The options available for plant-parasitic nematode management include sanitation, resistant and tolerant varieties, crop rotation, cover crops, conservation tillage and nematicides. In most cases, a combination of these practices will be needed to keep nematode numbers below the economic threshold. Only use nematicides when other options are not feasible or do not reduce populations below the economic threshold.
Sanitation
If plant-parasitic nematodes are not present in a field, take preventative measures to reduce the chance of introducing an infestation [11, 30]. Wash planting, cultivating and harvesting equipment to remove all soil residue when moving from field to field. For example, the spread of reniform nematodes has been linked to movement of equipment from field to field as producers enlarge their operations by leasing new lands. Soil clods clinging to equipment have been shown to transport reniform nematodes, with thousands occurring in a pint of soil. Reniform and soybean cyst nematodes can remain viable for three and 10 years, respectively, in dry soil.
Preventing the introduction of damaging nematodes eliminates the need to manage them. Once nematodes are present, they cannot be eradicated and must be managed. Wash all equipment to remove soil residues before use on nematode-free fields. Remove soil residues from all contract harvesting equipment as well as from any newly purchased equipment.
Resistant and Tolerant Varieties
Plant-parasitic nematodes are capable of reducing crop yields in an infested field. Therefore, it is important to maintain low nematode population densities. The most important and often most economical management tool is the use of resistant or tolerant cultivars [33]. Tolerance is defined as the ability of a crop to produce an adequate yield in the presence of the nematode. Resistance is the ability of a plant to limit nematode population increases and is determined in greenhouse studies that evaluate nematode populations over time. New cultivars are constantly being developed for all crops in various crop production regions. Evaluations of nematode tolerance in field crops are made through field trials that examine yield.
A resistant variety will not allow a nematode population to increase. Nematologists define resistance based on a nematode reproductive factor, Rf, which is the final nematode population, Pf, divided by the initial nematode population, Pi. Rf=Pf/Pi [6, 19]. A host variety with an Rf value less than one is considered resistant and does not allow the nematode population to increase. A value of greater than one indicates the nematode population increases in the presence of the host plant. However, plant breeders will often determine resistance as a ratio between the variety being evaluated and a standard cultivar [33]. If the variety has a lower final population than the standard, it is considered moderately resistant or resistant.
Nematode numbers following a tolerant crop will generally be higher than those following a resistant crop. Do not plant a tolerant variety in the same field two years in a row. In an Alabama study, PhytoGen 565 WRF produced a seed cotton yield of 3,133 pounds per acre with 1,585 root-knot eggs per gram of root [32]. This cotton variety is considered root-knot tolerant. It produces a good yield while supporting a nematode population above the established economic threshold. In the same study, PhytoGen 367 WRF produced 3,467 pounds per acre of seed cotton with only 382 root-knot eggs per gram of root. This variety was considered resistant and nematode numbers should be lower for the following season’s crop.
Many universities across the Southeast conduct variety trials on cotton, corn, soybeans and small grains in the various production regions of their states. The annually updated yield and disease ratings are published on their websites. These are some examples: www.alabamacrops.com and www.msucares.com. Variety trial information is available online at the American Phytopathological Society website: www.plantmanagementnetwork.org. (Search for plant disease management reports.)
The resistant reaction of crops to infection by plant-parasitic nematodes is complex. To better understand host resistance, new technologies are being used at the feeding site. Technologies such as laser capture microdissection, in concert with microarray analysis and other genomic analysis methods, are identifying genes that are specific to not only the susceptible or resistant reaction, but also to the different resistant reaction types [12]. These technologies allow for identification of common strategies that plants use to combat plant-parasitic nematodes. The impetus is the development of meaningful gene annotation databases that are publicly available and easy to mine so that many labs have the ability to explore the function of genes in functional genomics analyses [12]. Once this goal is met, solutions to agricultural problems presented by plant-parasitic nematodes will become available.
Crop Rotations
Crop rotations are effective in reducing nematode populations [7]. The production of corn, grain sorghum or peanuts for one year may sufficiently reduce reniform nematode numbers to allow the production of cotton or soybeans the following season [7, 9, 11, 13, 38]. Although rotations with corn, grain sorghum and peanuts will reduce reniform numbers, they may increase root-knot numbers. Knowing the type of nematode present and the crop host status is important for planning the cropping sequence (Table 12.1). Plant-parasitic nematode types and population levels shift with the crop grown. Therefore, planting a rotation crop that is resistant to one species may increase another nematode species that will become the dominant pathogen. Sampling is important in understanding the dynamics of the different species present in a field. Weed populations in all crops must be controlled to eliminate nematode increases on weed host plants [19, 27]. It is common to find an increase in reniform numbers during the non-host corn and grain sorghum rotation since these nematodes feed on weeds often present late in the growing season [19].
If a resistant cultivar is available, do not grow it in the same field for two or more consecutive years. If a resistant cultivar is grown in the same field for multiple years, it allows for the selection of nematode strains that will be able to feed and reproduce on the resistant cultivar. The number of these nematodes will increase in the field and the resistant cultivar will no longer have any resistance to the nematode strain. This practice is common in soybean production where a resistant soybean cultivar is rotated with a non-host such as corn or grain sorghum, or a susceptible-but-tolerant soybean cultivar [9, 31]. The susceptible-but-tolerant cultivar allows the nematode population to increase but still produces an economically acceptable yield. Multiple cropping sequences are possible, including summer and winter crops.
Cover Crops
Winter cover crops are typically sown after the fall harvest with such goals as reducing soil erosion, competing with weeds, increasing soil organic matter and providing a niche for nematode-antagonistic microflora. Microflora may consist of fungi, bacteria and predatory nematodes. The most common winter-cover grain crops are rye, wheat and oats, while vetch and clovers are the typical legumes employed. These winter cover crops do not effectively suppress all plant-parasitic nematodes.
Many winter cover crops are hosts of plant-parasitic nematodes and may actually increase populations of nematodes for the summer crop when soil temperatures warm in the spring [10, 37]. Root-knot nematode numbers were lower on corn following rye and oat winter cover crops in Florida tests. Cowpeas, crotalaria, joint vetch, and sunn hemp were shown to be poor hosts to root-knot nematode and are good winter cover crops in Florida and the Gulf Coast region [23]. In Georgia, “AU Early Cover” hairy vetch and common hairy vetch increased root-knot nematode numbers and subsequent cotton-root galling [35]. Rye and Cahaba white vetch did not increase root-knot galling on cotton.
In Alabama, 31 winter cover crops were evaluated for reniform nematode management in cotton [10]. Crimson clover, subterranean clover and hairy vetch were determined to be good hosts for the reniform nematode. They could increase reniform numbers if spring soil temperatures are warm before cotton planting. Although, in field trials over two years, cotton yields were not affected by the winter cover crops as compared to winter fallow.
Winter cover crops benefit nematode management by competing with host weeds and increasing soil organic matter that supports nematode-antagonistic microflora. The common winter cover crops rye, wheat and oats compete with weeds, suppressing alternate hosts that can increase nematode numbers in the spring. These grasses also are suspected to increase the natural microflora that can suppress but not eliminate plant-parasitic nematodes. Suppression of the nematodes is not below the economic threshold levels of these pests. Practices in combination with cover crops are required to reduce nematode numbers below economic thresholds.
Reduced-Tillage Practices
Plant-parasitic nematode populations are affected by reduced-tillage practices but results have been inconclusive and differ between species. Nematode numbers are also known to be affected by soil type, soil moisture, location and host crop, which are all factors that interact with tillage practices. In soybeans, nematode populations reach their peak in conventionally tilled soybean monocultures. Further studies have reported that soybean cyst nematode’s J2 (the infective stage) numbers in the soil were highest in conventionally tilled soybean monocultures. Soybean cyst nematode numbers were reduced by natural fungal pathogens more frequently in no-till systems than in disked or chiseled tillage systems [4]. Rotations with any winter or summer crop reduced soybean cyst nematode numbers in these infested fields [34]. Lesion nematode numbers were reported to decline with reduced tillage or no-till as compared to conventional tillage [8]. However, reports from Georgia corn fields indicated nematode numbers were not affected by tillage [1]. Soil type may be a determining factor in nematode population potentials. Reniform nematode numbers were reported to decline with conventional tillage, although the mechanical stirring of the soil facilitates the nematode’s spread across the field [11, 36]. Root-knot in corn may not always be affected by tillage systems, although tillage in the spring and fall reduced root-knot numbers in corn compared to a no-till or ridge-till system [22, 36]. However, lesion nematodes in these studies had greater numbers in tilled soils. In addition, common crop production practices will not reduce the populations to levels that will eliminate plant injury and yield reductions. Uprooting crops or turning the soil after harvest exposes nematodes to sun, and the drying reduces their numbers. Tillage and late-season applications of herbicides kill the regrowth of cotton and late-season weeds. These practices are effective in reducing the overwintering population. Note that organic residues increase the microbial biomass of the soil, which then increases the soil microflora, natural predators of soilborne nematodes [21]. No-till systems increase microbial biomass. Thus, as the soil organic matter increases from reduced tillage, plant-parasitic nematode numbers may decrease.
Nematicides
Nematicides are defined as chemicals that kill nematodes. They first became widely and economically available in 1943 with the discovery that a mixture of 1, 3-dichloropropene and 1, 2-dichloropropane was effective in controlling plant-parasitic nematodes [20]. This was coupled with an increase in crop yields. Ethylene dibromide and dibromochloropropane were reported in 1945 and 1954, respectively, to be effective in the management of root-knot nematodes. These discoveries led to the subsequent increases in the use of halogenated hydrocarbons and other volatile compounds for nematode management. In the late 1960s, volatile compounds were followed by a new generation of nematicides. These included the carbamates and organophosphates that were non-volatile and easier to apply. These compounds generally are active against both insects and nematodes, depending on the distribution of the material around the root.
There are four main chemical groups of nematicides: the halogenated aliphatic hydrocarbons, methyl isothiocyanate compounds, organophosphates and carbamates. More recently an additional group of nematicides have been introduced [17]. These are biological products that exhibit nematicidal activity toward plant-parasitic nematodes.
Nematicides can be further subdivided into two broad categories based on movement through the soil. The fumigant nematicides that include methyl bromide, chloropicrin, 1, 3-dichloropropene and metam sodium are chemicals that are formulated as liquids and vaporize after application. The gas moves through the soil pores and mixes with the soil moisture film surrounding the soil particles. The second category is the non-fumigant nematicides. These nematicides are either liquid or granular and move downward in the soil with water percolation. They may be contact or systemic nematicides. Contact nematicides kill nematodes by contact. Systemic nematicides are taken up by the plant and affect the nematodes when they feed. The non-fumigant nematicides include products such as Meymik, Mocap, Vydate and Counter.
Due to the inherent toxicity of nematicides to animals and the environment, only use them when other options are not available. This would include situations where there is a lack of cultivars with resistance or where crop rotation is not economically feasible. Nematicides also have their limitations. Nematicides do not give 100 percent nematode control [17]. Use them with crop rotations and other management practices in a total nematode management program [6].
Base the decision to use a nematicide on a strategy to reduce the initial nematode inoculums, to reduce the rate of nematode development, and to reduce the population density increases on the host plant [11, 17]. The initial nematode population is the nematode population that is in the soil from the previous year’s crop. They survived the winter and serve as the primary inoculums for the current year’s crop. The use of a pre-plant fumigant nematicide (Telone II, Vapam, Kapam), in-furrow products (Meymik, Counter, Velum Total) or seed treatment nematicides (Avicta, Aeris, Votivo, N-Hibit) at the time of planting are effective in reducing the initial nematode inoculum (Table 12.2).
Reducing the rate of nematode development during the season can be accomplished with post-plant nematicide applications. These are applied after the plant has sufficient leaf and root mass to allow root uptake of the product from the soil (side-dress Meymik). For foliar sprays (Vydate C-LV), foliar absorption is followed by downward translocation to the roots where it will affect the nematode’s feeding activity [17]. Each of the products mentioned will vary in their effectiveness. Consult local county and state agricultural officials to determine if the nematicide will work in a region. Microbial degradation has occurred with some nematicides so continuous use of a single nematicide is not advisable and may reduce its efficacy [16].
Variable-Rate Nematicide Applications
Precision agriculture has become a widely accepted practice in the Southeast. One important aspect of the technology is variable-rate applications of nematicides. In the field, plant-parasitic nematodes generally have a non-uniform, clustered spatial distribution [14]. The distribution varies with nematode species, soil texture and the crop grown. Variable-rate application or site-specific application is the application only to the areas where the nematode population has reached the economic threshold.
To implement a successful nematode management program, the nematodes present in the field and their location must be determined [14,15]. This is accomplished by collecting samples from a uniform, systematic grid across the field or through the use of zone sampling [26]. Zone sampling creates zones or areas of similarity from which samples are collected. Soil texture is one criterion for obtaining points from similar areas. Different nematode genera favor different soil textures, so soil texture will influence the damage resulting from infection. Each sample point is geo-referenced using a global positioning system (GPS). This type of sampling is popular because it maps the spatial information for a specific nematode pest [26].
One drawback to grid sampling is that it is time consuming and can be costly. Each sample must be processed to identify the nematode species present and the estimated total number present. Once the nematode population numbers are located and mapped, nematode contour maps can be developed to graphically represent nematode numbers in a field. The map can be overlaid with yield maps to determine problem areas in the field. Poor crop yields in combination with high nematode numbers are good indications that areas may require nematicide applications. A nematicide prescription map and predetermined application rates are then loaded into the application equipment’s computer. The specified amount of nematicide is applied to the selected areas as the equipment moves across the field. To monitor that the correct dose is delivered, an as-applied map is created during application.
A representative number of soil samples is the key to success for any nematode management program. This becomes essential for variable-rate application of nematicides. The smaller the sample grid size (0.025–0.5 acre), the more detailed the nematode distribution map, resulting in better placement of the nematicide [14]. However, the more samples, the higher the laboratory cost to process them.
Remote sensing is being examined as a way to detect and estimate plant-parasitic nematodes associated with crops [14]. Remote sensing is the characterization of an object without coming into physical contact with it. The technique results in contour maps that represent a nematode’s spatial distribution in a chosen area. From there, prescription maps are prepared. As the application equipment travels across the field, the rates are adjusted for each nematode management zone. These types of variable-rate applications are based on nematode population numbers.
The second means of variable-rate nematicide applications are based on soil textures and soil electrical conductivity [26]. Electrical conductivity is the ability of a material to conduct an electrical current, in this case soil. Recent studies have demonstrated that nematicides have been less effective in field locations with high clay content. Soil electrical conductivity data is collected utilizing a Veris soil electrical conductivity mapping system. The Veris cart is used in conjunction with a GPS receiver to georeference the collected data. The sensors measure a shallow soil electrical conductivity, 0–12 inches, and a deep soil electrical conductivity, 0–36 inches, and then store the data in the operating console. The data collected is converted to shape files for each soil depth and classed by specific electrical conductivity ranges. A nematicide prescription map is then developed based on management zones representing the electrical conductivity ranges. Preliminary results have shown that less nematicide is applied to the zones with higher electrical conductivity values.