• Topics: Entomology and Nematology | Nematode Management in Field Crops | Agronomic Crops | Soybean Pest Management | Kinloch, Robert A | Rich, Jimmy R | Nematodes | Dunn, Robert A

Soybean Nematode Management ¹

Nematodes that Attack Soybean

Soybean (Glycine max) has been grown relatively little in Florida in recent years, but the rise in value may cause some growers to consider growing the crop again. Part of that consideration should include knowledge of the severe nematode problems of soybean in Florida. Growers should be aware that nematodes are a major pest problem in soybean production, and in addition, seriously increase nematode risks to other crops that may follow soybean.

Nematodes attacking soybean include all the root-knot (Meloidogyne spp.) nematode species that occur in Florida. In addition, reniform (Rotylenchulus reniformis), lesion (Pratylenchus spp.) and sting (Belonolaimus longicaudatus) nematode populations are increased by soybean production. Each of these nematodes can be limiting factors in the production of soybean as well as other crops in Florida. Soybean cyst nematode (Heterodera glycines), important in previous years, only affects soybean among Florida agronomic crops. It has not been detected in Florida for many years since the decline in soybean production and the shift of much of the previous soybean acreage to cotton, a non-host of soybean cyst nematode.

Diagnosis

The presence or potential for nematode problems in soybean could be suggested by one or more of the following: 1) Cropping history of the field, e.g. two or more years of cotton production or other nematode-susceptible crops; 2) Above-ground symptoms including off-color and/or stunted plants in spots or large areas of a field; 3) Below-ground symptoms such as small knots on roots or stunted and swollen root tips.

Above-ground Symptoms

Above-ground symptoms of nematodes attacking soybean resemble those caused by many other kinds of root injury, such as disease or general nutrient deficiency. Irregular, oval shaped areas in the field may contain plants that are stunted and yellowed (Figure 1). Plants with nematode-damaged root systems wilt more easily under moderately dry conditions and recover more slowly than healthy plants when water becomes available. Severity of damage is unevenly distributed in the field. Near the end of the season, leaves of nematode-affected soybean plants may prematurely yellow and drop before those of healthy plants. Frequently, lines of these damaged plants may stretch along rows following the direction of usual soil cultivation.

Figure 1. 

Stunted plants in an oval pattern caused by root-knot nematodes.

[Click thumbnail to enlarge.]

Root Symptoms

All nematodes affecting soybean reduce feeder roots and cause root stunting. To clearly see root symptoms, roots should be carefully dug with a shovel for close examination. Below-ground symptoms differ, depending on the nematodes causing them, and are not always distinct enough to use as a sole basis for diagnosis.

Root-knot nematode infections cause the roots of soybean to swell into galls. These may not be obvious until the latter half of the soybean season. Root galling caused by nematodes on soybean may resemble Rhizobium nodules, and these can be confused when diagnosing a root-knot nematode problem. Rhizobium nodules on soybean roots grow on one side of the root, can be easily removed from the root and are a pink color inside. Root-knot nematode galls, on the other hand, grow around the small roots, are firmly attached, and are not pink on the inside.

Roots infected with soybean cyst nematode (SCN) usually have reduced Rhizobium nodulation and are short, sparse, and dark in color. Small period-sized dot-like cysts on the roots can be seen with very careful examination. These are the developing females of soybean cyst nematode and can range in color from white to creamy yellow to dark brown.

Other nematodes may cause soybean roots to be stunted or pruned, and the roots are usually darker in color than healthy roots. Sting nematodes stop root growth, giving the appearance that roots have been cut off. Reniform nematodes are the most difficult to diagnose in the field and produce overall stunted root systems with few feeder roots. Extensive root decay caused by common fungi or bacteria often follows nematode injury, especially by lesion nematodes. A laboratory analysis is usually necessary to confirm cyst, reniform, lesion, and sting nematodes on soybean.

Nematode Assays

Soybean is such an excellent host for many important plant-parasitic nematodes that a field planted to this crop should be sampled before any cash crop is planted at the same site. Samples should not be taken when the soil is dusty dry or soggy wet. Two sampling strategies may be employed. A general survey should be performed immediately after soybean has been harvested. A soil core to 10 inches deep should be taken for every 1 acre in a 10-acre block containing a uniform soil type and cropping history. In a more definitive strategy where a nematode problem is suspected, several soil cores from within and immediately around a poor growth site should be taken while soybean is still growing. Include portions of damaged roots with the soil sample.

Prior to taking samples, contact your County Extension agent for information concerning available sampling tools, shipment bags and proper procedures for submitting samples. The cores should be thoroughly mixed and a 1-pint sample extracted and placed in a sealed plastic bag and kept cool (not frozen) before immediate shipment to an advisory laboratory.

Figure 2. 

Ten acre sampling patterns. Take 10 to 20 soil cores.

[Click thumbnail to enlarge.]

The University of Florida/IFAS Nematology Assay Laboratory provides routine nematode identification to the general public. There is a charge for each sample. Sample kits available at County Extension offices contain instructions for collection and handling of samples, and packing and shipping materials. Payment must accompany samples when they are sent to the lab for analysis. Diagnostic laboratories in the private sector may have different procedures and fees, and provide different services.

Field Mapping

With the advent of GPS systems, some growers are now routinely collecting soil samples from their fields with sampling area sizes of 1-5 acres. These are then placed on maps and variable rate nematicide applications are made only where the nematicide is needed to improve yield. On one north Florida farm, intensive sampling and variable rate nematicide application have saved the grower up to 60% of nematicide costs by only targeting nematode problems areas in his fields.

Root-knot nematodes can be monitored by observing root galling on susceptible crops. Carefully lift plants at regular intervals throughout a field and note on a field map the extent of galling at each site. This provides far more detailed information about the distribution of problems than can be obtained from lab analysis of one or two composite soil samples. Crop species and varieties on which galls are found can also give valuable clues to which root-knot nematode species are present in each location.

Management

Nematode management for soybean and following crops depends upon identifying and monitoring nematode populations to choose tactics for each field. Careful choice and integration of crop rotation, other cultural practices, resistant varieties, and nematicides are required. The kinds of nematodes present and their distribution in fields must be known to:

1. identify fields that cannot be planted profitably, or where a different crop has better profit potential;

2. plan optimum crop rotations to minimize losses for following crops and reduce populations;

3. select appropriate resistant varieties; and

4. recognize fields where nematicide use might pay for soybeans or other crops.

Limiting Movement of Nematodes

Reniform nematode is not yet established in all soybean land in Florida, so preventing its further spread within a local area or even a single farm is a very economical way of avoiding ("controlling") losses to it. Nematodes are easily spread in soil and infected plants, but can move only a few feet per year under their own power. Soil and trash on tillage or harvest machinery is a common means of spread, so do not go directly from a contaminated area to a clean one. Work in clean fields before infested areas. Be sure that borrowed or hired equipment is clean of soil and plant material before it enters clean land. Remove soil from machinery, clothes, shoes, tools, etc. when leaving a contaminated area. Do not move whole plants with soil on their roots from contaminated or suspect soil.

Crop Rotation

Soybean generally is not a suitable rotational option for the nematode management in other crops. Soybean should not be planted either before or after cotton as both are hosts to important nematode pests including southern root-knot, reniform, and sting nematodes. The susceptibility of soybean to nematodes is so great that, even if planted as a forage, growing soybeans for more than one consecutive season could cause damaging levels of nematodes for subsequent crops. Unless sting nematodes are in a field, a crop of soybean should be followed by a summer planting of a grass crop such as field corn, sorghum, or bahiagrass, etc. These crops are the most likely to reduce populations of root-knot and reniform nematodes. Rotation is a good tool to help keep relatively low populations from becoming too high, or for gradually reducing high populations over several years. It should not be expected to abruptly reduce a nematode population, since:

• some of a nematode population will survive the winter without a host

• most crop plants can support at least a little nematode reproduction

• most planted fields have weeds that support nematode reproduction

Soybean is the only common agronomic crop that supports soybean cyst nematode (SCN). However, some leguminous weeds, some varieties of field peas (southern peas), and most edible beans apparently can support SCN reproduction, so avoid them in a rotation intended to reduce SCN and do not follow soybeans with these crops. An additional problem is that some of the eggs within a cyst of SCN may remain dormant for many years. There may well be enough (perhaps 5%-10% of the initial number) to initiate a new population 2 or 3 years after the cyst is produced, and a very few may well survive as long as 5 years. Therefore, rotation cannot be expected to reduce a high population of SCN rapidly, nor to eliminate a population.

Reniform nematode populations are likely to increase where cotton and soybean are grown frequently, and also will build up on many vegetables. Rotations to reduce reniform nematodes should include corn, sorghum, other grasses, and/or peanut. Unfortunately, rotations are of little value for reducing lesion or sting nematode numbers, because they live well on so many different plants, including most grasses, soybean, most vegetables, and cotton. Lesion nematodes reproduce well on peanut and tobacco in Florida; sting does not.

Soybean is often planted behind a winter small-grain crop such as rye, wheat, or oats. A winter grain crop can help prevent erosion and weed growth, and provides income from the field twice per year rather than once. However, root-knot nematodes can infect new roots of many small grains planted while the soil is warm enough (above 65^oF) to allow nematode infection of the grain crop before soil temperatures drop in winter. Rye, triticale, most wheat varieties, and barley can support root-knot nematodes; many varieties of oats support less or no reproduction. Reproduction in these grains may not actually increase root-knot nematode numbers, but may interfere with their normal overwinter decline. If planting is delayed until soil temperatures are lower, root-knot nematodes should not be able to reproduce in small grains.

Use of crop rotation systems that include bahiagrass has been increasing, and this perennial grass is a non-host for nematodes affecting soybean. However, weeds must be managed in the bahiagrass or nematode populations will be maintained in such a system resulting in damage to the following soybean crop. A two-year bahiagrass rotation is sufficient to manage plant-parasitic nematodes in a future soybean crop providing weeds are controlled early and regularly in the first year bahiagrass and this is continued throughout the life of the rotation.

Figure 3. 

Weeds in bahiagrass pastures maintain nematodes for future crops.

[Click thumbnail to enlarge.]

Resistant Varieties

Considerable effort has been expended over the years to develop soybean varieties with resistance to soybean cyst and root-knot nematodes. There are soybean varieties with resistance to one or more of the three common species of root-knot nematodes and some populations of soybean cyst nematode (SCN). Some varieties resistant to SCN also are resistant to the reniform nematode. However, no variety is "nematode proof", and mixed populations of more than one nematode make choosing resistant varieties more difficult. In root-knot nematode resistant soybean varieties, the resistance is considered "quantitative" and is governed by a large unknown number of genes. Resistant varieties, therefore, may allow root-knot nematodes to survive and reproduce but at a lower level than susceptible varieties. This does not cause the resistance in a particular cultivar to fail but a considerable soil population of root-knot nematodes will remain following cultivation of a resistant soybean. Alternatively, soybean cyst resistance in soybean is considered "qualitative." Here resistance is conferred by a few dominant soybean genes. Compared to root-knot nematode, essentially no soybean cyst nematodes develop within a resistant cultivar. If soybean cyst-resistant cultivars are monocultured for a few years, resistance-breaking cyst nematodes will cause failure of that particular line of resistance.

Soybean varieties or lines are no longer tested in Florida for nematode resistance traits. However, when buying seed, consult with seed dealers and soybean seed companies to determine variety resistance to the three root-knot nematodes (Javanese, peanut and southern) commonly found in Florida soybean fields.

Nematicides

Nematicides approved for nematode management of soybean are given in Table 1. There is little or no justification for using nematicides on soybeans produced for forage. For the grain market, growers should carefully consider costs and commodity price expectations before choosing nematicides for nematode management in soybean.

Temik 15G is available for use on soybean, but using this nematicide is subject to several use restrictions in general and more specifically in Florida. Before applying the product, read the label carefully, and in addition, request information from the Florida Department of Agriculture and Consumer Services (FDACS) on use restrictions and regulations for the product or go directly to their web site at: http://www.flaes.org/temik/index.html

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, Dean.

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