Plant-parasitic Nematodes in the Mid-South

Terry Kirkpatrick

University of Arkansas Southwest Research and Extension Center

Hope, AR

The mid-South, comprised mainly of the states of Arkansas, Tennessee, Mississippi, and Louisiana constitutes a significant agricultural region in the southern U.S. The Misssissippi River Delta provides the focal point for the majority of the agronomic crop production. This area is characterized by a diversity of soil types ranging from light, sandy loam soils to heavy clays. Crops that are routinely produced in this region include cotton, rice, soybean, wheat, and to a limited extent corn and grain sorghum. Throughout much of the region, however, cotton and rice drive the agronomic system, due both to the potential economic returns that are possible, and because of land ownership/lease arrangements.

With the exception of rice, plant-parasitic nematodes can be of economic concern in all of the major crops grown in the mid-South. Surveys conducted during the past ten years indicate that one species of the root-knot nematode (Meloidogyne incognita), the soybean cyst nematode (Heterodera glycines), and the reniform nematode (Rotylenchulus reniformis) are common throughout the region and may infest 50% or more of production fields in some areas. Although numerous other plant-parasitic nematode species may also be found associated with crops in the mid-South, only these three are considered to be of major economic importance.

Root-Knot Nematodes. The root-knot nematode is perhaps the easiest of the nematodes to diagnose in the field. Root-knot gets its name because infection of roots of susceptible hosts results in the formation of galls or swellings on the roots. These galls are readily visible to the naked eye from about mid-season through harvest time. Aboveground symptoms of root-knot also are relatively apparent in many fields. Symptoms include stunting and poor growth of plants in areas within infested fields. Heavily parasitized plants may exhibit nutrient or water stress symptoms, particularly during the latter half of the growing season. This nematode species is of major economic concern in both cotton and soybean. It is currently the most important nematode species on cotton in Arkansas, western Tennessee, and the Missouri bootheel, and it is also common in the sandy soils along the Mississippi River in Mississippi and Louisiana. Corn is also a known host for root-knot, although little work has been done to determine the economic significance of root-knot in corn production systems in the mid-South. Grain sorghum appears to be a poor host for most of the root-knot populations found in the region. Continuous monoculture of grain sorghum, however, may result in an increase in root-knot population densities over time (3-6 years).

Soybean Cyst Nematodes. The soybean cyst nematode continues to be the most important nematode pest of soybean throughout the region. Surveys in Arkansas indicate that approximately 70% of the soybean acreage is infested by this nematode. Fortunately, soybean cyst nematodes have a narrow host range that includes soybean and a few legume weeds. The nematode can, however, survive for 7-10 years in soil in the absence of a host. Soybean cyst nematodes are also relatively easily diagnosed in the field from midseason through harvest. The nematodes infect soybean roots and swell to form tiny (but visible with a hand lens) cysts that are attached to roots. Cysts are white in color early on, but as they age they change to a yellow and then a brown color. Cysts can be distinguished from other structures that may be seen on soybean roots such as nodules because they are much smaller and are lemon-shaped. Aboveground symptoms of soybean cyst nematode damage range from large irregular spots of poorly developing plants to virtually no readily apparent foliar symptoms. Even without visible plant symptoms, studies in western Tennessee have documented at least 10 bu/acre yield reductions due to the nematode without visible symptoms.

Reniform Nematodes. The reniform nematode is a relative newcomer to the mid-South. This nematode has historically been of concern primarily in tropical and sub-tropical areas of the world, and in 1960 reniform was thought to occur in the U.S. only in extreme southern Texas, Louisiana, and Florida. Incidence of the reniform nematode, unfortunately, has changed during the past 40 years–moving steadily northward. In 1999, the reniform nematode was named the most important nematode pest of cotton in the states of Mississippi and Louisiana. This nematode has now been identified in every cotton producing county or parish in these two states. In addition, since 1990, the reniform nematode has been identified in 10 major cotton producing counties in Arkansas, from the Louisiana line to the Missouri bootheel. The nematode has also been reported in western Tennessee, and appears to be on the increase there as well. At the present rate of spread, the reniform nematode is rapidly replacing root-knot as the most important nematode pest of cotton throughout the mid-South. Reniform nematodes also reproduce well on soybean, but there has been little definitive work to determine its economic importance in the crop. Neither corn, grain sorghum, nor rice appear to be a host for this nematode.

The most important single factor in managing a nematode problem in any crop is accurate diagnosis of the presence and identity of the species involved. Although visual inspection may many times indicate the presence of root-knot or soybean cyst nematodes, the reniform nematode is almost impossible to identify based on field symptoms or root inspection. Soil sampling for assay by a nematology laboratory is the most accurate and dependable means of determining whether or not a nematode problem exists. Public (university) or private nematology laboratories that provide assays on a fee basis are available in virtually all parts of the U.S. These laboratories can also provide advice on timing and methods for sampling fields and the procedures for handling and shipment of the samples. Nematodes are living organisms. They must remain alive until the assays are conducted. Consequently handling samples properly is as important as the sampling process itself. Results of nematode assays are always only as good as the sample that was submitted.

In the mid-South, nematode management is primarily accomplished through one or a combination of methods. Options include the use of resistant cultivars in some crops, crop rotation with resistant or poor hosts to lower nematode population densities, or the use of chemical nematicides. Selection of the method or methods for managing nematodes in any individual field must be determined based on the nematode species that is present, the crops that may be grown, and the economics of the management program. For most of the major crops, threshold levels have been established to aid in determining the magnitude of a nematode problem. These levels vary considerably from state to state, and should be used as general guides only. Experience with crop performance in individual fields should always be included in considerations when formulating nematode control strategies.

The least expensive, and many times most effective means of managing nematode damage to crops is through the use of nematode resistant cultivars. Well-adapted, high-yielding nematode resistant cultivars have been developed in some crops. Unfortunately, nematode resistant cultivars are not available for all major crops and all major nematodes. In cotton, only a few root-knot resistant cultivars have been developed, and none are widely grown in the mid-South because of a lack of sustained high yield across the region. Because historically the root-knot nematode has been associated with a disease complex that also involves the fungus Fusarium oxysporum f.sp. (Fusarium wilt) numerous wilt resistant cultivars have been developed, and many of these cultivars are marketed as "tolerant" to the root-knot nematode/Fusarium wilt disease complex. Unfortunately, while the genetic resistance to the wilt organism has been improved in these cultivars, they are still susceptible to the nematode and yield loss potential is high in fields with high root-knot infestations. There are no cotton cultivars currently available with resistance to the reniform nematode.

Both root-knot and reniform nematode resistant soybean cultivars are available, although few are highly resistant. Recent studies in Arkansas indicate that severe root-knot may suppress soybean yield by as much as 50%. These studies also indicated that selection of a moderately resistant cultivar resulted in yield improvement of 15-25 bu/acre depending on the infestation level. Numerous reniform nematode resistant soybean cultivars are also available, although none are highly resistant and some population increase can be expected during the season on these cultivars. Corn is not a host for the reniform nematode, but it is a very good host for root-knot. Dramatic root-knot population increases have been documented following a single season of corn. To date, no commercial hybrids that are popular in the mid-South have been found that are resistant to root-knot. Grain sorghum is also a non-host for the reniform nematode, and this crop does not appear to support high populations of root-knot during the first two years of production. However, because root-knot is capable of limited reproduction on grain sorghum, continued monoculture tends to increase populations over time.

complicating factor, however, in managing this nematode with resistant cultivars is the existence of races or biotypes of the nematode. There are currently 16 possible races of the soybean cyst nematode, each of which are characterized by their ability to reproduce on different resistant soybean types. Historically, soybean cyst nematode race 3 was predominant in much of the area, but within the past few years, races 4, 5, 6, 9, and 14 have increased in frequency. Continued production of any given resistant cultivar (or cultivar representing a particular type of resistance) may exert sufficient selection pressure on the local soybean cyst nematode population to force a change in race, resulting in the ineffectiveness of the resistant cultivar. Consequently, resistant cultivars must be used judiciously in combination with crop rotation to avoid undue selection pressure and loss of effectiveness of the resistant cultivar.

Crop rotation may be an effective means of lowering nematode population densities and the accompanying crop loss. Selection of rotation crops and cropping sequences must be determined based on the nematode species in question and on the economic feasibility of using the rotation program. Rice is perhaps the most effective rotation crop across all of the economic nematode species. Rice is a non-host for all three of the economic species. In addition, the flooded culture of rice appears to be effective in lowering nematode populations, presumably by lowering the oxygen concentration in the soil for long periods of time during the summer months. In the mid-South, a common rotation is a soybean-rice system. This rotation may be very effective in controlling root-knot and reniform nematodes, and is also somewhat effective in lowering soybean cyst nematode densities. Research has shown, however, that a significant number of soybean cyst nematodes may survive a year of rice. Combinations of rice-soybean rotation and use of resistant and susceptible soybean cultivars appear to be the most effective long-term solution to soybean cyst nematode problems.

As indicated earlier, grain sorghum may also be an effective rotational crop with either cotton or soybeans for lowering reniform, soybean cyst and root-knot. The length of the rotation (number of years needed) may vary according to the nematode population density that is present in a field. A single year of rotation may be very effective at lowering populations that were initially moderate to high. Two years may be necessary, however, in some fields with extremely high initial population densities. Where root-knot is the target species to be controlled, it is not recommended that grain sorghum be used more than two consecutive years. Unfortunately, with current commodity prices, grain sorghum may not be economically feasible for many growers. In situations where cotton is the primary crop, it may be more economically feasible to utilize root-knot or reniform nematode resistant soybean cultivars to help lower nematode populations. However, because most of these cultivars are not highly resistant, it is important that fields be monitored annually to determine the levels of nematodes that are present at the end of the rotation crop season.

Corn can provide a very effective and possibly more economically attractive rotation crop for growers who need to manage either soybean cyst or reniform nematodes in soybeans or reniform nematodes in cotton. As in the Midwest, corn-soybean rotations are becoming more popular in the mid-South. Before this rotation regime is adopted, an accurate identification of the nematode problem is necessary, because although corn will lower soybean cyst or reniform nematodes, it may increase the severity of a root-knot problem. Where root-knot is a problem in soybeans, use of resistant soybean cultivars may allow this rotation to be continued, although most soybean cultivars are not highly resistant to the nematode and may still show yield suppression if root-knot populations increase to high levels on the corn.

In general, rotation of summer crops tends to exert a greater influence on nematode populations than winter cover or agronomic crops. The use of wheat, oats, rye or other grain crops does not appear to influence nematode populations to a significant extent in the mid-South, presumably because they are growing when soil temperatures are too low for nematode activity and reproduction. In some areas, particularly in the southern portion of the region, use of nematode susceptible cover crops such as hairy vetch may actually be detrimental because soil temperatures warm in the spring early enough to allow reproduction by the nematode prior to cover crop destruction.

Chemical nematicides may be effective, and in some crops and situations, cost effective in managing nematodes. Because they are expensive, nematicide use in the mid-South is primarily restricted to cotton. There are currently three nematicides that are labelled for use in cotton in the region, aldicarb (Temik), fenamiphos (Nemacur), and 1,3-dichloropropene (Telone). Temik and Nemacur are both considered to be contact nematicides–which are applied to soil at the time of planting and work as a contact poison in the soil. These materials also provide systemic insecticidal activity and may be desirable for management of early season insect pests as well as nematodes. Telone is a soil fumigant. It can be very effective in controlling nematodes, but requires specialized equipment for injection into the soil and a waiting period of approximately two weeks after application before the crop can be planted because of potential phytotoxicity. In general, nematicide application can result in yield improvements in the range of 10-20% in cotton depending on the nematode population density and species. Efficacy, however, is affected greatly by environmental conditions, particularly soil moisture, and results tend to vary from year to year and even from field to field within the same year. Recently, research has been initiated to attempt to define areas within individual fields that have high nematode infestation levels for "zone" treatment with nematicides. This approach would allow nematicide expenditures to be focused within fields only where populations exceed threshold levels. At this time, however, the feasibility (both physically and economically) of using this approach to nematicide application has not been demonstrated.

three species are of major economic concern, root-knot, reniform, and soybean cyst nematodes. Symptoms of nematode parasitism are rather general and may mimic other problems such as nutrient deficiencies or water stress. Nematode management strategies depend on the accurate identification of the nematode species that are present and their infestation levels. The most accurate and dependable method for determining the species and levels is through soil assay by a nematology laboratory. Nematode management is best accomplished through a combination of methods that include use of resistant cultivars if available, crop rotation, and in some situations application of chemical nematicides.