🪱Nematode Diseases — The Invisible Root Destroyers

From Field to Lab — The Underground Menace

A tomato farmer in Nashik uproots a stunted, wilting plant and notices swollen, knotted roots that look like beads on a string — classic root-knot nematode (Meloidogyne) galls. A banana plantation in Kerala where trees topple over in mild wind despite healthy-looking leaves is under attack from the burrowing nematode (Radopholus similis). A wheat field showing patchy poor growth has cereal cyst nematode (Heterodera avenae) silently feeding inside the roots. These microscopic worms cause estimated losses of 10–15% of crop production globally, yet they remain the most overlooked pest group in Indian agriculture.

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Introduction to Nematology

Nematodes are microscopic, unsegmented roundworms that live in soil, water, and plant tissues. Many species are plant-parasitic and cause significant economic losses in agriculture worldwide.

• 1743 — Needham discovered the wheat seed-gall nematode (Anguina tritici), the first plant-parasitic nematode to come to the attention of early investigators.
• Father of Nematology — H.C. Bastian
• Father of American/Modern Nematology — N.A. Cobb
• Nematodes generally undergo four moultings (i.e., they have four larval stages)
• First scale to measure plant diseases — Cobb’s scale

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Classification by Feeding Habit

Plant-parasitic nematodes are classified based on how and where they feed on host plants.

Ectoparasites

Nematodes that feed on the outer surface of roots without entering the plant tissue. They use their stylet to puncture cells from outside.

• Examples: Dagger nematode (Xiphinema), Sting nematode (Belonolaimus), Needle nematode (Longidorus)

Endoparasites

Nematodes that enter and feed inside plant tissues. They are further divided into:

Sedentary Endoparasites — Enter the root, establish a permanent feeding site, and become immobile.

• Examples: Root-knot nematode (Meloidogyne), Cyst nematode (Heterodera, Globodera)

Migratory Endoparasites — Move freely within root tissues, feeding as they go.

• Examples: Lesion nematode (Pratylenchus), Burrowing nematode (Radopholus)

Semi-endoparasites

Nematodes that partially enter roots — the anterior portion feeds inside while the posterior remains outside.

• Example: Citrus nematode (Tylenchulus semipenetrans), Reniform nematode (Rotylenchulus)

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Antagonistic Crops

Certain crops produce chemicals or alkaloids as root exudates that repel or suppress plant-parasitic nematodes. These are called Antagonistic crops.

• Mustard — antagonistic to Cyst nematode
• Marigold — antagonistic to Pratylenchus, Rotylenchulus, and Meloidogyne spp.
• Other examples include neem

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Major Nematode Genera and Their Diseases

Complete Table of Plant-Parasitic Nematodes

Common Name	Scientific Name	Key Facts
Root-knot nematode	Meloidogyne spp.	Root knots are hard, deep-seated; controlled by Paecilomyces lilacinus
Rice root-knot nematode	Meloidogyne graminicola	Hook-like galls on roots; leaf distortion and crinkling; stunting in patches
Cyst nematode	Globodera spp. and Heterodera spp.	Female body becomes a cyst filled with eggs
Cereal cyst nematode	Heterodera avenae	Affects wheat and barley
Potato cyst nematode	Globodera rostochiensis	Major quarantine pest
Burrowing nematode	Radopholus similis	Causes toppling disease of banana
Citrus nematode	Tylenchulus semipenetrans	Causes slow decline/citrus decline and die-back
Reniform nematode	Rotylenchulus spp.	Kidney-shaped females; wide host range
Lesion nematode	Pratylenchus spp.	Causes root lesions; migratory endoparasite
Root lesion nematode	Pratylenchus sp.	Causes necrotic lesions on roots
Lance nematode	Hoplolaimus spp.	Migratory ectoparasite
Sting nematode	Belonolaimus	Large ectoparasite
Dagger nematode	Xiphinema spp.	Vector of plant viruses
Ring nematode	Criconemella spp.	Ectoparasite with cuticular annulations
Spiral nematode	Helicotylenchus spp.	Body curved in spiral shape
Needle nematode	Longidorus spp.	Sword-like stylet; transmits fan leaf virus of grape
Pin nematode	Paratylenchus spp.	Very small ectoparasite
Sheath nematode	Hemicycliophora spp.	Has a loose-fitting sheath
Stunt nematode	Tylenchorhynchus spp.	Ectoparasite causing stunting
Stubby-root nematode	Paratrichodorus and Trichodorus spp.	Causes short, stubby roots
Stem nematode of rice	Ditylenchus angustus	Causes Ufra disease of rice; twisting of leaf and leaf sheath
Stem and bulb nematode	Ditylenchus dipsaci	Causes onion bloat disease
Potato root nematode	Ditylenchus destructor	Causes rots on potato roots
Mushroom nematode	Ditylenchus mycelophagus	Feeds on mushroom mycelium
Seed-gall nematode	Anguina tritici	Causes wheat ear cockle; conversion of grains into cockles/galls
Foliar nematode	Aphelenchoides spp.	Feeds on leaves and buds
Rice root nematode	Hirschnynniella sp.	Affects rice roots

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Root-Knot Nematode (Meloidogyne spp.)

The most economically important group of plant-parasitic nematodes worldwide.

• Root knots/galls are hard in consistency and deep-seated
• Sedentary endoparasite — female becomes pear-shaped inside root tissue
• Wide host range — attacks vegetables, pulses, cereals, fruit crops
• Biocontrol agent: Paecilomyces lilacinus (egg parasite)
• Rice root-knot (M. graminicola) causes characteristic hook-like galls on roots, leaf distortion, and stunting in patches

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Cyst Nematode (Heterodera and Globodera spp.)

• Cereal cyst nematode — Heterodera avenae (attacks wheat and barley)
• Potato cyst nematode — Globodera rostochiensis (major quarantine pest)
• Sugarcane cyst nematode — Heterodera sacchari
• Dead female body forms a hardened cyst containing eggs that can survive in soil for years

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Burrowing Nematode (Radopholus similis)

• Causes toppling disease of banana
• Migratory endoparasite — burrows through root cortex causing extensive damage
• Affected banana plants topple over due to weakened root system

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Citrus Nematode (Tylenchulus semipenetrans)

• Causes slow decline / citrus decline and die-back
• Semi-endoparasite — female head embedded in root, body outside
• Most important nematode pest of citrus worldwide

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Reniform Nematode (Rotylenchulus reniformis)

• Named for the kidney-shaped (reniform) body of mature female
• Semi-endoparasite with very wide host range
• Important pest of cotton, soybean, and vegetables

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Seed-Gall Nematode (Anguina tritici)

Causes wheat ear cockle disease — one of the most frequently asked nematode facts in exams.

• Grains are converted into cockles or galls (dark, hard, irregularly shaped)
• Seedling leaves are twisted and crinkled
• Seedlings may be stunted and die
• Shows profuse tillering
• Tundu/Sehu or Yellow ear rot of wheat is caused by Anguina tritici + Clavibacter tritici (nematode-bacterium complex)

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Stem Nematode (Ditylenchus spp.)

• Ufra disease of rice — caused by D. angustus; symptoms include twisting of leaf and leaf sheath

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Pine Wilt Nematode (Bursaphelenchus xylophilus)

• Causes pine wilt disease — rapid wilting and death of pine trees
• Vectored by longhorn beetles (Monochamus spp.)
• Major quarantine concern for coniferous forests

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Foliar Nematode (Aphelenchoides spp.)

• Feeds on leaves and buds (above-ground parts)
• Causes angular leaf spots, leaf distortion, and bud necrosis
• Important on ornamentals like chrysanthemum and fern

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Needle Nematode (Longidorus spp.)

• Has a characteristic sword-like stylet
• Transmits fan leaf virus of grape
• Large ectoparasite

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Nematode Management

A. Chemical Control

Fumigants:

• DD (mixture of dichloropropane and dichloropropene)
• EDB (ethylene dibromide)
• MBr (methyl bromide)
• Nemagon (DBCP — dibromochloropropane)

Non-fumigant Nematicides:

• Carbamates: Aldicarb, Carbofuran (Furadan), Metham sodium (Vapam)
• Organophosphate: Phorate (Thimet)

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B. Physical Control

• Hot water treatment — 54°C for 10-15 minutes

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C. Cultural Control

• Crop rotation with non-host crops
• Use of antagonistic/trap crops (mustard, marigold, neem)
• Flooding — kills nematodes by asphyxiation
• Soil solarization — using transparent polyethylene sheets to raise soil temperature

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D. Biological Control

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E. Regulatory Control

• Adoption of quarantine regulations — e.g., potato tubers used as seed material have restricted movement from Nilgiri Hills (TN) to other parts of the country
• Certification of planting material as nematode-free

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Summary Cheat Sheet

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Miscellaneous Exam Facts

• Gram staining of bacteria was discovered by H. Grahm (1884)
• Parasitic nature of fungi was proven by Anton De Bary
• Concept of vertical and horizontal resistance — J.E. Vandar Plank
• Green ear disease of bajra first reported by E.J. Butler (1907)
• Crystallization of viruses first done by W.M. Stanley (1935)
• Gelatin plate technique — Robert Koch
• Germ theory — Louis Pasteur
• Oldest known viral disease of plants — Tulip color breaking
• Bordeaux mixture first used in India against leaf spot of groundnut in 1904 by Lawrence
• Solar energy treatment — Luthra & Sattar
• First plant virus whose genome was completely sequenced — Cauliflower mosaic virus
• First systemic fungicide — Carboxin
• Fungi named as weed of laboratory — Aspergillus niger
• ”Shepherd crook” symptoms — Fire blight of pears and apple
• Rice fever disease — Blast of rice