Unit 5: Nematode Pests of Crops

I. Orientation — Plant-Parasitic Nematodes and Crop Damage

Plant-parasitic nematodes are microscopic roundworms, usually 0.3–3.0 mm long, that feed on plant cells using a protrusible stylet. Crop loss results from direct tissue injury, disruption of water and nutrient uptake, and interactions with fungi and bacteria.

• Defining properties:
  • Feeding habit: Ectoparasites remain outside roots; endoparasites enter tissues. Migratory species move while feeding, whereas sedentary species establish permanent feeding sites.
  • Life cycle: The usual sequence is egg, four juvenile stages (J1–J4), and adult; the first moult commonly occurs inside the egg.
  • Plant symptoms: Root galls, lesions, necrosis, stunting, chlorosis, wilting, reduced tillering, and patchy field growth are typical but not individually diagnostic.
  • Survival structures: Eggs, seed galls, cysts, quiescent juveniles, and infected planting material permit persistence between crops.
  • Diagnosis: Identification combines field symptoms with extraction from soil, roots, seeds, or leaves and microscopic or molecular examination.
  • Management principle: Effective control integrates clean planting material, resistant cultivars, crop rotation, sanitation, organic amendments, biological agents, and registered nematicides.

II. Rice — Seed, Leaf, Stem, and Root Parasites

A. Nematode pests of rice

Rice nematodes occupy distinct ecological niches, ranging from seed-borne foliar parasites to root-invading species in flooded and upland soils.

• White-tip nematode: Aphelenchoides besseyi is carried beneath the rice glumes and becomes active in the moisture film around emerging leaves.
  • It causes white, twisted leaf tips, shortened panicles, distorted grains, and reduced seed weight.
  • Use nematode-free seed, resistant varieties, seed cleaning, and an appropriately validated hot-water seed treatment.
• Rice-stem nematode: Ditylenchus angustus causes ufra disease, especially in deep-water rice of South and Southeast Asia.
  • Symptoms include swollen nodes, twisted leaves, incomplete panicle emergence, and sterile or poorly filled grains.
  • Destruction of infected stubble and volunteers reduces carry-over.
• Rice-root nematode: Hirschmanniella oryzae is a migratory endoparasite adapted to flooded paddy soils; feeding produces brown root lesions and weak tillering.
• Rice root-knot nematode: Meloidogyne graminicola is particularly damaging in upland, direct-seeded, and intermittently flooded rice.
  • Characteristic hook-shaped or terminal root galls distinguish its injury.
  • Flooding, puddling, rotation with poor hosts, clean nursery soil, and resistant cultivars suppress populations.

III. Wheat — Cereal Seed, Cyst, and Lesion Nematodes

A. Nematode pests of wheat

Wheat is attacked by nematodes that replace grains with galls, form durable root cysts, or migrate through cortical tissues.

• Ear-cockle nematode: Anguina tritici converts developing kernels into dark, hard seed galls containing numerous dormant second-stage juveniles.
  • Contaminated seed spreads the pest; floating or mechanical separation removes lighter galls.
  • Seed gall nematodes can carry Rathayibacter tritici, associated with tundu or yellow-ear-rot disease.
• Cereal-cyst nematode: Heterodera avenae causes molya disease, notably in wheat-growing areas of Rajasthan and adjoining regions.
  • Infested crops show pale patches, poor tillering, and knotted or “bushy” roots; white females later become brown cysts.
  • Rotation with non-cereal crops and resistant wheat or barley cultivars interrupts reproduction.
• Root-lesion nematodes: Pratylenchus thornei and P. neglectus produce elongated cortical lesions and yield loss under moisture stress.
• Additional pest: Meloidogyne naasi causes cereal root-knot, with small galls and excessive root branching in temperate regions.

IV. Vegetables — Diverse High-Value Host Systems

A. Nematode pests of vegetables

Vegetable crops suffer severe damage because susceptible hosts are grown repeatedly and transplanted through infested nurseries.

• Root-knot complex: Meloidogyne incognita, M. javanica, M. arenaria, and temperate M. hapla attack tomato, brinjal, okra, cucurbits, carrot, and other vegetables.
  • Sedentary females induce multinucleate giant cells; visible root galls cause wilting, chlorosis, and poor marketable yield.
  • Grafting, resistant tomato cultivars, soil solarization, rotation, organic amendments, and biological agents such as Purpureocillium lilacinum support management.
• Potato-cyst nematodes: Globodera rostochiensis and G. pallida persist as cysts containing eggs and juveniles; they are major quarantine pests of potato.
• Stem-and-bulb nematode: Ditylenchus dipsaci causes swelling, distortion, and rotting in onion, garlic, and related hosts.
• Other parasites: Rotylenchulus reniformis and Pratylenchus species cause root decline, often without conspicuous galling.
• Sanitation: Clean seedlings, sterilized potting media, weed control, and removal of infested roots prevent nursery-to-field movement.

V. Pulses — Root and Cyst Constraints on Legumes

A. Nematode pests of pulses

Pulse nematodes damage roots and nodules, thereby reducing both nutrient absorption and biological nitrogen fixation.

• Root-knot nematodes: Meloidogyne incognita and M. javanica attack chickpea, mung bean, urd bean, cowpea, pea, and pigeonpea.
  • Root galling must be distinguished from beneficial rhizobial nodules, which are detachable and often pink internally when active.
• Pigeonpea-cyst nematode: Heterodera cajani forms lemon-shaped females on roots and durable brown cysts in soil.
• Chickpea-cyst nematode: Heterodera ciceri is important in chickpea and lentil production in parts of West Asia and the Mediterranean region.
• Reniform and lesion nematodes: Rotylenchulus reniformis and Pratylenchus species cause stunting, root necrosis, and poor nodulation.
• Management: Resistant cultivars, cereal rotations that are verified as non-hosts, clean seedling soil, summer ploughing, and removal of leguminous weeds reduce inoculum.

VI. Oilseed Crops — Nematodes of Groundnut, Soybean, and Sesame

A. Nematode pests of oilseed crops

Oilseed losses arise from root destruction, impaired pod development, and repeated cultivation of susceptible hosts.

• Groundnut pests: Meloidogyne arenaria produces galls on roots, pegs, pods, and shells; Pratylenchus brachyurus causes cortical lesions.
• Soybean-cyst nematode: Heterodera glycines is a globally important soybean pest; females appear as small white or yellow bodies on roots before becoming brown cysts.
  • Rotate resistant soybean cultivars with non-host cereals and monitor shifts in virulent populations.
• Sesame and sunflower pests: M. incognita, M. javanica, and Rotylenchulus reniformis cause galling, stunting, and premature senescence.
• Mustard and rapeseed: Root-knot and cyst nematodes may become important where cruciferous crops are repeatedly cultivated.
• Management: Weed-host removal, clean seed and machinery, resistant cultivars, crop rotation, and locally approved seed or soil treatments are complementary measures.

VII. Fiber Crops — Cotton and Jute Root Parasites

A. Nematode pests of fiber crops

Fiber-crop nematodes reduce stand establishment, root efficiency, boll formation, and stem biomass.

• Cotton root-knot nematode: Meloidogyne incognita causes conspicuous galls and interacts with Fusarium oxysporum f. sp. vasinfectum, increasing wilt severity.
• Reniform nematode: Rotylenchulus reniformis is a major cotton pest in tropical and subtropical soils; the immature female becomes semi-endoparasitic and kidney-shaped.
• Regional cotton pests: Hoplolaimus columbus and Belonolaimus longicaudatus injure roots in suitable sandy soils.
• Jute pests: M. incognita and R. reniformis cause root galling, stunting, and reduced stem and fiber yield.
• Management: Resistant cotton cultivars, non-host rotations, residue destruction, weed control, and prevention of soil movement suppress population build-up.

VIII. Citrus — Slow Decline and Root Deterioration

A. Nematode pests of citrus

Citrus nematode injury is chronic, with declining canopy vigor reflecting gradual loss of feeder-root function.

• Citrus nematode: Tylenchulus semipenetrans causes slow decline; females embed their anterior ends in feeder roots while the posterior remains exposed.
  • Soil particles adhere to the gelatinous egg mass, giving infected roots a characteristically dirty appearance.
  • Symptoms include sparse foliage, small leaves and fruit, twig dieback, and reduced yield.
• Burrowing nematode: Radopholus similis is associated with spreading decline of citrus in localized subtropical production areas.
• Management: Certified nematode-free nursery stock, tolerant or resistant rootstocks, clean irrigation and equipment, organic mulches, and preplant site assessment are central.
• Perennial-crop limitation: Established orchards cannot be rotated easily, so prevention is more effective than attempting eradication after planting.

IX. Banana — Burrowing, Lesion, Spiral, and Root-Knot Nematodes

A. Nematode pests of banana

Banana nematodes weaken the root–corm system, reducing bunch weight and increasing plant toppling.

• Burrowing nematode: Radopholus similis produces reddish-brown to black lesions in roots and corm tissues.
  • Severe root destruction causes “blackhead,” poor anchorage, and toppling under wind or bunch weight.
• Lesion nematodes: Pratylenchus coffeae and P. goodeyi migrate through roots; P. goodeyi is especially important in cooler highland banana systems.
• Spiral nematode: Helicotylenchus multicinctus feeds in and around cortical tissues, causing superficial necrosis and root decline.
• Root-knot nematodes: M. incognita and M. javanica produce galls and interfere with root development.
• Management: Tissue-culture plants, pared and sanitized suckers, clean nurseries, fallowing, non-host rotations, organic amendments, and resistant or tolerant genotypes reduce infection.

X. Tea — Nematodes of Plantation Feeder Roots

A. Nematode pests of tea

Tea nematodes are most damaging in nurseries and young plantations where feeder-root loss delays establishment.

• Tea root-lesion nematode: Pratylenchus loosi causes dark lesions, root pruning, chlorosis, dieback, and reduced shoot production.
• Other pests: Meloidogyne species, Radopholus similis, and Rotylenchulus reniformis may damage tea according to region, soil, and planting material.
• Spread: Rooted cuttings, nursery soil, drainage water, footwear, and implements transport nematodes between blocks.
• Management: Use clean mother plants and rooting media, remove infected nursery stock, improve drainage and organic matter, control weed hosts, and establish ground covers only after host suitability is known.

XI. Coffee — Root-Knot and Lesion Complexes

A. Nematode pests of coffee

Coffee nematodes cause root deformation and dieback, producing long-term decline in perennial plantations.

• Root-knot nematodes: Meloidogyne exigua is important in Latin America, while M. paranaensis, M. incognita, and M. coffeicola can cause aggressive root injury.
  • Symptoms include galls or cracking, corky roots, chlorosis, wilting, branch dieback, and poor berry production.
• Lesion nematodes: Pratylenchus coffeae and related species cause necrotic lesions and predispose roots to secondary microorganisms.
• Management: Nematode-free seedlings, resistant rootstocks or cultivars, nursery sanitation, shade and water-stress management, organic amendments, and removal of heavily infected roots reduce losses.
• Diagnostic point: Species-level identification matters because coffee cultivars and rootstocks differ markedly in resistance to individual Meloidogyne species.

XII. Coconut — Chronic Root-Parasitic Complexes

A. Nematode pests of coconut

Coconut commonly supports mixed nematode populations whose damage becomes serious under drought, poor nutrition, or other root stresses.

• Principal genera: Radopholus similis, Pratylenchus coffeae, Rotylenchulus reniformis, Helicotylenchus species, and Meloidogyne species may occur in coconut rhizospheres.
• Damage pattern: Root lesions, cortical decay, reduced feeder-root density, yellowing, weak growth, and lower nut yield develop gradually.
• Planting-material risk: Infested nursery seedlings and soil attached to roots are major pathways into clean plantations.
• Management: Raise seedlings in clean media, inspect roots before planting, maintain irrigation and balanced nutrition, add organic matter, sanitize tools, and manage intercrops and weeds according to their host status.
• Diagnosis: Nematode counts must be interpreted with root condition and associated pathogens because above-ground symptoms are nonspecific.

XIII. Biogeography, Dispersal, and Field Patterns

A. Distribution of nematode pests

Nematode distribution is determined by host availability, climate, soil properties, survival biology, and movement through human activities.

• Geographical pattern: Tropical regions favor M. incognita, M. javanica, R. reniformis, and R. similis; cooler areas favor pests such as M. hapla, Globodera species, and D. dipsaci.
• Soil influence: Texture, temperature, moisture, aeration, and organic matter affect movement and survival; many mobile ectoparasites are particularly damaging in coarse, sandy soils.
• Field distribution: Populations usually occur in aggregated patches rather than uniformly, producing irregular zones of stunting or chlorosis.
• Natural spread: Movement occurs through water, wind-blown soil, root contact, and limited active crawling in soil-water films.
• Human-assisted spread: Infested seed, tubers, suckers, seedlings, soil, irrigation water, footwear, tools, and machinery move pests rapidly across fields and regions.
• Sampling implication: Multiple soil cores from the crop root zone should be combined into a representative sample; roots or affected plant parts should accompany soil where possible.
• Quarantine significance: Cyst nematodes, burrowing nematodes, seed-gall nematodes, and other regulated species require certified planting material, inspection, containment, and exclusion from pest-free areas.