The Southern Root Knot Nematode

By Amy Dismukes, Nursery Extension Area Specialist, TSU, Otis L. Floyd Nursery Research Center

Plant parasitic nematodes are tiny, microscopic, “worm-like” animals (Fig. 1) that live in the soil and plant tissues. In Tennessee, the most troublesome species to the nursery industry are those that feed on plant roots and live in the soil. These nematodes can be challenging to diagnose, difficult to control, and can be easily spread from site to site in soil that may be carried on tools, on equipment and tires, on mud-caked boots, and within or on infested plant material.

Fig. 1. Newly hatched (2nd stage) juvenile root knot nematodes

(Photo Credit, Alan Windham)

Fig. 1. Newly hatched (2nd stage) juvenile root knot nematodes

(Photo Credit, Alan Windham)

The Southern root knot nematode (RKN), Meloidogyne incognita, is a major pest in nurseries. RKN can infest the roots of many woody ornamentals including, but not limited to, boxwood (Buxus spp.) (Fig. 2), catalpa (Catalpa speciosa), cedar (Cedrus spp.), Japanese quince (Chaenomeles spp.), euonymus (Euonymus spp.), gingko (Ginkgo biloba), Althea (Hibiscus syriacus), hydrangea (Hydrangea macrophylla), Chinese holly (Ilex cornuta), jasmine (Jasmium primulinum), walnut (Juglans spp.), mulberry (Morus spp.), nandina (Nandina domestica), stone fruit (Prunus spp.), fire thorn (Pyracantha spp.), oak (Quercus spp.), and viburnum (Viburnum spp.).

Fig. 2. Field-grown boxwood shrubs infested by root knot nematodes may present symptoms similar to water stress or nutrient deficiency

(Photo Credit, A. Dismukes)

Root knot nematodes damage plant roots through their feeding activity, which is achieved by piercing root cells using a needle-like mouthpart called a stylet. Once attached to the root, feeding activity results in gall and clubbed-root formation on root tissues (Fig. 3). When nematode levels become high enough (Fig. 4), gall induction can disrupt root function. The extent of plant damage depends on the environment, nematode species, populations within the plant following reproduction, and host plant tolerance.

Fig. 3. Galled and clubbed root tips caused by root knot nematode feeding on root tissues

(Photo Credit, A. Dismukes)

Fig. 4. To see galls on infected plant roots, wash soil or substrate from roots. Root tips may break with rough handling, but galls are not easily rubbed off by hand.

(Photo Credit, A. Dismukes)

Root knot nematodes develop from egg to adult through six life stages that can be completed within 21 to 28 days. RKN deposit eggs in the soil (Fig. 5), and after the eggs hatch, the 2nd-stage juveniles infest root tips. Within the infected root, cells enlarge and galls are formed (Fig. 6). Immature and adult male RKN remain vermiform (worm-like) and will eventually leave the roots. Adult RKN females expand into a swollen, pear shape and remain within the root, depositing eggs into the soil through the root surface. Once in the soil, the reproductive cycle continues until the environment becomes unfavorable. Adult female RKN and root knots are visible without a microscope, however, juveniles and males are too small to be seen without additional magnification.

Fig. 5. A mass of root knot nematode eggs being deposited into soil

(Photo Credit, Alan Windham)

Fig. 6. A close up of cleaned boxwood roots shows galls formed in response to nematode feeding.

(Photo Credit, A. Dismukes)

Symptoms

Because above-ground symptoms of RKN activity are not always obvious in colonized plants, infection and reproduction can go undetected. Symptoms of RKN infestation and feeding can be confused with plants that have impaired root function resulting from other causes. Appearance often mimics drought stress, nutrient deficiency, or stress associated with root pathogens. In above ground plant tissues, visible symptoms to the plant include wilting (particularly when soil moisture is adequate), loss of vigor, bronzing and yellowing of leaves (Fig. 7), etc. Plants will produce fewer and smaller leaves and fruits. Plants will not grow as quickly as uninfested plants and may defoliate earlier than normal. Under heavy RKN infestation plants can die. Damage is often worst in warm, irrigated, sandy soils.

Fig. 7. Bronzing and yellowing of foliage is characteristic of root knot nematode infection on a heavily infected boxwood plant.

(Photo Credit, A. Dismukes)

Injury to infected plant roots can be more difficult to diagnose. In extreme cases, root galls can grow up to 1-inch in diameter on some host plants, but galls are usually much smaller. Unlike nitrogen-fixing nodules on legumes, RKN galls are true swellings that cannot be rubbed off by hand. In addition to damaging the conductive abilities of infected plant roots, galls can predispose host plants to infection by other soil-borne pathogens, thereby increasing potential losses in susceptible crops.

Diagnosis and Detection

If you believe that your crops may be infected with nematodes, contact the TSU Nursery Research Center in McMinnville, Extension agents, or specialists with the University of Tennessee Soil, Plant & Pest Center. An appropriate specialist can help conduct assays using plant tissue, root, and soil samples to identify and positively confirm nematode presence. The best time to monitor nematodes in soils is immediately before or at crop harvest, when seasonal nematode populations have reached their peak. Samples collected in the spring will normally have reduced levels of nematodes due to unfavorable reproductive conditions, but if damage is severe, may be capable of detecting presence of RKN.

Management and Control

Once RKN are diagnosed, growers and landscape managers may have several options to assist them in limiting further damage to crops and landscape specimens. Root knot nematode can be difficult to manage long-term with chemical nematicides because nematodes persist in soil and can infect a wide range of host plants. In production systems, fumigants can be applied as pre-plant treatments to reduce populations. To be effective, however, the fumigant must penetrate a large volume of soil. Many fumigants volatilize quickly, so treated soil must be covered to retain the nematicide after application. Nervous system toxicants, like carbamate and organophosphate products, are not toxic to plants and may provide a measure of control against RKN, but these broad-spectrum pesticides are extremely toxic to non-target organisms and also present threats to health of applicators. When misapplied, nervous system toxicants can cause significant damage to the human nervous system.

Biological control methods are an important strategy in effective nematode management. There are many kinds of nematophagous (nematode-eating) fungi, plus beneficial bacterial antagonists that are usually present already within a healthy soil. Some fungal species rely on spores to capture RKN, others have specialized structures that develop on hyphae, while other species directly penetrate nematode surfaces and can parasitize eggs and females. Pasteuria penetrans is a bacterial RKN antagonist that attaches to the cuticle of a juvenile nematode and then consumes it. Although a number of commercial biocontrol agents are available for RKN management, they may not be economically feasible for treating large production areas. To encourage these nematode antagonists in field operations and landscapes, proactive nursery growers and landscape managers should focus on maintaining optimal soil health with high organic content, which tend to have lower RKN populations.

Cultural control methods to manage nematode populations are potentially the most successful method for limiting RKN damage. Successful cultural control benefits from careful planning. If the grower can identify an alternative non-host plant species, or a resistant crop that provides a high economic return, crop rotation can be very successful. RKN will increase the damage potential of plants that are water-stressed. Frequent irrigation can also help reduce damage. Populations will still be present but they will cause less damage. Cover crops provide organic matter to soil that will also help to reduce nematode populations. Cover crops can be grown in a fallow year or outside of the normal growing season for nursery crops. Some cover crops are directly antagonistic to nematodes. For example, both sudangrass and marigolds produce chemicals that are toxic to nematodes, while also providing the added benefits of conserving topsoil and improving soil quality. In warm climates and when a field can be removed from cultivation for long periods, flooding and soil solarization have also been used to control nematodes. Finally, peat, aged manure, and composts can be added to manage good soil health. These amendments provide organic matter and are useful for increasing the water and nutrient holding capacity of soils. Taken together, these steps all lessen the impacts of nematode injury.

A combination of management tactics and tools can help reduce RKN to population levels that do not exceed acceptable damage thresholds. It is important to remember that once RKN are present in a soil system, RKN will always be present; eradication efforts will not be successful. Once a susceptible host plant is transplanted and begins to grow in infested soils, however, its roots will support RKN reproduction, which in turn will result in expanding nematode populations and a greater infection rate. In RKN infested habitats, the management goal shifts toward actions that optimize plant health and reduce plant and crop damage from nematodes.