Nematodes: Management Guidelines for Kansas Crops

Timothy C. Todd: Nematologist, Plant Pathology
Douglas J. Jardine, Extension Specialist, Plant Pathology
Cooperative Extension Service, Kansas State University, Manhattan

Nematodes are nonsegmented, usually microscopic roundworms that inhabit soil, water, and plant tissues. The average acre of soil in Kansas contains more than 10 billion of these tiny invertebrates. Many are beneficial, contributing to decomposition and nutrient cycling, but typically one-third to one-half of the population are obligate herbivores. These nematodes feed on plant tissues using a hollow, needle-shaped stylet to puncture plant cells (Figure 1). Following injection of digestive enzymes that are frequently phytotoxic, ingestion of the cell contents occurs.

 Figure 1. General morphology of a plant-parasitic nematode.

Most plant-parasitic nematodes inhabit the soil or plant roots but a few types occupy aboveground parts of plants. Nematodes that remain in the soil and feed at or below the root surface are referred to as ectoparasites, while those that penetrate plant tissues to feed internally are called endoparasites (Figure 2).

Figure 2a. Sting nematodes are migratory ectoparasites that selectively feed on root tips. Since they do not enter the roots, recovery is from the soil (photograph courtesy of C. C. Russell).

Figure 2b. Lance nematodes are migratory semi-endoparasites that partially penetrate root tissue and can be recovered from both roots and soil (photograph courtesy of C. C. Russell).

Figure 2c. Lesion nematodes are migratory endoparasites that feed and lay their eggs in the root cortex. During the growing season, most of the populations will be recovered from roots.

Figure 2d. Cyst nematodes are sedentary semi-endoparasites. Adult females become swollen and immobile with most of the eggs retained within the body of the mature female (cyst) which is visible on the root surface. Juveniles and cysts can be recovered from the soil.

Figure 2e. Root-knot nematodes are sedentary endoparasites. Adult females become swollen and immobile and are typically encolosed in root galls with eggs deposited in a gelatinous mass within the gall or on the root surface. Juveniles are recovered from the soil.

Symptoms of nematode injury include reduced plant growth and vigor, with characteristic root abnormalities sometimes present (Figure 3). Most above ground symptoms result from reduced nutrient and moisture availability due to damaged roots and consequently are often attributed to fertility-or drought-related stresses. Yield losses may also occur in the absence of distinctive symptoms. Therefore, diagnosis of a nematode problem must ultimately be based on the types and numbers of nematodes present.

Figure 3a. Irregular patches of severely stunted corn plants are common in sandy soils infested with sting or needle nematodes.

Figure 3b. Uneven, chlorotic stands of soybeans are typical of cyst nematode damage. Symptoms may resemble nitrogen or potassium deficiencies.

Figure 3c. Rapid wilting and death results from infestations of susceptible pine trees by the pinewood nematode.

Figure 3d. Stubby, discolored root tips with excessive branching and few feeder roots result from feeding by sting nematodes. Similar types of root pruning (generally with less discoloration and more swelling of the root tips) are associated with stubby-root and needle nematodes as well as aluminum and herbicide toxicity.

Figure 3e. Root lesions can develop following feeding by several nematode species. Secondary invasion by root-rotting fungi frequently contributes to lesion enlargement and severity (photograph courtesy of C. C. Russell).

Figure 3f. Root galls are produced primarily by root-knot nematodes, although swellings of root tips can result from feeding by other nematodes.

Tolerance Limits and Economic Thresholds

The extent of crop damage generally depends on the number of plant-parasitic nematodes present at planting time. Most plants will tolerate low to moderate levels of many nematode species. The number of nematodes below which damage does not occur is referred to as the tolerance limit or damage threshold. Above the tolerance limit, damage increases and crop yield decreases in a linear fashion as the number of nematodes increases.

 The population density of the nematode at which predicted yield loss is sufficiently high to warrant a management response (normally 5-10 percent) is considered the economic threshold Tolerance limits and economic thresholds not only vary among nematode species but also are affected by soil texture and other environmental conditions.

 In general, tolerance limits will be lower in coarse-textured soils, such as alluvial sands, due to the inherently limited availability of moisture and nutrients. Also, since most fields contain several species of plant-parasitic nematodes, damage may reflect interactions among nematode species as well as associations with other root pathogens.

 Tables 1 and 2 list threshold levels and recommended management responses for plant-parasitic nematodes associated with the major crops in Kansas.

 Threshold levels are provided to assist interpretation of nematode assay results from the KSU Plant Disease Diagnostic Clinic or other laboratories that have been adjusted to reflect extraction efficiencies.

 Thresholds are based on field research in Kansas but they are not absolute and are subject to change as new data emerge. They apply primarily to population densities immediately before planting. Populations increase as the growing season progresses, and high numbers of nematodes at the end of the season are not necessarily associated with significant yield loss. Population densities for the following season can be estimated from late-season assays if overwinter survival potential is known. Average survival rates in Kansas range from 20 percent for lesion nematodes to 80 percent for the soybean cyst nematode.

Table 1. Nematodes associated with turfgrasses, vegetables, and ornamentals in Kansas.

Table 2. Nematodes associated with agronomic crops in Kansas.

Sampling for Nematode Assay

 There are two basic objectives for nematode sampling at the commercial level in Kansas: (1) diagnosis of a nematode problem and (2) advisory purposes. Separate sampling strategies required for each category follow. In all cases, samples should consist of pint-sized volumes of soil and associated roots or foliage (see Tables 1 and 2) in sealed plastic bags.

 Promptly submit samples with all relevant information to a County Cooperative Extension Office or the KSU Plant Disease Diagnostic Clinic.

 Detailed information on nematode sampling and assay procedures are available from County Extension Offices (refer to the Extension Fact Sheet Sampling Nematode Populations). There is a nominal fee of $10 per sample to help offset laboratory expenses.

Diagnosis: A comparison of nematode populations from healthy and affected areas is essential for a reliable diagnosis of nematode damage. Collect several soil and root samples to a depth of 6 to 8 inches near the periphery of damaged areas as well as from nearby healthy areas. Soil cores are then bulked by area, mixed, and subsampled. Post-plant management options are limited.

Advisory: Each field of uniform soil type and cropping history should be divided into quadrants. From each section, submit a sample consisting of at least five soil cores taken to a depth of 6 to 8 inches. This type of sample will normally be collected before planting, but results from late-season samples may also be useful as a basis for management recommendations for the following season.

Nematode Management

Management strategies for nematodes generally are designed to reduce population levels below damage thresholds since eradication is rarely feasible in field situations. The most effective and economical practices include: (1) crop rotation when the target nematode has a limited host range and (2) resistant varieties when available. Cultural practices, such as maintaining good soil fertility and moisture are necessary to maximize yield potentials and may increase crop tolerance levels.

 Nematicides also can be effective in reducing nematode populations, and although control is short-lived (nematode populations at the end of the season are often greater than in nontreated areas), crops will usually have sufficient time to establish healthy root systems.

 For information on registered nematicides, see Extension publication MF-1038, Nematode Management in Kansas: Chemical and Biological Control.

Cooperative Extension Service, Manhattan, Kansas MF-1063 May 1993
Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State University, County Extension Councils, and United States Department of Agriculture Cooperating, Walter R. Woods, Director. All educational programs and materials available without discrimination on the basis of race, color, national origin, sex age, or handicap. 5-93- 2M
File Code: Plant Pathology-5,4