Unit 6: Nematode Management - Subjective Questions

Nematode management is the planned use of compatible practices to keep plant-parasitic nematode populations below the economic damage threshold while maintaining crop productivity and environmental safety.

Major principles:

• Correct diagnosis: Identify the nematode species, estimate its population density and determine its distribution through representative soil and root sampling.
• Prevention: Avoid introducing nematodes through infested planting material, soil, irrigation water, tools and machinery.
• Population reduction: Use crop rotation, resistant varieties, organic amendments, biological agents, physical treatments and need-based nematicides.
• Host avoidance: Grow non-host crops, adjust planting dates or use fallow periods to prevent nematode multiplication.
• Protection of planting material: Treat seeds, seedlings, tubers, bulbs and nursery soil before planting.
• Monitoring: Compare initial and final nematode populations. The reproduction factor is expressed as , where is the initial population and is the final population.
• Integration: Combine complementary methods rather than relying on a single tactic.
• Economic and ecological considerations: Select practices that are affordable, practical, residue-safe and compatible with beneficial organisms.

The objective is generally management rather than complete eradication, because nematodes are difficult to eliminate from infested fields.

Crop rotation involves growing a sequence of crops with different host responses so that a target nematode is deprived of a suitable host for one or more seasons.

Mode of action:

• Non-host or poor-host crops prevent or greatly reduce nematode reproduction.
• Natural mortality lowers the population during the absence of a suitable host.
• Certain rotation crops may release suppressive compounds or support antagonistic microorganisms.

Factors determining success:

• Host range: Rotation is most effective against nematodes with a narrow host range and less effective against highly polyphagous species.
• Correct identification: The nematode species or race must be known because crops differ in susceptibility.
• Duration: Long-lived eggs or survival stages may require rotations lasting several seasons.
• Weed control: Susceptible weeds and volunteer crops can maintain the nematode population.
• Cropping sequence: A poor-host crop must not be followed by another crop that multiplies the same nematode.
• Economic suitability: Rotation crops should be locally adapted and economically acceptable.
• Population monitoring: Soil and root samples should be examined before planting a susceptible crop again.

Thus, rotation works best when supported by sanitation, weed control, resistant cultivars and periodic nematode assessment.

Field sanitation, ploughing and fallowing reduce nematode survival and prevent their spread.

• Field sanitation:

  • Remove and destroy heavily infected roots, tubers, bulbs and crop residues.
  • Eliminate volunteer plants and weeds that serve as alternate hosts.
  • Clean soil from farm tools, machinery, footwear and containers before moving them between fields.
  • Use nematode-free nursery plants and clean irrigation water.

• Deep summer ploughing:

  • Exposes infected roots, egg masses and nematode stages to sunlight, heat and desiccation.
  • Breaks and dries crop residues that may protect endoparasitic nematodes.
  • Improves the effect of subsequent solarization or organic amendment application.

• Fallowing:

  • Keeps land free from suitable host plants for a defined period.
  • Causes population decline through starvation and natural mortality.
  • Requires strict weed control because weeds can maintain nematode populations.

These practices rarely eradicate nematodes, but they lower inoculum and improve the effectiveness of other management measures.

Organic and crop-based practices suppress nematodes through several direct and indirect mechanisms.

Organic amendments:

• Farmyard manure, compost, oil cakes and crop residues improve soil structure and microbial activity.
• Their decomposition may release ammonia, organic acids and other compounds toxic or repellent to nematodes.
• They support predatory and parasitic microorganisms and may improve plant tolerance through better nutrition.
• Only well-processed, contaminant-free materials should be used to avoid phytotoxicity or pathogen introduction.

Green manures and cover crops:

• Poor-host species prevent multiplication of the target nematode.
• Brassicaceous residues may release isothiocyanates during decomposition, a process called biofumigation.
• Marigold and some other plants produce compounds that suppress particular nematodes.

Trap crops:

• A trap crop stimulates nematode penetration or egg hatch but is destroyed before the nematode completes reproduction.
• Correct timing of crop destruction is essential; otherwise, the trap crop may increase the population.

Effectiveness depends on the nematode species, amendment dose, soil temperature, moisture, decomposition rate and local cropping system.

Crop management practices can modify nematode activity, plant exposure and the ability of plants to tolerate injury.

• Planting time: Sowing may be adjusted so that vulnerable crop stages do not coincide with temperatures most favorable for nematode penetration and reproduction.
• Early harvesting: Short-duration crops may be harvested before several nematode generations are completed.
• Irrigation: Proper irrigation reduces drought stress and helps injured roots function, whereas movement of contaminated water can spread nematodes. Waterlogging may also damage roots and increase disease complexes.
• Nutrient management: Balanced fertilization promotes root regeneration and improves tolerance. Excessive or unbalanced nitrogen should be avoided because it may increase susceptibility or reduce crop quality.
• Intercropping: Including non-host or antagonistic plants may reduce the density of susceptible roots and influence soil microbial communities.
• Weed management: Weeds must be controlled because they may act as hidden hosts even when the main crop is resistant or non-host.

These methods primarily reduce damage or reproduction and should be combined with resistant cultivars, sanitation and population monitoring.

Soil solarization is a hydrothermal method in which moist soil is covered with transparent polyethylene during the hottest period to trap solar radiation and raise soil temperature.

Procedure:

1. Prepare a fine, level seedbed and remove undecomposed debris.
2. Irrigate the soil to near field capacity because moisture improves heat conduction and stimulates vulnerable biological activity.
3. Cover the soil tightly with thin, transparent polyethylene and seal the edges.
4. Maintain the cover, commonly for several weeks during intense sunshine.
5. Remove the sheet carefully and avoid bringing untreated subsoil to the surface.

Effects:

• Elevated temperatures kill or weaken eggs, juveniles and adults in the heated soil layer.
• Heat may also suppress soil-borne pathogens and weed seeds.
• Changes in soil microorganisms can produce a period of enhanced biological suppressiveness.

Factors affecting effectiveness:

• Solar intensity, ambient temperature and treatment duration
• Soil moisture, texture and heat conductivity
• Thickness and transparency of the sheet
• Depth at which nematodes occur
• Quality of sealing and absence of tears

Solarization is most suitable for nurseries, seedbeds and warm regions, but it is less effective at greater soil depths.

Heat treatments are designed to expose nematodes to lethal temperatures without seriously injuring the plant or planting material.

• Hot-water treatment: Seeds, bulbs, tubers, corms, roots or planting stocks are immersed in water maintained at a prescribed temperature for a specific period. It is useful against nematodes carried inside or on propagative material.
• Steam treatment: Steam is applied to nursery soil, greenhouse beds, potting mixtures or equipment. It penetrates soil and destroys nematodes, pathogens and many weed propagules.
• Dry heat: Hot air may be used for selected seeds, tools or storage materials, although it generally penetrates tissues more slowly than hot water or steam.

Precautions:

• Temperature and exposure time must be standardized for each crop and nematode.
• Plant material should be graded by size to ensure uniform heating.
• Overheating may reduce germination or damage tissues, whereas insufficient heating permits nematode survival.
• Treated material must be protected from recontamination.

Heat treatment is particularly valuable for high-value planting material and nursery media, where treatment can be applied uniformly.

Cultural methods alter crop production practices or the cropping environment to reduce nematode reproduction and damage. Examples include crop rotation, sanitation, fallowing, weed control, organic amendments, trap cropping and adjustment of planting time.

Physical methods use physical agents or barriers to kill, remove or exclude nematodes. Examples include soil solarization, hot-water treatment, steam sterilization, dry heat and physical removal of infected roots.

Major differences:

• Cultural methods act mainly through host deprivation, habitat modification or improved plant tolerance.
• Physical methods act mainly through lethal temperature, desiccation or physical exclusion.
• Cultural methods are generally suitable for large fields but often act slowly.
• Physical methods can provide rapid disinfestation but may be expensive or practical only for nurseries, protected cultivation and planting material.

Advantages:

• Both methods reduce dependence on chemical nematicides.
• They are generally residue-free and compatible with integrated management.

Limitations:

• Cultural practices require accurate knowledge of host range and cropping history.
• Physical treatments may not reach nematodes in deep soil and can injure plants if improperly applied.

The two approaches are complementary and may be combined, such as summer ploughing followed by solarization.

Biological control of nematodes is the reduction of nematode populations or their damaging activity through living organisms, their products or manipulation of the soil biological community.

Principal mechanisms:

• Parasitism: Fungi or bacteria infect eggs, juveniles, females or cysts and obtain nutrients from them.
• Predation: Predatory fungi, mites, microarthropods and other organisms capture or consume nematodes.
• Antibiosis: Microorganisms produce toxins, enzymes, antibiotics or volatile compounds that inhibit hatching, movement, penetration or survival.
• Competition: Beneficial organisms compete for nutrients and colonization sites in the rhizosphere.
• Induced resistance: Some rhizobacteria and fungi stimulate plant defense pathways, reducing nematode establishment or reproduction.
• Plant growth promotion: Improved root growth and nutrient uptake help plants tolerate nematode injury.
• Modification of the rhizosphere: Biological agents may alter root exudates and microbial communities, making the environment less favorable to nematodes.

Successful biological control depends on agent quality, soil moisture, temperature, organic matter, compatibility with pesticides and establishment in the root zone.

Nematophagous fungi suppress plant-parasitic nematodes by attacking eggs, females, cysts or mobile stages.

Important functional groups:

• Egg- and female-parasitic fungi: Species such as Pochonia chlamydosporia and Purpureocillium lilacinum colonize egg masses or sedentary stages and reduce egg viability.
• Nematode-trapping fungi: These produce adhesive networks, knobs or constricting rings that capture mobile nematodes before fungal hyphae penetrate them.
• Endoparasitic fungi: Spores attach to or are ingested by nematodes, germinate and develop within the body.
• Toxin-producing fungi: Some fungi release metabolites that immobilize or kill nematodes.

Application methods:

• Seed or seedling-root treatment
• Nursery-bed and soil application with compost or farmyard manure
• Application around the root zone through suitable formulations

Advantages: They are residue-safe, potentially self-perpetuating and compatible with organic amendments.

Limitations: Field performance can vary with temperature, soil moisture, native microbes, formulation quality and pesticide use. Therefore, fungi should be applied preventively and integrated with cultural methods rather than expected to provide immediate eradication.

Bacterial antagonists reduce nematode infection directly and may also improve plant defense and growth.

Role of Pasteuria:

• Pasteuria species are obligate bacterial parasites of nematodes.
• Endospores attach to the nematode cuticle, germinate and penetrate the body.
• Bacterial development reduces nematode mobility, feeding and reproduction and may eventually fill the host body with new spores.
• Their spores can persist in soil, but activity may be highly specific to particular nematode hosts or populations.

Plant-growth-promoting rhizobacteria:

• Bacteria such as Bacillus and Pseudomonas may produce enzymes, antibiotics, toxins and volatile metabolites.
• They can interfere with egg hatching, orientation, root penetration and development.
• Root colonization may induce systemic resistance and enhance nutrient uptake or root growth.

Constraints:

• Variable establishment under field conditions
• Sensitivity to soil environment and incompatible pesticides
• Need for viable, stable formulations
• Host specificity in some bacterial parasites

Bacterial agents are most dependable when applied early, supported by organic matter and combined with resistant crops and sanitation.

Conservation biological control protects and encourages antagonists already present in the soil. It involves adding organic matter, minimizing unnecessary broad-spectrum pesticides, maintaining suitable moisture and adopting practices that favor a diverse soil food web.

Augmentative biological control increases antagonist populations by applying mass-produced organisms or their propagules. Examples include treating nursery soil or roots with formulations of Pochonia, Purpureocillium, Trichoderma, Bacillus or Pseudomonas. Repeated applications may be required because introduced agents do not always persist.

Classical biological control involves introducing and establishing a natural enemy from another region to obtain long-term suppression. This approach is less commonly used for plant-parasitic nematodes because soil organisms are difficult to establish and non-native agents require rigorous biosafety assessment.

Comparison:

• Conservation is usually inexpensive but depends on the existing antagonist community.
• Augmentation offers a defined product and targeted application but depends on formulation quality and environmental conditions.
• Classical control may provide durable effects, but establishment is uncertain and ecological risks must be evaluated.

In practice, conservation and augmentation are commonly integrated with cultural practices.

Chemical nematicides may be classified primarily as fumigant and non-fumigant products.

Fumigant nematicides:

• Applied to soil, where they form toxic gases or volatile compounds.
• Move through soil air spaces and may control nematodes, some soil-borne pathogens and weed propagules.
• Require careful soil preparation, suitable moisture and temperature, sealing and a waiting period before planting.
• Their performance is strongly affected by soil texture, organic matter and leakage.

Non-fumigant nematicides:

• Usually formulated as granules, liquids or seed-treatment products.
• Move mainly in soil water and act by contact or systemic activity.
• Many interfere with nematode movement, orientation, feeding, development or reproduction.
• They are generally easier to apply than fumigants but require placement in the active root zone.

Nematicides may also be described according to their mode of action, systemic behavior, formulation and time of application. Product selection must follow local registration, crop-label directions, target-nematode biology and environmental safety requirements.

Fumigant nematicides:

• Movement: Disperse mainly as gases through soil pores.
• Application: Injected or incorporated into well-prepared soil, often followed by sealing or covering.
• Action: Generally broad-spectrum and lethal to exposed nematodes.
• Advantages: Can treat a relatively large soil volume and may suppress multiple soil pests.
• Limitations: Require specialized application, suitable soil conditions and a pre-plant waiting period. Volatility, worker exposure and effects on non-target organisms are concerns.

Non-fumigant nematicides:

• Movement: Dissolve and move mainly in the soil-water phase; some may be absorbed by roots.
• Application: Applied as granules, drenches, in-furrow treatments, seed treatments or through irrigation where permitted.
• Action: May kill nematodes or temporarily disrupt movement, feeding and reproduction.
• Advantages: More targeted placement and, for some products, use near or after planting.
• Limitations: Distribution can be uneven in dry soil, and activity may be affected by leaching, adsorption and microbial degradation.

Both groups must be used only according to approved labels and as components of integrated nematode management.

Safe and effective application principles:

• Confirm the nematode species and population before deciding on treatment.
• Select only a product registered for the crop, target nematode and application method.
• Follow the label dose, timing, placement, re-entry interval, pre-harvest interval and disposal instructions.
• Calibrate equipment and apply uniformly in the root zone.
• Consider soil texture, organic matter, moisture and temperature because they affect movement and persistence.
• Wear prescribed personal protective equipment and prevent contamination of skin, water sources, food and animal feed.
• Maintain application records and store products in secure, labeled containers.
• Avoid repeated use of the same mode of action where alternatives are available.

Need for integration:

• Chemicals may not reach nematodes inside roots, cysts, egg masses or deep soil.
• Repeated use can increase cost, residues, non-target effects and resistance-selection pressure.
• Sanitation, rotation, resistant cultivars, biological agents and organic amendments reduce the initial population and improve treatment reliability.

Therefore, nematicides should be used on the basis of diagnosis and economic need, not as a substitute for preventive management.

Plant quarantine is the legal and administrative regulation of the movement of plants, plant products, soil and related materials to prevent the introduction or spread of harmful pests, including plant-parasitic nematodes.

Importance:

• Many nematodes spread over long distances through infected seedlings, tubers, bulbs, roots, seed lots, adhering soil and contaminated machinery.
• Newly introduced species may lack natural enemies and may threaten crops that have little resistance.
• Preventing entry is generally cheaper and more effective than eradicating an established nematode.
• Quarantine protects pest-free production areas, export markets and the supply of healthy planting material.

Major quarantine measures:

• Prohibition or restriction of high-risk commodities
• Requirement of phytosanitary certification
• Inspection, sampling and laboratory diagnosis at entry points
• Treatment, detention, re-export or destruction of infested consignments
• Post-entry quarantine for planting materials that require observation
• Domestic restrictions to contain outbreaks within a country
• Surveillance and delimiting surveys around detected infestations

Quarantine succeeds only when regulations are supported by accurate diagnostics, traceability, public awareness and strict enforcement.

A quarantine decision should follow a systematic sequence.

Inspection and diagnosis:

1. Examine permits, origin, commodity identity and phytosanitary certificates.
2. Inspect the consignment for adhering soil, root galls, lesions, cysts, abnormal roots and poor plant growth.
3. Collect representative samples of soil, roots, seeds, tubers or packaging material.
4. Extract nematodes using methods appropriate to mobile stages, eggs or cysts.
5. Identify the organism through morphology, morphometrics and validated biochemical or molecular tests.
6. Maintain chain-of-custody records and prevent sample contamination.

Pest risk analysis:

• Determine whether the organism is absent, limited in distribution or under official control.
• Evaluate pathways of entry and the probability of survival during transport.
• Assess establishment based on host availability, climate and soil conditions.
• Estimate potential spread and economic, environmental and social consequences.
• Identify suitable risk-reduction options and evaluate their feasibility.

Possible actions: Release the material, prescribe treatment, hold it in post-entry quarantine, re-export it, destroy it or impose emergency containment. Measures should be scientifically justified and proportionate to risk.

• Resistance: The inherited ability of a plant to suppress nematode penetration, development or reproduction compared with a susceptible plant under similar conditions. A resistant plant may still be penetrated or show limited symptoms.
• Susceptibility: The inability of a plant to restrict nematode infection and reproduction. A susceptible host permits successful feeding, development and multiplication.
• Tolerance: The ability of a plant to maintain acceptable growth and yield despite nematode infection. A tolerant cultivar may support high nematode reproduction and can therefore increase risk to the following crop.
• Immunity: The complete inability of a nematode to infect or reproduce on a plant. True immunity is absolute and is less common than resistance.

Key distinction: Resistance concerns the effect of the plant on the nematode, whereas tolerance concerns the effect of the nematode on plant performance. Thus, a resistant plant may sometimes show damage at high inoculum, while a tolerant but susceptible plant may yield well and still leave a large nematode population in the soil.

Plant resistance may operate before penetration or after the nematode enters the root.

Pre-infection mechanisms:

• Root exudates may be unattractive or may fail to stimulate egg hatch.
• Physical barriers such as thick cell walls, suberization or unfavorable root-surface features may hinder penetration.
• Preformed chemicals may repel, immobilize or kill infective stages.
• Root architecture or short periods of susceptibility may reduce nematode contact.

Post-infection mechanisms:

• Rapid recognition of nematode effectors can activate localized defense responses.
• A hypersensitive response may cause death of cells near the infection site, preventing feeding-site establishment.
• Cell-wall strengthening, callose deposition, lignification and production of phenolics or defensive proteins restrict development.
• Failure to form or maintain giant cells or syncytia deprives sedentary nematodes of food.
• Some resistance allows early development but prevents maturation, egg production or normal sex development.

Resistance may be controlled by major genes or multiple genes. Its expression can be influenced by nematode species or race, plant age, temperature and inoculum density.

Development of resistant cultivars:

• Identify resistance sources in cultivars, landraces, wild relatives or induced mutants.
• Confirm resistance against relevant nematode species, races or pathotypes.
• Transfer resistance through conventional crossing, backcrossing and selection.
• Use marker-assisted selection, genomic approaches or biotechnology where permitted.
• Combine resistance with desirable yield, quality and adaptation traits.

Evaluation:

• Conduct controlled screening with a known initial population.
• Record penetration, galling or lesion indices, numbers of females, eggs or cysts and final population density.
• Calculate the reproduction factor as .
• Include resistant and susceptible checks and test across temperatures, soils and locations.
• Evaluate yield under both infested and non-infested conditions to distinguish resistance from tolerance.

Durable deployment:

• Avoid continuous cultivation of one resistance gene because virulent nematode populations may be selected.
• Rotate resistant cultivars with non-host crops and cultivars carrying different resistance genes.
• Pyramid multiple genes where possible.
• Control weeds and volunteers that maintain the nematode.
• Monitor population density and shifts in virulence.

Resistant cultivars are economical and environmentally safe, but they should remain part of an integrated programme.