Part I: Cool-season turf, Diagnosing and managing nematodes
Plant-parasitic nematodes-microscopic worms that feed on plant roots-occur widely in soils and attack a variety of plants including turfgrasses. The geographic region, the host-plant species and the soil environment all play a role in the potential harm that may result from infection. Here, I'll address nematode problems and their management on cool-season turfgrasses. Researchers in the New England region developed much of the information presented here. However, it applies generally to areas with similar climate and turfgrass species.
Nematodes parasitize nearly all turfgrasses, but significant injury does not occur unless the population is high. Nematodes thrive in sandy soils, and this is where you usually see damaging populations. In the northern United States, many soils are clayey and do not support high populations of nematodes. Also, low temperatures during the winter months reduce nematode populations. Nevertheless, putting greens in the North often have populations of nematodes high enough to cause a decline in turf health. In addition to putting greens, high populations may develop in other sites where soils have sandy texture.
During the past 20 years, many putting greens that were originally established on native soils have received annual applications of sand-based topdressing in an effort to manage surface uniformity and thatch, and improve infiltration. This practice has created an upper soil layer that is 70 to 90 percent sand and, in some cases, 4 inches deep. Sandy soil texture provides channels for air and water, so most putting greens, which usually receive irrigation daily, drain well. While these conditions are important for the maintenance of healthy turf, they also provide an ideal environment for the development of high populations of nematodes. If you want to start a farm for the production of nematodes, construct a putting green!
The nematodes The nematodes that occur most commonly on turfgrasses in New England are the ring (Criconemella), stunt (Tylenchorhynchus), spiral (Helicotylenchus), lance (Hoplo-laimus), needle (Longidorus), cyst (Heterodera) and root-knot (Meloidogyne) nematodes. Some of these genera do not appear to be damaging, even at relatively high population levels, while others cause significant injury. Plant-parasitic nematodes cause injury by inserting a stylet, a hollow spear-like mouth part, into the plant. Ring, spiral and stunt nematodes remain on the outside of the root as they feed. The lance nematode may feed from the outside but also may enter into the root (see photo, opposite page) resulting in considerable damage. Root-knot and cyst nematodes enter the root as juveniles and feed continuously at one site. Root-knot nematodes cause swelling of the infected root. The cyst nematode becomes swollen and fills with eggs-the majority of the body develops outside the root (see photo, at right).
Except for root-knot nematodes, symptoms that occur on roots are not obvious. Above-ground symptoms include wilting and decline, but these symptoms are not unique to nematodes. Therefore, to determine if nematodes are contributing to a decline in turf health, you must carry out a soil or root assay. To collect a representative sample and understand the results of the nematode assay, you must first understand some basics about population dynamics.
Population dynamics The suitability of the host plant (available food), soil texture and temperature are the primary factors that govern the rise and fall of nematode populations over the course of a year. Soil texture governs the amount of air and water present, and temperature influences the reproductive rate of the nematode.
Nematodes are obligate parasites, meaning that they are absolutely dependent on a living host to complete their life cycle. Therefore healthy turf is necessary for a significant nematode population to develop. If the turfgrass has root rot, it will not be a good host for the nematode. In the absence of a host, the nematode becomes dormant and survives on its stored food reserves.
In the northern United States, nematode populations decline during the winter but vary at other times of the year. Graph 1 (page G 24) shows population fluctuations of three species during a season. A common scenario is that the population is low in the spring, increases to a maximum during mid-summer and then declines. The duration of development from egg to egg-bearing adult varies. If we consider a typical life-cycle length of about 30 days, a peak in the population would reflect good growing conditions for the month before. Likewise, when the population is low, it reflects earlier conditions that were unfavorable for nematode reproduction. Even if you've applied a nematicide, the nematode population does not drop immediately. An assay several weeks after the application of a nematicide will not show a reduction because the nematodes are still in the soil, even though they are unable to feed.
Nematode population-dynamics studies have revealed optimum nematicide application timing for achieving best results. For example, in one 4-year study conducted on a golf course in Bedford, N.Y., I found that juvenile cyst nematodes hatch about mid-April (see Graph 2, page G 24). The cyst nematode is only susceptible to nematicides after the juveniles hatch from the egg and are moving to the root. Once inside the root, they are protected. In the egg stage, they also are invulnerable within the mature cyst. However, a second opportunity to control this nematode occurs about August 1 when another hatch of juveniles begins.
Nematode populations generally include several different species-three to six genera usually are present in a golf green. Each genus has a different pathogenic potential and may respond differently to environmental conditions. Graph 1 shows stunt, ring and spiral nematodes on the same green. Note that the population fluctuations were similar for each nematode but not exactly the same.
Horizontal and vertical distribution of nematodes in soil When you are sampling, it is important to know that nematodes are not evenly distributed horizontally. Therefore, a single soil sample can be misleading. Graph 3, opposite page, demonstrates how nematode counts vary widely, even between samples taken just 1 foot apart. Due to a relative lack of mobility, ring nematodes have an even more clumped distribution than others. In most cases, a composite sample of 15 to 20 cores provides a fairly reliable estimate of the nematode population.
Nematode distribution also varies considerably by depth. For this reason, it is important that you take soil samples at a consistent depth. For putting greens, I recommend that you take cores to a 4-inch depth (a more detailed description of sampling techniques is given in Part II of this series, page G 28).
Diagnosis of nematode problems High populations of plant-parasitic nematodes impair root systems, which in turn affects the health and vigor of turfgrass. Symptoms include wilting, decline and slow response to water and fertilizer. However, other pathogenic agents and cultural and environmental problems can cause these symptoms as well. To make a diagnosis, it is important to rule out fungal and bacterial pathogens and soil-related problems. In addition, review your cultural practices as a possible cause of symptoms.
Finally, it is essential to have a qualified laboratory assay a soil sample for nematodes. You should take a composite sample from an apparently healthy area to compare with the problem area. Although you can take a single plug from the affected area with a cup cutter (if nematodes are causing the problem, the sample should have a high population), it is best to take multiple samples throughout the affected area with a soil probe. For the assay, select a nematology lab that has experience with nematode problems in turf. It is a good idea to call the lab before sending the sample. If you split samples to send to different labs, do not be surprised if the results are somewhat different. The extraction process is not 100-percent efficient.
Predicting nematode problems Populations of soil-inhabiting nematodes may rise and fall and may be a problem in some years and not in others. However, the species composition of nematodes present at a particular site remains stable. For example, if you have ring, stunt and spiral nematodes on green No. 6, they will always be there, and it is not likely that new nematodes will show up and become established. An exception is the cyst nematode, which can spread via goose droppings. Newly constructed sand-based greens should not develop significant nematode populations for several years. Eventually, however, equipment such as aerifiers will probably introduce nematodes through contaminated soil.
You can learn a great deal by taking a composite sample throughout a putting green four or five times during the growing season. You need to perform a comprehensive analysis such as this only once. The analysis will tell you which nematodes are present. It may be that several species occur on a particular putting green, but they may not be important species. Thus, when problems occur in the future at this site, you could rule out nematodes as a likely cause. Conversely, you should assay a green that has relatively pathogenic nematodes whenever a decline occurs. By taking samples throughout the growing season on all of the greens during the same year, you can compare the relative carrying capacity of the greens.
Because nematode populations are primarily dependent on soil texture, moisture, temperature and host, individual greens have unique abilities to propagate nematodes. For example, soil temperature is a function of shade, slope and position toward the sun. Height of cut also affects soil temperature (but this will be common to all greens on a course). The lower the height of cut, the higher the soil temperature will be. The data from a comprehensive analysis will allow you to evaluate which greens are candidates for nematode problems.
Managing nematodes It is not possible or necessary to eliminate nematodes from established turf. Low populations are not a problem. Unfortunately, nematode populations often peak in mid-July to August, just when traffic wear and heat stress are bearing down. For this reason, you should apply a nematicide by mid-June. This will prevent the population from peaking during the high-stress period.
It is a fact that healthy turfgrass promotes larger nematode populations than unhealthy grass. Nevertheless, healthy grass can better withstand nematode infestations, as well as other stresses. If possible, raise the height of cut during periods of high nematode activity. Other cultural practices that minimize stress and increase turfgrass vigor are also helpful.
Manufacturers are developing a number of novel products as alternatives to nematicides. Some have shown some promise when tested under controlled conditions, but they generally have not provided consistent and significant suppression of nematodes. However, it is also true that traditional nematicides sometimes can be inconsistently effective, and, in some cases, soil microorganisms quickly render them ineffective. In spite of this, conventional nematicides still are the most effective available treatment for suppression of nematodes in turf (see table on page G 29 for a listing of products registered for nematode control in turf).
By applying this information to your nematode-control program, you can improve your nematicide's effectiveness. Biologicals are also entering the picture. Just as biologicals are receiving attention for their potential to control weeds and disease, such is the case for nematodes too.
To prevent damage to a putting green when taking soil samples, here is a method you can use when sampling for nematodes. Take 4-inch-deep soil cores, tearing the top 0.5 inch of turf/thatch from the cores, placing them next to the holes. After collecting and bulking the desired number of cores together, fill the holes nearly to the top with a topdressing mix and push in the grass plug. A watering can with a narrow spout is perfect for filling the holes with topdressing. It is best to sieve the topdressing before putting it into the watering can to prevent clumps and debris from plugging the spout. The topdressing also needs to be dry. If you take samples in the heat of the day, it may be necessary to water them to help them take hold. After you mow the green, the patches are difficult to detect. An added bonus to this method is that nematology labs prefer soil samples without the turf-they are easier to sieve.
Dr. Robert L. Wick is professor of plant pathology at the University of Massachusetts-Amherst.
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