Nematodes on northern turfgrasses

Nematodes are everywhere. They’re found in all but the most extreme environmental conditions, in soils, oceans, rivers, streams, on plants, in plants and even in animals and humans. Fortunately, most nematodes are innocuous, eating bacteria and occasionally each other. Of the thousands of nematode species, few are pathogens. But despite the small proportion that is pathogenic, all plants grown in natural soils are attacked by nematodes, no matter where they are grown.

Nematodes are most severe in tropical and subtropical climates. Because winter temperatures in these areas usually are above freezing, nematodes can remain active in the soil for extended periods and do not freeze.

In areas where the ground does freeze, such as the northern United States, nematodes have some protection against sub-zero temperatures, and eggs often can remain in soil unaffected for months by temperature. In addition to winter conditions, the growing season in areas like the southern United States is longer than in the North, which translates into more nematode generations and more plant damage. One significant exception is the soybean cyst nematode, which is widely prevalent and causes substantial devastation from Minnesota to Florida.

Turfgrass parasitic nematodes are similarly more destructive in southern regions than in northern areas, but northern nematodes still cause substantial damage when population numbers are high.

Nematodes of the North

Four plant-parasitic nematodes in the northern U.S. regularly cause noticeable damage to turf: the stunt nematode (Tylenchorynchus spp.), the lance nematode (Hoplolaimus galeatus), the barley root-knot nematode (Meloidogyne naasi) and the spiral nematode (Helicotylenchus spp.). Frequently, plant-parasitic nematodes are not identified as to species. There currently are 111 identified species of Tylenchorynchus, and species identification can be extremely difficult and time consuming. Other nematodes are found parasitizing northern turfgrasses, but they tend to be less common and it’s unclear how much damage they cause. These include the ring nematode (Criconema spp., Figure 1), the needle nematode (Longidorus spp.), the cyst nematode (Heterodera iri), the lesion nematode (Pratylenchus pentrans), the pin nematode (Paratylenchus spp) and the sheath nematode (Hemicycliophora spp). And still others regularly attack roots but have not been cited as causing economic damage.

Figure 1 The ring nematode (Criconema spp.) gets its name from the pattern of distinct rings that comprise the nematodes cuticle, or outside layer similar to a pressurized skin. While relatively common, this microscopic nematode rarely reaches high levels in turfgrass stands, and it is unclear how high nematode levels need to be to observe economic damage.

This demonstrates a major point when addressing turf-parasitic nema-todes: Their mere presence doesn’t mean they are causing symptoms or that damage levels are of concern. Stunt and spiral nematodes nearly are universal on northern courses; however, levels of these nematodes must be relatively high before we observe damage symptoms. Lance and root-knot nematodes tend to be more sporadic in appearance, but lance causes severe damage at lower levels than the other nematodes. Lance nematodes spend some of their time outside of the root, but also burrow through roots throughout the year (Figure 2). This can cause catastrophic damage and makes it difficult to count their true population levels simply by soil extractions.

Figure 2 Lance nematodes are particularly destructive because they enter and swim through the root cortex. In this image, three nematodes can be observed buried in a creeping bentgrass root. In the middle of the micrograph, the head and stylet of a lance nematode is clearly visible with the tails of two other lance nematodes above and below it.

Damage determination

To determine if a nematode population is causing damage to turf, we extract them from the soil and count their numbers. If the number exceeds a set threshold, they may be a problem. Use caution when interpreting thresholds, however, because thresholds are just estimates usually derived using specific environmental conditions (type of grass, season, rooting depth, height of cut, etc.).

While 3,000 stunt nematodes per 100cc of soil likely place significant stress on Poa annua plants growing in a push-up soil with only one-half inch of root in the middle of July, the same number of nematodes is unlikely to have any effect on a sand-based creeping bentgrass (Agrostis stolonifera L.) green with a rooting depth of 5 inches in the fall. Consider nematode population numbers and nematode thresholds in the context in which they were developed. As previously mentioned, there are 111 species of stunt nematode. Do they all cause the same amount of damage at the same population levels? It’s unlikely, but despite this, we’re stuck with the generic term “stunt nematode.”

A superintendent once asked me, “Aren’t thresholds just guesses?” The answer isn’t simple. In one respect a threshold is an educated guess as to when nematodes are likely to be a problem. But again, every course is different and so is every green. Thresholds stop being educated guesses when they coincide with visual damage.

Unfortunately, symptoms of nematode damage are difficult to identify and are non-discrete. Every superintendent can identify a disease like dollar spot, brown patch or pink snow mold. But nematode damage can dramatically vary in appearance. Turf-parasitic nematodes attack plant roots (with few exceptions), so it stands to reason that symptoms will appear as root damage. But root rots generally are not associated with nematode injury. Shallow, weak rooting is a common nematode symptom. Damage from lance nematodes frequently results in wilted patches, while stunt nematode damage can mimic nutrient deficiencies (Figure 3). Root-knot nematodes also produce patch symptoms.

Figure 3 Severe damage to a mixed annual bluegrass/creeping bentgrass green caused by high levels of lance nematodes. Damage had been observed repeatedly for a number of years in the same location, typically during mid-summer, but had gone undiagnosed. The superintendent increased levels of aeration and seeding to damaged areas, but nothing less than chemical application would solve the issue.

Nematode damage frequently is most severe in areas with the most traffic, damage or poor conditions. Damage that may appear to be solely walk-on/walk-off injury can be exacerbated by high nematode populations. Frequently, nematodes are implicated after greens have been thoroughly drenched with fungicides but turf quality continues to decline and quality no longer responds to fertilizer and water applications.

Determining that a green does not have a nematode issue is straightforward if numbers are low. If fewer than 100 stunt nematodes per 100cc soil are detected, they are not of concern. Conversely, it’s easy to determine that a green has a nematode issue when numbers are high. If stunt nematodes are recorded at 40,000 nematodes per 100cc of soil, nematodes are a major problem, and most of the turf probably is dead. In 2010, I counted a population this high from a putting green. The guesswork comes in when numbers are between 1,000 to perhaps 4,000 stunt nematodes.

If we use a threshold of 800 stunt nematodes per 100cc of soil, a value of 1,000 stunt nematodes per 100cc of soil should be a problem. But it may not be, depending upon the inherent health of the turf, the depth of roots and management practices. For this reason, having a single number often is insufficient to determine if a green has a bona fide nematode problem (Table 1).

Table 1 Nematode thresholds vary considerably based on the species of grass being grown, the time of the year, management practices and soil composition. The numbers presented below are typical guidelines we use to determine when nematode populations could be considered problematic, based on a composite sample. It should be noted that nematode populations increase throughout the season, so a population at threshold in April will definitely be much higher in July if no attempt is made to control the nematodes.

Sampling and alternative products

In a recent conversation, a superintendent described how he once applied Nemacur (fenamiphos) on the approach side of a green based on nematode counts I forwarded to him from a declining area (approximately 15,000 stunt nematodes per 100cc of soil, which is well over any established threshold). The treated turf quickly perked up and looked better than it ever had. The treated area looked so good, he noted, that members wondered if something was wrong with the untreated area. The untreated area looked acceptable but had moderate nematode numbers that the superintendent decided not to treat because of the scarcity of Nemacur. If both sides had been treated, the green likely would have been in the best condition members had ever seen.

Prior to 2009, few superintendents undertook regular and repeated nematode sampling. If nematodes were suspected and nematode counts were above threshold, superintendents used Nemacur because it was widely available, inexpensive and usually provided season-long control.

In addition, organophosphates like Nemacur work quickly when turf experiences genuine nematode-induced symptoms. A superintendent generally sees a dramatic improvement in turf quality three to five days after an organophosphate application if nematodes are a legitimate problem, resulting in empirical evidence that’s easy to demonstrate to members. Although not labeled for turf nematodes, superintendents who apply chlorpyrifos (an organophosphate insecticide previously known as Dursban) for annual bluegrass weevil also have managed to suppress nematode populations as a non-target effect. When an organophosphate application for nematodes has no effect on turf quality, nematodes probably are below threshold.

The production of Nemacur was halted in 2009. Existing stocks were distributed, and with the help of the Golf Course Superintendents Association of America, superintendents have until Oct. 6, 2017 to use it up or legally dispose of it. As an organophosphate, Nemacur is extremely toxic and affects just about anything with a nervous system. Approximately 200 mg of fenamiphos, perhaps equivalent to a small ibuprofen pill, would kill a 200-pound adult. But it is an important and effective tool when used carefully and cautiously. Its most significant drawback was its ability to migrate into sandy soils, with the potential to contaminate drinking water in locations with shallow water tables. Even careful use could not prevent it from migrating into these types of sensitive areas.

With the final cancellation of Nemacur, superintendents battling nematodes need to find alternate nematicides. Fortunately, new products have been developed and show promising results, but none are as effective as Nemacur. They are, however, generally far safer to the environment and applicators than organophosphates and carbamates.

The new products are not without drawbacks, and three primary issues have arisen with newer materials.

First, some of them have a limited control spectrum. A nematicide may work on one genus of nematode but not another. Secondly, the product may bind to thatch or get locked up in soils with high organic matter. Finally, the amount of nematicide required to achieve control may exceed what can practically be applied, or it may cause damage to the turf when applied at high rates or inappropriately. Before using any product, superintendents should gather as much information as possible about their specific nematode problem.

Sample analysis

Superintendent usually count nematodes after extraction from the soil. They sometimes are counted in the roots, but our common thresholds are not calibrated for those types of counts, and only lance or root-knot enter the roots.

The soil extraction procedure typically uses a sugar solution to float nematodes out of soil. The nematodes are lighter than the sugar solution while soil is heavier than the solution, so separation is relatively easy. However, every lab uses different equipment for extraction and often uses slightly different techniques. This can result in different nematode counts from different labs. Consequently, it may be difficult to compare thresholds from one lab to another. If labs use substantially similar techniques, nematode counts will be relatively close, but never the same.

Superintendents frequently ask how to send a nematode sample for analysis. The best answer: Ask the lab. Labs can determine nematode counts from a cup-cutter plug of soil. Unfortunately, most damage thresholds are determined using composite samples, so cup cutter-sized soil plugs may be misleading.

If a superintendent suspects nema-todes in turf decline, a cup-cutter soil plug from a damaged area is a good way to start, but if there’s an indication of a potential problem, it needs a follow-up with a composite sample. Composite samples are averages across a green. In this process, the superintendent takes 20 to 30 cores from a green and combines them into a single composite sample. A composite sample allows for a more consistent count and reduces the variability involved if a sample comes from a nematode hotspot.

Take composite sample cores only 2 inches to 3 inches deep. Nematodes only survive where roots are present, so core depths of deeper than 4 inches aren’t useful.

Obviously, the depth of the core can change nematode counts by diluting dense populations of nematodes at the surfaces, with empty soil deeper down. Also, composite samples should not be random. Conduct the sampling systematically, using either a grid or zig-zag pattern.

It’s important to sample for nema-todes at the appropriate time of year. The best time to look for most nematodes is spring through early summer. However, you can detect root-knot nematodes only in the early spring, while juveniles are present in the soil (Figure 4). Once root-knot nematodes migrate into roots, they cannot be extracted with soil and cannot be controlled by chemical means.

Figure 4 Nematode populations increase and decrease throughout the season. Most turf-parasitic nematode populations peak in the height of the summer or early fall. However, root-knot nematodes peak in early spring and can only be controlled when the nematodes are present and countable in the soil. Once they enter plant roots, little can be done to manage them.

Nematodes continue to be a serious issue for turfgrass managers, and while southern regions tend to be the hardest hit, high nematode populations also cause significant damage on northern golf courses. Fortunately, a new set of modern nematicides have been developed to help turfgrass managers deal with these intractable pests.

Nathaniel Mitkowski, Ph.D., is a turfgrass pathologist at the University of Rhode Island. You may reach him at mitkowski@uri.edu for more information.

Photos: Nathaniel Mitkowski

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