When Dealing With Turf Problems in the Southeast

Turfgrasses are subject to many and varied stresses that makes the successful culture challenging. In troubleshooting turfgrass problems on golf courses, sod farms, athletic fields and home lawns, there is usually a litany of suspected causes of the turf problem. Some of these factors include soil pH problems, nutrient imbalances, improper cultural practices, and diseases. And, when confronted with diagnosis of a plant problem, most diagnosticians have been trained to say, when prompted, “Take a soil sample” as in, “take an aspirin and call me in the morning” from an M.D. In many cases, the soil sample will reveal potential causes of poor turf health. But, a major cause of turf problems in the Southeastern United States is frequently overlooked as a cause or contributor to turf stress until all other possibilities have been exhausted. This potential cause is from plant parasitic nematodes.

The Southeastern United States is nematode heaven. The soils are frequently highly weathered, low in fertility, and sandy in many areas. The climate is warm and humid, and even tropical or semitropical along coastal environments. Even in the Piedmont or regions of higher elevations, where native soils may be heavy, containing a high percentage of clay, nematodes can do quite well in sand-based putting greens or athletic fields. A sand-based putting green is like a large Petri dish for nematodes. A nematologist (a person who gets their kicks out of nematodes) could not design a more perfect system for their culture. To make things even better, from the nematologist’s viewpoint, we frequently plant these greens or athletic fields with vegetative plant materials, such as sprigs or sod of bermudagrass, that may have the nematodes already present. And, there is no charge for the nematodes!

Nematodes are interesting organisms. They are animals, in their own Phylum, the Phylum Nemata. They are among the most abundant animals in the world and have adapted to exploit most habitats. In fact, Nathan Cobb, the Father of Nematology, was quoted thusly in 1915:

"If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits we could investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes."

Few of us are as eloquent as Cobb was in 1915, but his quotation illustrates the fact of the abundance of nematodes in a variety of habitats. In fact, the truth be known, we could probably not live without them, as many are beneficial organisms. One area of research is the study of entomophagous, or insect-feeding, nematodes as potential biological control agents of insects. Also, the free-living nematode, Coenorhabditis elegans, is one of the “white rats” of genetic research and the genetic control of organism differentiation.

As one can see from the table above, plant parasites constitute only about 7% of the total known genera of nematodes. However, if you are the one with greens that are infested with one or more damaging species, they are important enough!

Nematodes are simple-appearing, worm-like organisms. They have a head region, with sensory organs located there to assist in recognition of potential food sources, they have digestive systems, nervous systems, and reproductive systems. The plant parasites are obligate parasites, meaning that they must feed on living plant materials in order to survive and reproduce. Some nematodes, however, can survive long periods of time in a quiescent, or dormant state (some lance nematodes have been known to survive years in dry soil in the absence of active feeding). Plant parasites feed on plants with the use of an organ called a stylet, which is a hollow, needle like spear in the head. They use this tool to tear plant cells or penetrate to deeper tissues such as root meristems. Most of the plant parasites feed on roots, but some feed on above-ground plant parts. In turf, the root parasites are by far the most important.

This figure shows the head region of a ring nematode (Criconemella sp.), feeding on a root epidemis cells. The stylet is penetrating the uppermost cells of the root epidermis.

Feeding Habits. As cell contents are destroyed, they are pumped into the body of the nematode by way of a muscular “median bulb”, and the nutrition is used by the nematode for growth and reproduction. Plant parasitic nematodes are frequently categorized roughly into 2 groups based on their feeding habits: ectoparasites and endoparasites. The ectoparasites feed on roots, with the majority of their bodies remaining outside the root, as in the figure of the ring nematode feeding. Some ectoparasites are “grazers” and feed along the root surface. Others have preference for certain feeding sites, like root tips. Sting nematodes are examples that prefer to feed on root meristems. Endoparasitic nematodes actually penetrate roots with their bodies. Some migrate within the root as they feed, e.g. lesion nematodes; others are more or less sedentary and set up specialized feeding sites, such as the root knot nematodes.

Reproduction. Most plant parasitic nematodes are bisexual, meaning there are male and female nematodes. Eggs are produced by cross fertilization, but in some instances fertile eggs are produced by parthenogenesis (without males). Eggs hatch in response to environmental signals, and a J2 or second-stage larva, emerges from the egg and ready to feed. The larval nematodes feed and grow in size, through several molting stages, until they become sexually mature adults. In many cases, the time from egg to adult can be achieved in as little as 30 days.

Plant parasitic nematodes vary tremendously in their capacity to reproduce. Some, such as the root knot nematodes (Meloidogyne spp.), may reproduce quite rapidly, with each female nematode capable of producing 400 to 500 eggs, each hatching to produce a larva. Some nematodes, such as the sting nematodes (Belonolaimus sp.) have a lower capacity to reproduce, with each female nematode producing only a few eggs at a time.

Distribution Vertically and Horizontally in Soil. Nematodes are largely soil borne organisms. The plant parasites that cause problems in turf and other crops are distributed unevenly in the soil. Their vertical distribution is not uniform, with higher numbers of nematodes being present usually in the upper soil profiles in comparison to the lower soil profiles. This makes sense when one considers that the nematodes are likely to be more concentrated where root systems are most concentrated. But, nematodes are frequently also not uniform in a horizontal fashion… nematodes may be concentrated in patches in greens, lawns or athletic fields. The symptoms of nematode damage may, more or less, correspond to the unequal concentrations of nematodes in patchy distributions.

Diagnostic and Routine Soil Samples for Nematodes

The clustered or patchy distribution makes it more complicated in sampling a soil for suspected nematode damage. If one does not know if the patchy, unhealthy areas of turf might be caused by nematodes, then it is suggested that samples be taken at the margins of unthrifty areas and compared to similar samples taken from more or less healthy turf. The results of the nematode assay can then be compared and if one or more genera of damaging species are more abundant along the margins of the damaged turf, then nematode involvement may be evident. However, consider if you already know that you’ve had nematode problems in a particular site, but you want to determine if a control strategy was successful in lowering populations. In that case, routine samples are taken, and an average population is monitored by taking samples across the areas in a zig-zag pattern. The zig-zag pattern increases the chances that both areas with high and lower populations are sampled. It takes about 25 soil cores taken with a standard 1-inch soil probe, at a 4 inch depth, to accumulate enough soil for an adequate assay. The soil cores can be collected in a bucket, gently mixed, and placed in a plastic bag to keep the soil from drying excessively. In either case, it is suggested that the soil sample be treated as you would a carton of milk: put it in an ice chest, label it with the pertinent information, and get it to a reliable lab quickly in good shape. Do not ship over a weekend or when a holiday might be scheduled. If the lab does not get the sample in a timely fashion, the nematodes may die due to heat, drying, etc. and the results obtained will not be reliable.

Damage Thresholds. Many states publish damage thresholds whereby plant managers can determine the relative capabilities of different nematodes to cause damage to specific crops. These thresholds can reflect the relative susceptibility/tolerance of specific crops and they can reflect the relative virulence/ aggressiveness of the nematode species. Clemson publishes damage thresholds for most of the common plant parasites. For instance, on all grasses the threshold for noticeable damage caused by sting nematodes is no more than 20 nematodes/100 cc of soil. But, on bermudagrass turf, spiral nematodes usually must exceed 1000 nematodes per 100 cc of soil to damage it. Other nematodes have thresholds set for the different turfgrass species. Persons utilizing thresholds should be aware that some states publish thresholds based on 100 cc of soil, and some based on 500 cc of soil. Concerning different labs also, there are varied techniques whereby nematodes are extracted from soil or plants and enumerated. A user should find a lab that is capable of good, reproducible efficient extraction of nematodes, and stick to that lab. In all cases, however, plant managers should take the results of nematode assay as a guideline to assist in diagnosis or monitoring of populations of nematodes. A sample that contains multiple species of nematodes, with several at or above threshold indicates that nematodes are likely a significant source of biological stress. Sometimes, even a single species, particularly the sting nematode, if present at or above threshold levels indicates a problem. However, assessment of other potential stresses that may contribute to the problem should be identified. These may include compacted soils, lack of suitable nutrient levels or their availability, inadequate irrigation, etc.

Procedures to use Clemson University’s Nematode Assay Lab. The standard assay for nematodes includes the identification to genus and enumeration of population levels of the following nematodes: Awl - Dolichodorus sp., Cyst - Heterodera sp., Dagger - Xiphinema sp., Foliar - Aphelenchoides sp., Lance - Hoplolaimus sp., Columbia Lance - Hoplolaimus columbus, Lesion - Pratylenchus sp., Needle - Longidorus sp., Pin - Paratylenchus sp., Ring - Criconemella sp., Root Knot - Meloidogyne sp., Sheath - Hemicycliophora sp., Spiral - Helicotylenchus sp., Sting - Belonolaimus sp., Stubby Root - Trichodorus sp., and Stunt - Tylenchorhynchus sp.

submission form must accompany all samples (detailed sampling instructions are on

the back of the form). For turfgrasses, sample 3 to 5 inches deep with a soil probe

populations are highest. After mixing the soil, place one quart in a plastic bag. Do

not leave samples in the trunk of a car or other places where temperatures in the

bags can exceed 80F. Since nematodes are not uniformly distributed in a field, a single

Cherry Road, Clemson University, Clemson, SC 29634-0113. Specialists can aide in interpretation of sample results, if needed.

Management of nematode problems. Nematodes are extremely difficult pests to manage in turf as well as other plant commodities. One reason is that they are in the soil environment, and getting an active agent efficiently to the pest for suppression is difficult, whether that active agent is biological or chemical. An elegant form of control is through host plant resistance or tolerance; unfortunately, very little research has been conducted on the host suitability of new turfgrasses to important nematodes. This is an area of research worthy of attention. Similarly, biological control is an attactive control strategy due to the perception that biological products are potentially more environmentally acceptable than chemicals. Some biological products have included chitin/urea mixtures, sesame-based products, and recently a fungal metabolite that is nematicidal. The latter product is labeled as Ditera, from Abbott Labs, and is a fermentation product of the fungus Myrothecium. With all of these products, consistent reliable results have been difficult to attain for varying reasons. Users should be aware that although a product may be labeled as a nematicide, EPA does not require data of relative effectiveness of the products. If users wish to evaluate these products or any product they should leave untreated areas, take nematode samples before and after the materials are used, and be aware that short term nitrogen effects may give a temporary greenup unrelated to nematode suppression. However, we should continue to test and evaluate these and newer materials as they are developed.

Chemical nematicides have been used extensively over the years for successful nematode management. In fact, due to the early success of certain chemical nematicides, research in biological and resistance/tolerance in turf was not a high priority. In 1984, EDB (ethylene dibromide) was cancelled for use. Prior to its cancellation, this fumigant was used in turf with excellent results and was cheap. Afterwards, DBCP (Nemagon or Fumazone) was utilized, as well as DD (dicloropropane + dicloropropene). These are all no longer available now. In golf course turf, we still have 2 contact nematicides Nemacur and Mocap. These cannot be used legally on sod farms or sites other than golf courses.

Recently, Dr. Horace Skipper, Clemson University, documented cases of enhanced biodegradation of Nemacur on several SC golf courses. These are cases whereby soil microorganisms utilize the chemical as a carbon source, leading to its more or less rapid degradation. Degradation occurs rapidly and control is lessened extensively. If a golf course has problems with enhanced biodegradation, they must rotate to another chemical and/or do not use the pesticide at all for 2 or more years.

Sod production farms cannot use Nemacur or Mocap. However, they can use Telone II (1,3 dichloropropane). Telone is a liquid fumigant nematicide that is highly effective against sting nematodes. The material is injected behind a chisel plow or it can be knifed into bermudagrass sod behind vertical coulter injection rigs. A chisel plow application would likely be conducted preplant, and places the fumigant 10 or more inches deep. The liquid volatilizes to a gas, which dissolves in films of water to kill nematodes. Sod production farms may also still use methyl bromide, Vapam (metam sodium), or Basamid. These latter chemicals are more or less nonselective toxicants that have activity against weeds, some insects, and nematodes. They must be used preplant only.

Lawn care companies have few alternatives other than the biological products. Basamid is a restricted use chemical, although it contains a Warning signal word. Again, it must be used preplant only, so its use on existing sodded sites is not an option unless the turf is being completely replaced.

There have been no new reliably effective nematicides registered for turf in the past 20 years and there are few indications from pharmaceutical companies that any are in the “pipeline”. So, our tools for management of nematodes are extremely limited.

However, current efforts at the Pee Dee Research and Education Center are underway to screen standard and new grasses and breeding lines for nematode resistance/tolerance. Similarly, soil amendments of varying types have been found to be somewhat suppressive to nematodes in the past, but little research has been conducted for turf systems. The tolerance of nematode damage is very low in the marketplace. The needs for new research and approaches have never been greater, particularly given the increased demands on high quality turf with few blemishes.

First printed in the South Carolina Turfgrass Foundation Newsletter, July-September 2000