (the most damaging species in the northeastern U.S.) lives both ectoparasitically and endoparasitically.
Nematodes That Parasitize Turfgrasses
Golf greens provide an ideal environment for nematodes, providing them with a uniform host and a long growing season. The soil is 75 to 95% sand, which provides ideal pore spaces for oxygen, water, and nematode mobility, and the greens are watered frequently. Golf greens can have some of the highest populations of nematodes found in agricultural soils. The most common plant parasitic nematodes in turfgrasses in the northeastern U.S. are ring, stunt, lance and spiral.
Symptoms and Diagnosis
Nematodes injure turfgrass roots by either feeding on or by burrowing through tissues. Depending on the host and the nematode involved, symptoms on roots may include inhibition of root elongation, swollen tips, galls, lesions, and shortened, stubby roots. In many cases, no symptoms are observed on roots. Symptoms occurring above-ground result from root dysfunction and may include patchy areas of wilting, thinning, and decline where nematode populations are high. Because the above-ground symptoms they cause are not unique to nematodes, they are of little value in diagnosing nematode problems in turf. An accurate assessment of the pathogenic potential of nematodes can only be determined by a nematode assay.
Collection of Soil Samples for Nematode Assay
Nematode populations are unequally distributed in soil, so they are most accurately estimated with a composite sample. Use a ¾” or 1” diameter soil probe, or something similar, and sample to a depth of four inches throughout the site. This depth is a compromise but represents the population distribution of different species relatively well. After removing the soil core, tear the top ½” of turf and thatch off the core. Fill the hole with dry top dressing leaving a ½” space an replace the turf plug.
If a portion of the turf appears unhealthy, collect 15 to 25 subsamples from throughout the affected area and bulk them as one sample. For comparison, a composite sample should also be taken from an adjacent area where the turf appears healthy. A cup-cutter sample taken within the declining turf is acceptable, but a composite sample is preferred.
Threshold Levels for Turfgrass Nematodes
The results of a nematode assay should be evaluated in the context of previous assay results as well as current and past conditions on the green. Consider the species and numbers of nematodes present, the depth of the soil the sample taken, species composition of grasses, depth of root system, soil texture, symptoms, extent of damage, and presence of other stresses to the turf. Threshold levels (the nematode population levels which justify the implementation of control measures) are dependent on these variables. Currently, there is very little experimental data to establish threshold levels for golf greens in the northeast. The numbers listed below are based on information gleaned from experience as well as from case-history information and survey results compiled in the northeast. They are to serve as a guide only.
Common Name | Scientific Name | Feeding Mode | N.E. Region Thresholds/100cc* | New Jersey Thresholds** |
|---|---|---|---|---|
| Ring | Criconemella | Ectoparasite | 1500 | 1500 |
| Stunt | Tylenchorhynchus | Ectoparasite | 800 | 300 |
| Lance | Hoplolaimus | Ecto/endo | 400 | 150 |
| Spiral | Helicotylenchus | Ectoparasite | 1500 | 600 |
| Root-knot | Meloidogyne | Endoparasite | 500 | 100 |
| Longidorus | Ectoparasite | 100 | -- | |
| Cyst | Heterodera | Endoparasite | 500 | -- |
| Lesion | Pratylenchus | Endoparasite | 100 | 150 |
| Stubby root | Trichodorid | Ectoparasite | 100 | 100 |
| Dagger | Xiphinema | Ectoparasite | 200 | 200 |
*Developed by Robert Wick, PhD, University of Massachusetts. These threshold levels are based on field observations and laboratory assays from the New England region. Extenuating circumstances such as geographic location, method of sample collection, assay methods, host, and prevailing environmental conditions, can affect the interpretation of threshold levels.
**From: An Integrated Approach to Pest Management in Turfgrass: Nematodes, Fact Sheet FS1014: 2010. Rutgers Cooperative Extension.