Unit 5: Nematode Pests of Crops - Subjective Questions

Ufra disease of rice is caused by the stem nematode Ditylenchus angustus. It is particularly serious in deep-water and low-lying rice-growing areas.

Symptoms:

• Young leaves become twisted, wrinkled, and chlorotic.
• Brown lesions may appear on leaf sheaths and stems.
• Panicles may fail to emerge completely, producing the characteristic swollen boot symptom.
• Emerged panicles are often distorted and contain sterile or partially filled grains.
• Severely affected plants become stunted and may die.

Disease cycle:

• The nematode survives in infected rice stubble, ratoons, weeds, and volunteer rice plants.
• It becomes active under flooded and humid conditions.
• Nematodes move through surface water and enter plants between overlapping leaf sheaths.
• They feed ectoparasitically on young tissues and multiply rapidly.

Management:

• Destroy infected stubble and volunteer rice plants.
• Remove susceptible grass weeds that serve as alternate hosts.
• Practice crop rotation and summer ploughing where feasible.
• Avoid movement of contaminated seedlings and irrigation water.
• Use tolerant or resistant cultivars when available.
• Synchronize planting and maintain proper field sanitation to reduce carry-over populations.

White-tip disease and rice root nematode disease differ in their site of attack and epidemiology.

Management of white-tip disease:

• Use certified nematode-free seed.
• Apply recommended hot-water seed treatment after pre-soaking.
• Remove heavily infected plants and avoid saving seed from diseased fields.
• Grow resistant or tolerant varieties where available.

Management of rice root nematodes:

• Destroy infected stubble and weeds.
• Drain and dry fields between crops where agronomically possible.
• Rotate rice with suitable non-host upland crops.
• Raise seedlings in nematode-free nurseries.
• Apply organic amendments and approved nematicidal treatments when economically justified.

Thus, seed sanitation is central to white-tip management, whereas water, soil, and root-residue management are more important against Hirschmanniella.

Ear-cockle disease of wheat is caused by the seed-gall nematode Anguina tritici.

Symptoms and signs:

• Infected seedlings may show distorted, twisted, or crinkled leaves.
• Plants are often stunted, with shortened internodes.
• Normal grains are replaced by hard, dark-brown seed galls or cockles.
• A gall contains numerous dormant second-stage juveniles.
• Galls are lighter than healthy wheat grains and can often be separated by flotation.

Disease cycle:

1. Seed galls fall to the soil or are planted with contaminated seed.
2. Under moist conditions, second-stage juveniles emerge from the galls.
3. Juveniles climb seedlings in a film of water and feed on growing points.
4. They enter floral primordia and develop into adults.
5. Females lay eggs in the developing ovary, which is transformed into a nematode gall.
6. Juveniles remain dormant inside the dry gall for several years.

Association with tundu disease:

• Tundu or yellow-ear-rot develops when A. tritici carries the bacterium Rathayibacter tritici to wheat floral tissues.
• The bacterium produces yellow, sticky slime on spikes and causes rotting and distortion.
• Therefore, the nematode acts as a vector and creates infection courts for the bacterium.

Management:

• Sow clean, gall-free certified seed.
• Separate galls by brine flotation, followed by thorough washing and drying of seed.
• Rotate with non-host crops for at least two to three years.
• Remove volunteer wheat and maintain field sanitation.
• Prevent movement of contaminated seed lots between regions.

Molya disease of wheat and barley is commonly caused by the cereal cyst nematode Heterodera avenae.

Symptoms:

• Diseased plants occur in irregular patches in the field.
• Plants show stunting, yellowing, poor tillering, and premature drying.
• Roots become shortened and excessively branched, producing a bushy or knotted appearance.
• White females may be visible on roots during the growing season; they later turn into brown cysts.
• Grain number and grain weight are reduced.

Life cycle:

• Eggs remain protected within durable cysts in soil.
• Second-stage juveniles hatch in response to suitable temperature, moisture, and host-root stimuli.
• Juveniles penetrate roots and establish specialized feeding sites called syncytia.
• Females become swollen and remain attached to roots, while males become vermiform.
• The dead female body forms a cyst containing eggs, enabling long-term survival.

Management:

• Rotate wheat and barley with non-host crops such as pulses or suitable oilseeds.
• Use resistant or tolerant cereal cultivars.
• Destroy volunteer cereals and grassy hosts.
• Follow deep summer ploughing and improve soil organic matter.
• Avoid transporting infested soil on machinery, seedlings, or irrigation water.
• Use approved nematicides only as part of an integrated management programme.

Root-knot nematodes, mainly Meloidogyne incognita, M. javanica, M. arenaria, and M. hapla, attack tomato, brinjal, okra, cucurbits, beans, carrot, chilli, and many other vegetables.

Symptoms:

• Characteristic galls or knots develop on roots.
• Above-ground symptoms include stunting, chlorosis, wilting during hot periods, and poor response to fertilizers.
• Infected plants produce fewer and smaller fruits.
• Carrot and other root vegetables may become forked, cracked, or deformed.
• Root-knot infection increases susceptibility to wilt and root-rot pathogens.

Life cycle:

1. Eggs are deposited in a gelatinous matrix near the root surface.
2. The second-stage juvenile is the infective stage.
3. It penetrates near the root tip and migrates to the vascular region.
4. Secretions induce multinucleate giant cells, which serve as permanent feeding sites.
5. Females become pear-shaped and remain sedentary; males, when present, become vermiform.
6. A generation may be completed in about three to six weeks under favourable warm conditions.

Integrated management:

• Use nematode-free seedlings and resistant cultivars or rootstocks.
• Rotate with poor-host or antagonistic crops such as marigold, depending on the Meloidogyne species.
• Solarize moist nursery or field soil during the hottest period.
• Apply well-decomposed farmyard manure, neem cake, or other organic amendments.
• Use biological agents such as Pochonia chlamydosporia, Purpureocillium lilacinum, or Pasteuria penetrans where effective formulations are available.
• Remove infected roots and control weeds.
• Use approved nematicides according to local recommendations.

Successful control requires combining preventive, cultural, biological, and resistant-host measures.

Potato cyst nematodes are Globodera rostochiensis, the golden cyst nematode, and G. pallida, the pale cyst nematode. They are internationally important quarantine pests.

Symptoms:

• Poor crop emergence and irregular patches of stunted plants.
• Yellowing, early senescence, and reduced tuber size and number.
• Excessive formation of fine lateral roots.
• White, cream, or golden females may be observed on roots; mature females become brown cysts.

Persistence and spread:

• Cysts protect eggs in soil for many years.
• Spread occurs through infested seed tubers, soil attached to machinery, footwear, containers, and irrigation or flood water.

Management:

• Enforce quarantine and use certified pest-free seed tubers.
• Prevent the movement of contaminated soil and sanitize farm machinery.
• Grow resistant potato cultivars selected against the locally occurring species or pathotype.
• Follow long rotations with non-host crops.
• Destroy volunteer potato plants and solanaceous weed hosts.
• Use trap crops, biofumigant crops, organic amendments, or approved nematicides as supplementary measures.
• Monitor cyst density before planting to guide management decisions.

Root-knot and reniform nematodes are sedentary plant parasites, but they differ in morphology, symptoms, and host relationships.

Common management measures:

• Use clean seedlings and maintain weed-free fields.
• Rotate with verified non-host crops.
• Apply organic amendments and suitable biological control agents.
• Use resistant cultivars where available.
• Confirm the nematode by root and soil examination because above-ground symptoms are nonspecific.

Pulse crops such as chickpea, pigeonpea, mungbean, urdbean, cowpea, pea, lentil, and common bean are attacked by several nematode groups.

Major nematodes:

• Root-knot nematodes: Meloidogyne incognita, M. javanica, and M. arenaria cause root galls, stunting, and reduced nodulation.
• Pigeonpea cyst nematode: Heterodera cajani causes patchy yellowing, poor growth, and white females or brown cysts on roots.
• Root-lesion nematodes: Pratylenchus species produce brown cortical lesions and root decay.
• Reniform nematode: Rotylenchulus reniformis causes root injury, chlorosis, and reduced yield, especially in warm regions.
• Stem and bulb nematodes: Ditylenchus dipsaci may damage pea and certain other legumes in temperate areas.

Effects on pulses:

• Reduced root growth and nutrient uptake.
• Suppression of Rhizobium nodulation and biological nitrogen fixation.
• Increased incidence of wilt and root-rot complexes.
• Reduced pod formation, seed size, and total yield.

Integrated management:

• Use clean seed and seedlings and avoid infested fields for seed production.
• Grow resistant or tolerant cultivars where available.
• Rotate with cereals or other confirmed non-host crops.
• Remove weed hosts and destroy infected root residues.
• Apply well-decomposed organic manure, neem cake, and balanced fertilizers.
• Treat seed with compatible biological agents such as Trichoderma and apply nematode antagonists where recommended.
• Use soil solarization in nurseries and approved nematicides only when population levels justify treatment.

The selected rotation must be based on correct nematode identification because many root-knot species have broad host ranges.

The pigeonpea cyst nematode, Heterodera cajani, is an important pest of pigeonpea and can also reproduce on some other legumes.

Biology:

• Eggs are retained inside a protective brown cyst formed from the dead female body.
• Infective second-stage juveniles hatch from eggs and penetrate young roots.
• They establish sedentary feeding sites called syncytia near the vascular tissues.
• Adult females become lemon-shaped and protrude from the root surface.
• After egg production, the female dies and becomes a durable cyst.

Symptoms and damage:

• Plants are stunted and yellow, often in distinct field patches.
• Root systems are poorly developed and nodulation is reduced.
• Small white females may be visible on roots before they mature into brown cysts.
• Early and heavy infection reduces branching, pod setting, seed weight, and yield.
• Damage may be confused with drought or nutrient deficiency.

Management:

• Rotate pigeonpea with non-host cereals.
• Grow resistant or tolerant varieties where locally available.
• Remove volunteer pigeonpea and susceptible weeds.
• Incorporate organic amendments and maintain balanced soil fertility.
• Prevent movement of cyst-infested soil through machinery and planting materials.
• Diagnose infestation through cyst extraction and population assessment before planting.

Oilseed crops are attacked by nematodes that vary with crop, climate, and region.

Important crop–nematode associations:

• Soybean: Soybean cyst nematode, Heterodera glycines; root-knot nematodes, Meloidogyne species; and lesion nematodes, Pratylenchus species.
• Groundnut: Meloidogyne arenaria, M. javanica, Pratylenchus brachyurus, and the testa nematode Aphelenchoides arachidis in affected regions.
• Rapeseed and mustard: Root-knot nematodes and cyst nematodes, including Heterodera schachtii in suitable production zones.
• Sunflower: Meloidogyne incognita, M. javanica, Rotylenchulus reniformis, and lesion nematodes.
• Sesame and castor: Root-knot and reniform nematodes are frequently important.

Damage:

• Root galls, lesions, cyst formation, and root pruning reduce water and nutrient absorption.
• Crops become stunted and chlorotic and show poor seed filling.
• Groundnut infection may reduce pod development and contaminate seed or shells.
• Nematode injury may interact with fungal root-rot and wilt pathogens.

Management:

• Use certified, nematode-free seed and planting material.
• Rotate with non-host crops chosen according to the nematode species.
• Use resistant cultivars, especially against soybean cyst and root-knot nematodes.
• Control weeds and volunteer hosts.
• Incorporate oil cakes, compost, and green manures.
• Apply biological agents and approved seed or soil treatments where recommended.
• Adopt quarantine and seed-health measures against seed-associated nematodes such as A. arachidis.

Regular soil testing is essential because yield loss can occur before obvious above-ground symptoms appear.

The soybean cyst nematode, Heterodera glycines, is one of the most destructive pests of soybean.

Life cycle:

• Eggs survive inside brown cysts in soil.
• Second-stage juveniles hatch and penetrate soybean roots.
• They establish syncytial feeding sites and become sedentary.
• Females swell and appear as small white or yellow bodies on the root surface.
• After reproduction, the female dies and her body becomes a brown cyst filled with eggs.
• Several generations may develop during a cropping season under favourable conditions.

Symptoms:

• Irregular patches of stunted and chlorotic plants.
• Poor root growth and reduced nodulation.
• Premature maturity and reduced pod number and seed weight.
• White females on roots provide an important diagnostic sign.

Management:

• Rotate soybean with non-host crops such as maize or small grains.
• Grow resistant soybean cultivars and rotate resistance sources to slow virulence selection.
• Control volunteer soybean and susceptible weeds.
• Maintain balanced fertility and soil health.
• Prevent the movement of infested soil on machinery.
• Monitor egg and cyst densities because symptoms may be mild even when economic loss occurs.

Cotton is mainly damaged by the southern root-knot nematode Meloidogyne incognita and the reniform nematode Rotylenchulus reniformis. Lesion nematodes and sting nematodes may also be locally important. Jute and other fiber crops can likewise be attacked by root-knot and reniform nematodes.

Root-knot nematode in cotton:

• Causes galls on roots, reduced root growth, stunting, chlorosis, and premature wilting.
• The infective second-stage juvenile induces giant cells in the vascular region.
• Injury can predispose cotton to Fusarium wilt and other soil-borne diseases.

Reniform nematode in cotton:

• The immature female partially penetrates the root and becomes kidney-shaped after feeding.
• It causes root necrosis, poor feeder-root development, stunting, and delayed maturity.
• Distinct galls are generally absent, so diagnosis requires extraction from soil and roots.

Effects on production:

• Reduced boll number, boll size, lint yield, and fiber quality.
• Uneven plant growth and early senescence occur in heavily infested patches.

Integrated management:

• Use resistant or tolerant cotton cultivars when available.
• Rotate with poor-host cereals or resistant crops, considering the nematode species present.
• Control weeds and volunteer cotton.
• Use nematode-free nursery material for transplanted fiber crops.
• Apply organic amendments, cover crops, and biological control agents.
• Use seed treatment or approved nematicides according to local thresholds.
• Manage associated wilt pathogens and avoid movement of infested soil.

Slow decline of citrus is caused by Tylenchulus semipenetrans, a sedentary semi-endoparasitic nematode.

Host and feeding behaviour:

• It attacks citrus species and several related rutaceous plants.
• The young female inserts its anterior end into the root cortex, while the posterior portion remains outside.
• Eggs are laid in a gelatinous matrix on the root surface.

Symptoms:

• Gradual loss of tree vigour rather than sudden death.
• Sparse foliage, small and chlorotic leaves, twig dieback, and reduced fruit size and yield.
• Infected feeder roots appear dirty because soil particles adhere to egg masses.
• Root cortex deteriorates, reducing water and nutrient uptake.
• Symptoms become severe under drought, poor nutrition, salinity, or infection by other root pathogens.

Disease development and spread:

• Populations build slowly in perennial orchards.
• Spread occurs through infested nursery plants, contaminated soil, irrigation water, and farm implements.

Management:

• Establish orchards with certified nematode-free nursery plants.
• Use resistant or tolerant rootstocks suited to the local citrus species and nematode population.
• Prevent contaminated soil from entering clean nurseries and orchards.
• Apply organic mulches, compost, adequate irrigation, and balanced nutrition to improve root health.
• Remove severely affected roots or trees when necessary.
• Use approved pre-plant or post-plant nematicidal treatments based on population estimates.
• Monitor feeder roots and rhizosphere soil regularly.

Toppling disease of banana is mainly caused by the burrowing nematode Radopholus similis, a migratory endoparasite of roots and corm tissues.

Symptoms:

• Reddish-brown lesions initially appear in the root cortex.
• Lesions enlarge and become dark or black as tissues die and secondary organisms invade.
• Feeder roots are destroyed, reducing anchorage and absorption.
• Plants show stunting, yellowing, delayed bunch emergence, and reduced bunch weight.
• Heavily infected plants topple, especially during wind or after heavy rain.

Biology:

• Females and juveniles enter and migrate through root tissues.
• Eggs are laid within roots, and several stages feed on the cortex.
• The nematode survives in infected suckers, corms, roots, and alternate hosts.
• Long-distance spread occurs mainly through infested planting material.

Management:

• Plant certified nematode-free tissue-culture plants or clean pared suckers.
• Pare corms to remove discoloured tissue and use recommended hot-water treatment where appropriate.
• Remove and destroy infected roots and old banana residues.
• Rotate with non-host crops or maintain a clean fallow before replanting.
• Control weed hosts and avoid moving contaminated soil.
• Apply neem cake, compost, and effective biological agents such as Purpureocillium or Pochonia formulations.
• Use resistant or tolerant cultivars when available and apply approved nematicides only when needed.

Several nematodes may occur together in banana roots, so diagnosis should combine symptoms with laboratory extraction.

Diagnostic approach:

• Examine washed roots for galls and the colour, shape, and depth of lesions.
• Split roots and corm tissue to detect internal necrosis.
• Extract mobile nematodes from fresh roots and soil.
• Stain roots when necessary to observe sedentary females and egg masses.
• Identify nematodes morphologically or by molecular methods.
• Consider plant age, wind damage, waterlogging, and fungal root rots before attributing toppling solely to nematodes.

Tea is a perennial crop in which nematodes can accumulate over many years. Important pests include the root-lesion nematode Pratylenchus loosi, the burrowing nematode Radopholus similis, root-knot nematodes such as Meloidogyne species, and occasionally spiral and ring nematodes.

Symptoms:

• Nursery plants show poor rooting, stunting, and chlorosis.
• Mature bushes exhibit gradual decline, sparse foliage, reduced shoot growth, and low leaf yield.
• Pratylenchus and Radopholus produce brown or black cortical lesions and root decay.
• Meloidogyne species produce galls and swollen feeder roots.
• Secondary fungi may invade damaged roots and accelerate decline.

Management:

• Raise seedlings and cuttings in sterilized or nematode-free nursery media.
• Inspect planting material and reject plants with root lesions or galls.
• Use tolerant clones where available.
• Remove heavily infested bushes and as much infected root material as possible.
• Maintain shade, drainage, organic matter, balanced nutrition, and adequate moisture.
• Control alternate weed hosts and use suitable cover crops.
• Apply organic amendments, biological antagonists, and approved nematicides according to local recommendations.
• Prevent movement of infested nursery soil and tools to clean areas.

Coffee is seriously affected by root-knot nematodes, including Meloidogyne exigua, M. coffeicola, M. incognita, and related species, as well as root-lesion nematodes such as Pratylenchus coffeae.

Root-knot nematodes:

• Induce galls, swellings, or corky lesions depending on the species and coffee genotype.
• Cause root-tip injury, reduced feeder roots, chlorosis, wilting, and poor berry production.
• Females establish permanent feeding sites composed of giant cells.

Root-lesion nematodes:

• Enter and migrate through the root cortex.
• Produce elongated brown lesions, tissue necrosis, and root pruning.
• Wounds facilitate invasion by fungi and other soil microorganisms.

Field symptoms:

• Affected coffee plants occur in patches.
• Young plants may fail after transplanting.
• Mature plants show thinning canopies, branch dieback, reduced berry size, and gradual decline.

Management:

• Produce seedlings in sterilized media and use certified nematode-free plants.
• Use resistant cultivars or resistant rootstocks where available.
• Graft susceptible high-quality cultivars onto resistant rootstocks when locally recommended.
• Remove severely infected roots and control weed hosts.
• Improve drainage, organic matter, mulching, and balanced nutrition.
• Apply biological agents and approved nematicides based on nursery or field population levels.
• Clean tools and prevent the transfer of infested soil between plantations.

Because Meloidogyne species and populations differ in host response, accurate identification is necessary before selecting resistant material.

Coconut roots may be attacked by Radopholus similis, Meloidogyne species, Pratylenchus species, and several ectoparasitic nematodes. In tropical America, red-ring disease caused by Bursaphelenchus cocophilus is also important.

Burrowing and root-damaging nematodes:

• Radopholus similis migrates through the root cortex and produces reddish-brown lesions and necrosis.
• Root destruction reduces nutrient and water uptake, resulting in yellowing, poor growth, and reduced nut yield.
• Root-knot nematodes may cause galls, especially in nurseries and young palms.

Red-ring disease:

• Bursaphelenchus cocophilus invades palm tissues and produces a reddish or brown ring visible in cross-sections of the stem.
• External symptoms include yellowing, wilting, leaf collapse, and eventual palm death.
• It is transmitted mainly by the palm weevil Rhynchophorus palmarum.
• The disease is geographically restricted and should not be assumed to occur in all coconut-growing regions.

Management:

• Use healthy seedlings raised in clean nursery soil.
• Avoid movement of infested soil and planting material.
• Improve drainage, nutrition, and organic matter to support root recovery.
• Remove severely infected roots or palms where required.
• For red-ring disease, destroy infected palms promptly and manage the palm-weevil vector using sanitation, trapping, and locally approved measures.
• Apply quarantine regulations to prevent introduction into pest-free areas.

The distribution of plant-parasitic nematodes is determined by interactions among the nematode, host, environment, and human activity.

1. Host distribution:

• Nematodes become established where suitable crop, weed, or alternate hosts are present.
• Continuous monocropping encourages population build-up.
• Resistant cultivars can restrict reproduction and alter local distribution.

2. Climate:

• Temperature influences hatching, development, reproduction, and survival.
• Soil moisture is required for movement, but prolonged flooding suppresses some species while favouring rice nematodes such as Hirschmanniella.
• Rainfall and humidity influence nematodes attacking aerial plant parts.

3. Soil properties:

• Texture, pore size, aeration, organic matter, salinity, and pH affect nematode activity.
• Many mobile ectoparasites are favoured by light sandy soils, while some sedentary endoparasites occur across a wider range of soils.

4. Biological factors:

• Competition, predation, parasitic fungi, bacteria, and host-root microorganisms regulate populations.
• Dormant eggs, cysts, seed galls, and anhydrobiotic stages permit persistence during adverse conditions.

5. Natural dispersal:

• Nematodes move short distances through soil-water films.
• Irrigation, drainage, floods, wind-blown soil, animals, and surface runoff increase local spread.

6. Human-assisted spread:

• Infested seedlings, tubers, suckers, seed, nursery soil, machinery, footwear, and packaging materials are major pathways.
• International trade can introduce exotic species or pathotypes.

7. Cropping practices:

• Rotation, tillage, irrigation, sanitation, and pesticide use alter field populations.
• Distribution is often patchy because nematodes spread slowly from initial infestation points.

Consequently, distribution maps must be interpreted as dynamic and should be updated through systematic surveys and reliable identification.

A nematode-distribution survey should use standardized sampling, identification, and recording procedures.

Survey procedure:

1. Define objectives: Decide whether the survey is for detection, delimitation, quarantine, or estimation of crop loss.
2. Stratify the region: Divide it by crop, soil type, climate, irrigation system, and administrative area.
3. Select fields: Use random or systematic selection, with additional sampling of fields showing suspicious patches.
4. Collect samples: Take multiple soil cores from the active root zone in a zigzag or grid pattern and combine them into a representative composite sample. Collect fresh feeder roots or other affected tissues.
5. Label and store: Record location, coordinates, crop, cultivar, crop history, symptoms, soil type, and date. Keep samples cool and prevent desiccation or overheating.
6. Extract nematodes: Use sieving and decanting, Baermann funnels, centrifugal flotation, root maceration, or cyst-extraction methods as appropriate.
7. Identify and quantify: Identify specimens by morphology and morphometrics, supported by biochemical or molecular tests when required. Express density per standard quantity of soil or root.
8. Confirm pathogenicity: Where necessary, reproduce symptoms on a suitable host under controlled conditions.
9. Map records: Enter validated coordinates and population data into a geographic information system to prepare occurrence or density maps.
10. Interpret and report: Relate distribution to soil, climate, host, and management, while distinguishing confirmed absence from areas not adequately sampled.

Repeated surveys are necessary because nematode populations and their geographical ranges can change over time.