Nematodes are roundworms found in nearly every environment, including soil. Most are harmless or beneficial, feeding on bacteria, fungi, or other pests. However, plant-parasitic nematodes (PPNs) use a specialized, needle-like mouthpart called a stylet to puncture plant cells and feed. This damage poses a significant agricultural threat, causing billions of dollars in crop losses annually for both commercial growers and home gardeners.
Recognizing a Nematode Infestation
Identifying a nematode infestation is challenging because symptoms often resemble nutrient deficiencies, drought stress, or other root diseases. Above-ground indicators include general stunting, yellowing of the leaves (chlorosis), and premature wilting, especially during hot periods.
Below-ground symptoms are more definitive, involving physical changes to the roots. Root-knot nematodes, the most common type, cause irregular swellings or galls on the root structure. Other PPNs can cause root lesions, excessive branching (“hairy root”), or the formation of visible cysts. Since visual assessment is insufficient for treatment planning, definitive identification requires submitting a soil sample to a professional diagnostic laboratory. This analysis confirms the specific nematode species and population density, which determines if intervention is warranted.
Cultural and Physical Control Methods
Modifying the soil environment through cultural and physical methods manages nematodes by making the soil inhospitable or disrupting their life cycle.
Soil Solarization
Soil solarization uses the sun’s heat to disinfect the topsoil. This involves covering moist, tilled soil with a clear plastic tarp during the hottest months for four to six weeks. The plastic traps solar energy, raising the soil temperature high enough to kill PPNs and their eggs in the top six to eight inches. Temperatures of 52°C for 30 minutes can be lethal to root-knot nematode eggs.
Cultural Practices
Crop rotation is a cultural practice that starves nematodes by planting non-host crops in an infested area for a season or more. Non-host examples for common root-knot nematodes include corn, small grains, garlic, and specific legumes.
Increasing the soil’s organic matter content is also beneficial. Incorporating compost or manure improves soil structure and water retention, helping plants tolerate damage. High organic matter also fosters beneficial microflora that naturally suppress PPN populations. Fallow periods can also limit PPN populations by interrupting their ability to feed and reproduce.
Utilizing Biological and Targeted Interventions
Targeted interventions use specific biological agents or resistant plant genetics to manage nematode populations. Biological controls utilize naturally occurring organisms to attack or suppress PPNs. A prominent example is the fungus Purpureocillium lilacinum, a soil-dwelling parasite.
This beneficial fungus actively seeks out and parasitizes nematode eggs and juveniles. It attaches to the life stages and secretes powerful enzymes, such as chitinase, that break down the protective barriers of the egg shell and cuticle. Applied as a soil drench, the fungus establishes itself and provides long-term nematode suppression.
Resistant plant varieties are an effective targeted intervention. These cultivars are bred to either limit nematode reproduction (resistance) or tolerate their presence while still producing an acceptable yield (tolerance). Seed packets often use codes, such as the letter ‘N’, to indicate resistance against common species like Meloidogyne.
Another specialized technique is biofumigation, which uses specific cover crops to release natural nematicidal chemicals. Certain Brassica plants, such as mustards and oilseed radish, contain high levels of glucosinolates. When the plant material is incorporated into the soil, the glucosinolates break down and release toxic isothiocyanates (ITCs). These ITCs act as a natural, short-term fumigant that suppresses PPN populations.
Limitations of Chemical Nematicides
The use of chemical nematicides is restricted, especially for home garden use. Many older compounds, including fumigants like 1,3-dichloropropene, are classified as restricted-use pesticides due to their high toxicity and environmental concerns. These chemicals pose risks to beneficial soil organisms and can contaminate groundwater.
For most home applications, chemical nematicides are unavailable or impractical to apply safely. Commercial use requires strict regulatory oversight, specialized equipment, and licensed applicators. Furthermore, chemical treatments are often expensive and provide only short-lived control, necessitating annual reapplication.