Nematodes of turfgrass

Charles Overstreet  |  12/2/2016 3:42:10 PM

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The picture above shows just how damaging nematodes can be to grass commonly found on a golf course. Turfgrass growers have had a long battle with nematodes in the past and will continue in the future. Nematodes are considered to be an enigmatic pest primarily because they are often difficult to recognize and equally difficult to control. Nematodes have only fairly recently been recognized as pests of turfgrass (during the past 50 years). There are still a lot of unanswered questions about these pests.

Symptoms

One of the most troublesome things about nematodes is simply the fact that they are difficult to recognize as a pest. Most of the common types that damage turfgrass are root feeders. This damage is below ground and has to be recognized by the symptoms expressed above ground. Careful examination of the root system will often show that the roots are stunted, off-color, or necrotic. Even then it is difficult to positively identify the damage as being a nematode problem.

Other injuries to either the roots or foliage can be confused with the damage caused by these parasites. Foliage or above ground symptoms can often simply be an off-coloring, slowness in normal growth and development, thinning of stand, and failure to respond to either fertilizer or watering. Often spots appear in the grass first giving an indication that a problem is developing. These spots can be from a few feet in diameter to a fairly large area. Damage shows up in areas that are weak or in a stressed condition first. Nematode injury is nothing more than a stress on a plant which will be enhanced if poor conditions prevail. Symptoms may grow worse during a growing season and tend to spread. Injury can show up at any time during the growing season. Symptoms usually are in response to high populations. The greatest injury tends to be when high populations have survived the winter and early spring, and attack the roots early in the growing season. Nematode areas may tend to show up in the sample locations each year. If you had a problem this year in an area, expect more problems in the future. Populations tend to go up or down with environmental conditions or just normally during the growing season and winter months. However, don't expect nematodes to just go away.

Types

There have been numerous nematodes associated with declining turfgrass. Some are obviously serious pathogens and cause extensive damage. Some have been suspected as being a problem but it is still not clear as to how much damage they actually cause to plants. Still others occur in fairly high levels but are not thought to cause serious injury. Most of the time there are two or more types together making it difficult to determine whether individual types alone are causing the problem or whether it is a complex of nematode types that makes up the problem.

Sting nematode (Belonolaimus sp.) is considered to be one of the most damaging nematodes to turfgrass. This pathogen is so bad that even detecting any at all would strongly suggest a serious threat to the turfgrass. Sting nematode is an ectoparasite meaning that it uses a long stylet to penetrate the roots of the plant and feeds from the outside of the root. Sting is also thought to inject some type of toxin or poison into the roots because of the extensive damage done by this nematode. Sting nematode is generally found throughout the South and is found in only very sandy soils. In Louisiana this pest has only been found in association with turfgrass. It is extremely limited in its native distribution in this state, having been found in only Grand Isle, Ouachita Parish, and Claiborne Parish. Unfortunately for golf green people, this nematode apparently has come in on sod that came from other states which have this pest. It is a very large nematode and apparently needs a very high sand content to survive (80% or greater). This is why it is not a problem in most of the state. Golf greens are one of the few soils that are deliberately altered to fall within this range of soil in our state. Apparently even small amounts of silt, clay, and/or organic matter may severely limit the population buildup. Keep this in mind if sting nematode becomes a problem and if modifying the soil is a possibility.

Sting nematode is very easy to recognize in the laboratory because of its large size and the presence of an extremely large stylet. It does tend to survive the winter months in the egg stage. Samples should be collected anytime during the growing season but are best not collected in late winter and early summer. It may be difficult to find the juveniles or adults at this time.

There are several races or different populations of sting nematode that have been identified. These have been identified primarily because they seem to attack different crops. All the different types of sting nematode do attack some common plants. These include grasses such as Johnson grass, hairy crabgrass, centipede grass, rye, and wheat. Bermudagrass is one of the crops that apparently all types of sting don't attack. However, if it turns out that it is present in this grass, you can probable assume that it will attack it. Since most of the reports of sting have come from bermudagrass greens, it would appear that this is the common type found here.

Stubby-root nematode (Paratrichodorus sp.) is another nematode that is fairly widespread in Louisiana. Nematodes of this type are also ectoparasites which usually aggregate at the root tip. Since their feeding is in this location, it is not surprising that root growth and development is severely affected. Although this nematode is quite destructive, it usually takes higher populations than sting before damage becomes apparent. Roots may eventually become discolored or necrotic but this apparently comes from other organisms rather than secretions from the nematode. The most noticeable foliar symptom of this nematode may be the decline or reduction in a stand. Stubby-root nematode prefers coarser textured soils such as sandy soils or occasionally sandy-loams. They don't seem to be quite as particular in the amount of sand as sting nematode since it is widespread throughout the state in a number of crops. This nematode may be very variable in its distribution either within a field or area as well as the depth of soil where it is found. Stubby-root nematode has been reported as a major pest on bermudagrass, St. Augustine grass, centipede, bentgrass, and zoysia grass. This nematode has also been found to have a rapid population rebound after treatment.

Lance nematodes (Hoplolaimus sp.) are another fairly large nematode that causes problems with turfgrass. Lance nematode feeds on epidermal and cortical cells. Unlike the previous nematodes, this pathogen actually may move completely inside a root and act as an endoparasite. This nematode reduces shoot and root production, water uptake, and the number of root hairs as well as nutrient uptake. This may be expressed in the field as stand decline or reduced vigor. Populations of this nematode may vary only slightly with rainfall but are usually smaller after dry periods and larger after a period of rainfall. A sandy loam is considered to be more suitable for reproduction than silty loams or loamy sands. Lance nematode has been found associated with bermudagrass, St. Augustine, zoysia, and centipede grass.

Ring nematode (Criconemella sp.) is another extremely common nematode found associated with turfgrass. This nematode is an ectoparasite and feeds on a number of plants besides turfgrasses. Unlike some of the previous nematodes, populations usually have to be fairly high before injury occurs to plants. Turfgrasses that have been found to be both good hosts and damaged by this nematode include bermudagrass, St. Augustine, zoysia, and centipede grass. Apparently the most susceptible grass is considered to be centipede and much lower levels of the nematode can cause damage to this grass. Foliar symptoms associated with this nematode are either a slight yellowing or stunted growth.

Root-knot nematode (Meloidogyne sp.) is another nematode that is a widespread pest in turfgrass. Although in many cases it is not clear exactly how damaging this pathogen is to turfgrass, it certainly is a serious problem to many other crops such as vegetables, cotton, and ornamentals. One species, M. incognita is widespread in Louisiana and has also been shown to damage bermudagrass. Several other grasses such as orchardgrass, Italian ryegrass, or Kentucky bluegrass are also considered to be susceptible to root-knot. Common bermudagrass, carpet grass, or crabgrass appears to be resistant to our common species of this nematode. On most crops root-knot nematode produces very distinctive galls or knots on the roots. On turfgrass these galls are very small and often difficult to recognize. Don't count on being able to identify root-knot in turfgrass by this symptom.

Spiral nematode (Helicotylenchus sp.) is one of the most common nematodes associated with plants. They are widely found in golf greens, lawns, or pastures. Generally, there have been few reports linking this nematode with serious injury to turfgrass. During periods of stress, nematodes such as spiral may cause injury to golf greens if populations are large. Golf greens that are well maintained with proper water, nutrients, and cultural practices should have little trouble from this pest.

There are a number of other types of nematodes that have been associated with turfgrass in Louisiana. These include cyst (Heterodera sp.), lesion (Pratylenchus sp.), sheath (Hemicycliophora sp.), dagger (Xiphinema sp.), and pin (Paratylenchus sp.). Although fewer problems have been associated with these nematodes, they can still cause some problems if present in high enough populations and conditions are favorable for disease development.

Managing nematodes

Turfgrass is capable of supporting some populations of most nematodes. Healthy grass can often support much higher levels of some kinds before damage becomes apparent. There are a number of things that should be done to turfgrass to keep it healthy and reduce the amount of stress placed on it. Stressed grass will show nematode injury much quicker than more severe than well growing grass. Deep and infrequent watering encourages the development of a deep root system. Roots are then able to have a larger area to draw from water and nutrients. Daily watering means very shallow roots that are concentrated in a very limited area. Fertilization is also very critical. Nutrients should be readily available but not excessive. A soil test should be conducted to determine the levels that are present and also the pH of that soil. Avoid excessive nitrogen which tends to cause lush growth and encourages maximum nematode reproduction. Since stress can be such an important factor in nematode problems, anything cultural that can be done to eliminate stress should be done. This could include practices such as mowing at the correct height or mowing often to remove only a limited amount of foliage at each time. Either one of these is important because the plant has to have sufficient foliage for photosynthesis. Other cultural practices would include aeration of soil, dethatching or sanding to improve root growth, or breaking up hardpans. Be careful with the application of pesticides such as herbicides which can place unnecessary stress on plants if improper rates or incorrect usages are made. Often there are problems caused by fungi which attack plants weakened by nematodes. Try to manage fungal diseases, insects, or weed pests before they become a serious problem. Home owners may be able to change the type of grass that they are able to plant if a nematode problem becomes apparent. Centipede appears to be the most sensitive grass to ring nematode. Bahiagrass appears to be fairly tolerant of most nematodes and is not as likely to be injured as some grasses. Common bermudagrass appears to be resistant to the common root-knot nematode. Unfortunately, as was mentioned earlier, many nematodes such as sting, stubby root, and lance attack a wide range of different grasses. It will not help to replace a susceptible grass with a similar one.

Chemical control

There are currently a number of nematicides that can be used in turfgrass. Most of these nematicides are labeled for use only in golf courses or sod farms. There are a few that are labeled for use in residential landscapes. Always follow the label when applying nematicides.

Table 1. Nematicides that can be used on established turfgrass and locations that can be treated in Louisiana.

Nematicide

Golf Greens

Sod Farms

Sports fields

Home lawns

Nortica

Yes

Yes

Yes

Yes

Multiguard Protect

Yes

Yes

No

No

Nimitz Pro G

Yes

Yes

Yes

Yes

Indemnify

Yes

Yes

Yes

Yes

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