Nematodes are microscopic, unsegmented roundworms that inhabit the soil in vast numbers. A single teaspoon of healthy soil can contain hundreds of these organisms. Most soil nematodes are beneficial, feeding on bacteria, fungi, or other soil microorganisms, and are thus helpful to plant health. However, a small but destructive group known as plant-parasitic nematodes possess a specialized needle-like mouthpart, called a stylet. They use this stylet to pierce plant cells and feed on root tissue, causing significant damage to gardens and crops. This article focuses exclusively on strategies to eliminate these plant-parasitic types.
Identifying the Infestation
Successful management of a nematode problem begins with accurate diagnosis, as above-ground symptoms often mimic issues like nutrient deficiency or drought stress. Plants infested with parasitic nematodes commonly exhibit symptoms such as wilting during the hottest part of the day, general stunting, or chlorosis (yellowing of the foliage). These generalized symptoms occur because the feeding activity damages the root system, impairing the plant’s ability to absorb water and nutrients efficiently.
The most definitive evidence of an infestation lies beneath the soil surface, requiring the careful excavation of roots for inspection. Root-knot nematodes (Meloidogyne spp.) cause characteristic swellings or galls on the roots, which are abnormal growths resulting from the permanent feeding site. Other types, such as lesion nematodes (Pratylenchus spp.), cause darkened, dead tissue spots, while migratory ectoparasites may result in stubby, pruned root tips. To confirm the presence, species, and population density of harmful nematodes, a professional soil sample analysis by a diagnostic laboratory is necessary before selecting a control method.
Non-Chemical Control Methods
Cultural and physical practices are often the first line of defense against plant-parasitic nematodes, offering sustainable and accessible methods for home gardeners. One effective physical treatment is soil solarization, which utilizes the sun’s radiant heat to disinfest the topsoil layer. This process involves thoroughly wetting the soil, removing all vegetation, and then tightly covering the area with a clear, thin plastic sheeting. The plastic traps solar energy, raising the temperature of the top six to eight inches of soil to levels often exceeding 140°F, which is lethal to many nematodes and soilborne pathogens after four to six weeks in hot, sunny weather.
Rotating susceptible crops with non-host plants is an effective management technique that starves the nematode population by depriving it of a food source. Growing specific non-host grains like rye or certain cultivars of French marigold (Tagetes patula) can significantly reduce populations of root-knot nematodes. Marigolds suppress nematodes because their roots release chemical compounds, known as thiophenes, that are toxic to the pests. This rotational strategy requires planting the non-host crop in the infested area for an entire growing season to achieve a meaningful reduction in pest numbers.
Integrating specific organic soil amendments can create an unfavorable environment for nematodes through chemical and biological action. The addition of chitin, a complex carbohydrate found in crustacean shells, stimulates soil microbes to produce chitinase enzymes that break down nematode eggshells. Incorporating mustard seed meal or other Brassica plant residues into the soil releases volatile compounds called isothiocyanates upon decomposition. These naturally occurring chemicals act as biofumigants, suppressing nematodes and offering a non-synthetic alternative to chemical fumigation.
Utilizing Biological Controls
Biological controls harness living organisms to manage plant-parasitic nematode populations, offering a targeted and environmentally conscious approach. Certain beneficial fungi and bacteria are commercially available and can be introduced into the soil to actively parasitize or inhibit pest nematodes. Fungi such as Paecilomyces lilacinus target and destroy nematode eggs and sedentary adult females by penetrating their protective structures. Bacterial antagonists, notably species of Bacillus, produce nematicidal compounds, including lipopeptides and chitinase enzymes, that dissolve the nematode’s cuticle and disrupt their life cycle.
Some beneficial nematodes can be used as a biological control agent, although these species target insects, not plants. These entomopathogenic nematodes, primarily from the genera Steinernema and Heterorhabditis, are applied to the soil to hunt and infect soil-dwelling insect pests, such as grubs. Once inside the insect host, these predatory nematodes release symbiotic bacteria, which produce toxins that rapidly kill the host. While they do not directly attack plant-parasitic nematodes, their introduction supports a healthier soil ecosystem by controlling other pests and increasing the overall biological diversity that naturally suppresses harmful nematodes.
Nematicide Application
Chemical nematicides represent a strong management option, typically reserved for severe infestations where cultural and biological controls have proven insufficient. These agents are broadly categorized as fumigants, which are volatile chemicals that diffuse through the soil to kill nematodes, or non-fumigants, which act by contact or are absorbed systemically by the plant. Fumigants, such as 1,3-dichloropropene, are tightly regulated due to their toxicity and are generally restricted to commercial applicators.
For the general public, products based on naturally derived compounds offer a lower-risk chemical solution. These include botanical extracts like azadirachtin, derived from neem seeds, which disrupts the nematode’s growth and reproduction. Newer, less volatile synthetic non-fumigants are sometimes available and provide a more targeted kill with reduced environmental impact compared to older chemicals. Regardless of the product type, nematicides demand strict adherence to all label instructions, including application rates and the use of personal protective equipment, due to the inherent toxicity of these agents.