Plant-parasitic nematodes
The microscopic worms that attack roots: how they feed, the main groups from root-knot to cyst nematodes, how to diagnose them, and how to manage them.
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Nematodes are tiny roundworms. Most of the countless millions in a handful of soil are harmless or helpful, feeding on bacteria, fungi or other small animals. But a minority feed on plant roots, and they are among the most underestimated causes of crop loss in the world: invisible, slow, and easily mistaken for poor soil, drought or nutrient shortage.
Global estimates of yield losses caused by plant-parasitic nematodes are commonly put at around a tenth of world crop production. They are a standard part of plant pathology and nematology syllabi, and an important consideration in quarantine and seed certification.
What a plant-parasitic nematode is
- Size and shape: unsegmented, worm-like, usually 0.3 to 3 mm long; females of some groups swell into pear or lemon shapes.
- The stylet: a hollow, needle-like mouth spear that the nematode thrusts into plant cells to inject secretions and withdraw their contents. It is the feature that distinguishes plant-parasitic nematodes from free-living ones under the microscope.
- Life cycle: egg, four juvenile stages (the first moult usually inside the egg) and adult. The second-stage juvenile (J2) is typically the infective stage. Many species complete a generation in three to six weeks in warm soil, so several generations occur per season.
- Habitat: soil water films. Nematodes move only centimetres to metres a year on their own; they spread with soil on machinery and boots, irrigation water, and above all infected planting material.
How they feed
| Habit | What happens | Examples |
|---|---|---|
| Ectoparasites | stay in the soil and feed from outside on root cells | dagger nematodes (Xiphinema), needle nematodes (Longidorus), stubby-root nematodes (Trichodorus); some carry plant viruses |
| Migratory endoparasites | enter the root and move through it, killing cells and leaving lesions that fungi and bacteria invade | lesion nematodes (Pratylenchus), the burrowing nematode (Radopholus similis) of banana and citrus |
| Sedentary endoparasites | the juvenile enters, settles at one site, and induces special feeding cells; the female becomes immobile | root-knot nematodes (Meloidogyne), cyst nematodes (Heterodera, Globodera), the reniform nematode |
| Above-ground feeders | attack stems, leaves, bulbs or seeds | stem and bulb nematode (Ditylenchus dipsaci), rice white-tip nematode, the pine wood nematode carried by beetles |
Root-knot nematodes
Meloidogyne species are the most widespread and damaging nematodes in the world, especially in warm climates, and attack thousands of plant species: vegetables, pulses, cotton, tobacco, banana, coffee, fruit trees and many weeds. The juvenile settles near the vascular tissue and induces a few root cells to grow into multinucleate giant cells that feed it for the rest of its life. The surrounding tissue swells into characteristic galls ("root knots"). Eggs are laid in a gelatinous mass on the root surface.
Cyst nematodes
In Heterodera (cereal, soybean and beet cyst nematodes) and Globodera (potato cyst nematodes), the female swells until her body bursts out of the root, and when she dies her body wall hardens into a brown cyst packed with hundreds of eggs. Cysts protect the eggs for many years, often decades, which is why cyst nematodes are so hard to eliminate and why potato cyst nematodes are quarantine pests in many countries. Eggs of potato cyst nematodes hatch mainly in response to chemicals released by host roots.
Symptoms and diagnosis
Nematode damage is easily mistaken for something else:
- Above ground: patches of stunted, yellowing plants, often oval and elongated in the direction of cultivation; wilting in the heat of the day despite moist soil; poor response to fertiliser; nutrient deficiency symptoms, because damaged roots take up less.
- Below ground: galls (root-knot), tiny white, yellow or brown cysts on roots (cyst nematodes), dark lesions and rotting (lesion and burrowing nematodes), stubby or excessively branched roots.
Diagnosis needs soil and root samples sent to a laboratory. Nematodes are extracted, counted and identified:
- Baermann funnel and its tray variants: active nematodes swim out of soil or roots into water.
- Sieving and decanting methods for soil.
- Flotation (for example the Fenwick can) to separate cysts, which float when dry.
- Identification by morphology under the microscope and increasingly by molecular methods.
Damage depends on the population density at planting relative to a crop's tolerance: below a threshold density there is no measurable loss; above it, yield falls as density rises.
Managing nematodes
Eradication from a field is rarely possible; the aim is to keep populations below damaging levels. Integrated management combines:
- Exclusion: clean, certified planting material (seed potatoes, banana plantlets from tissue culture, nursery stock); quarantine; washing soil off machinery.
- Crop rotation with non-host or poor-host crops. Because many nematodes have narrow host ranges and their populations decline without a host, rotation is the backbone of control for cyst nematodes. Root-knot nematodes, with very wide host ranges, are harder to rotate against.
- Resistant and tolerant varieties: for example tomato varieties carrying the Mi gene against several root-knot species (its resistance weakens at high soil temperatures, above about 28 °C), and potato varieties resistant to particular cyst nematode pathotypes.
- Trap crops that stimulate hatching but do not let nematodes complete their life cycle.
- Organic amendments, biofumigation (incorporating brassica residues that release toxic compounds) and soil solarisation (covering moist soil with clear plastic in hot weather).
- Biological control with nematode-parasitic bacteria and fungi, some of which are sold as biological products.
- Chemical nematicides: many older fumigants and nematicides have been banned or restricted because of toxicity; newer, more selective products exist in some countries. Use only products registered locally, as directed.
How long a rotation needs to be
A potato field has 100 viable potato cyst nematode eggs per gram of soil, and potatoes should not be grown until the population falls below 10 eggs per gram. Without a host crop, the population declines by about 30 per cent a year through natural mortality and spontaneous hatching (a commonly used figure; the actual rate varies with species and conditions).
After n years: 100 × 0.7n < 10, so 0.7n < 0.1, and n > ln(0.1) ⁄ ln(0.7) = 2.303 ⁄ 0.357 ≈ 6.5 years.
So about seven years without potatoes or other host crops would be needed, which is why long rotations, resistant varieties and testing soil before planting are combined in practice.
Key points
- Plant-parasitic nematodes are microscopic roundworms with a stylet; most damage is to roots, often unseen.
- Feeding habits: ectoparasites, migratory endoparasites (lesion, burrowing), sedentary endoparasites (root-knot, cyst) and above-ground feeders.
- Root-knot nematodes induce giant cells and galls and have very wide host ranges; cyst nematodes leave long-lived egg-filled cysts.
- Symptoms are patchy stunting, wilting and poor fertiliser response; diagnosis needs laboratory extraction from soil and roots.
- Some ectoparasitic nematodes transmit plant viruses.
- Management: clean planting material, rotation, resistant varieties (such as the Mi gene in tomato), trap crops, solarisation, biofumigation, biological control and, where permitted, nematicides.
What the exams ask
- School-leaving and entrance papers: what nematodes are, the root-knot and cyst nematodes, and symptoms such as galls.
- Degree and research-fellowship entrances: the stylet, life cycle, feeding habits with examples, giant cells and syncytia, extraction methods, and virus vector nematodes.
- Plant protection and quarantine examinations: potato cyst nematode regulation, damage thresholds, sampling, and integrated management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do nematode-damaged plants wilt even in wet soil?
Damaged roots cannot take up water fast enough when the plants transpire strongly in the heat of the day. They recover at night, then wilt again the next afternoon.
Do marigolds really control nematodes?
Some marigold species reduce root-knot and lesion nematode numbers when grown as a dense cover or rotation crop, because their roots release compounds toxic to nematodes and are poor hosts. A few marigolds planted among vegetables have little effect.
Are all nematodes in soil harmful?
No. Most are free-living and feed on bacteria, fungi and other organisms, helping to recycle nutrients, and some are used as biological control agents against insect pests.
Sources
- Perry, R. N. and Moens, M. (eds) Plant Nematology, 2nd edition. CABI, 2013.
- Jones, J. T. et al. "Top 10 plant-parasitic nematodes in molecular plant pathology." Molecular Plant Pathology 14, 946–961 (2013).
- Nicol, J. M. et al. "Current nematode threats to world agriculture." In Jones, J., Gheysen, G. and Fenoll, C. (eds), Genomics and Molecular Genetics of Plant–Nematode Interactions. Springer, 2011.
- EPPO. Globodera rostochiensis and Globodera pallida, PM 9/26 and diagnostic protocol PM 7/40. European and Mediterranean Plant Protection Organization.
- Agrios, G. N. Plant Pathology, 5th edition. Elsevier Academic Press, 2005. Chapter on plant diseases caused by nematodes.
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