Unit 4: Symptomatology and Disease Interaction - Subjective Questions

Symptomatology is the study of symptoms produced in plants as a result of nematode infection and the physiological disturbances caused by nematode feeding.

Major symptoms are classified into:

• Above-ground symptoms:
  • Stunting and poor plant growth
  • Yellowing or chlorosis
  • Premature wilting, especially during hot periods
  • Reduced leaf size and sparse foliage
  • Dieback and decline
  • Poor flowering, fruiting, and yield
• Below-ground symptoms:
  • Root galls or knots
  • Root lesions and necrosis
  • Excessive root branching
  • Stubby or shortened roots
  • Root-tip injury
  • Cysts or egg masses on roots
  • Root rotting in association with secondary pathogens

Most above-ground symptoms are nonspecific. Therefore, accurate diagnosis requires examination of roots, soil, and plant tissues for nematodes and characteristic signs.

Above-ground symptoms appear on shoots, leaves, flowers, or fruits and usually result from impaired root function.

• Yellowing, stunting, and reduced vigour
• Wilting despite adequate soil moisture
• Nutrient-deficiency-like symptoms
• Patchy growth within a field
• Premature senescence and reduced yield

Below-ground symptoms occur directly on roots, tubers, bulbs, or other underground plant parts.

• Root galls caused by Meloidogyne species
• Cysts produced by Heterodera or Globodera species
• Brown or black lesions caused by Pratylenchus species
• Stubby roots associated with Trichodorus and Paratrichodorus
• Excessive root branching or a hairy-root appearance
• Necrosis, rotting, and destruction of feeder roots

Below-ground symptoms are generally more useful for preliminary diagnosis, although laboratory confirmation is often necessary.

Nematode symptoms often occur in irregular patches because nematodes are not distributed uniformly in soil.

• Patchy distribution: Nematode populations are concentrated around infested planting material, previous host roots, soil clods, irrigation channels, or areas reached by contaminated machinery.
• Stunting: Feeding destroys root cells and reduces root elongation, limiting water and nutrient uptake.
• Chlorosis: Damaged roots absorb less nitrogen, magnesium, iron, and other nutrients, producing deficiency-like yellowing.
• Wilting: Loss of feeder roots and disruption of vascular tissues reduce water absorption and transport.
• Environmental influence: Symptoms become more severe under drought, high temperature, poor fertility, or other stress conditions.

The gradual spread of nematodes through cultivation, irrigation, runoff, and infected propagating material causes patches to enlarge over successive seasons.

• Root-knot nematodes, Meloidogyne spp.: Cause galls or knots through hypertrophy and hyperplasia of root tissues. Infected roots may become distorted, excessively branched, and inefficient in water uptake. Egg masses may be visible on the root surface.
• Cyst nematodes, Heterodera and Globodera spp.: Induce specialized feeding structures called syncytia. Adult females become swollen and later form durable brown or white cysts attached to roots. Plants develop reduced and poorly branched root systems.
• Lesion nematodes, Pratylenchus spp.: Migrate through cortical tissues, causing elongated brown or black necrotic lesions. Lesions may merge, resulting in extensive root decay.
• Stubby-root nematodes, Trichodorus and Paratrichodorus spp.: Feed near root tips and suppress root elongation. Repeated injury produces short, thickened, blunt roots with excessive lateral branching.

These symptoms reflect differences in nematode feeding behaviour and parasitic habit.

Direct symptoms develop at the nematode feeding or invasion site because of mechanical injury, enzymatic activity, and modification of plant cells.

Examples include:

• Root galls induced by Meloidogyne
• Necrotic lesions produced by Pratylenchus
• Stubby roots caused by trichodorid nematodes
• Cysts attached to roots after infection by cyst nematodes
• Seed or leaf galls caused by Anguina species

Indirect symptoms result from impaired plant functions or interactions with other pathogens.

Examples include:

• Chlorosis due to reduced mineral absorption
• Wilting caused by poor water uptake
• Stunting and yield reduction
• Increased susceptibility to fungal or bacterial diseases
• Viral symptoms following transmission by vector nematodes

Thus, direct symptoms reflect local tissue damage, whereas indirect symptoms represent systemic physiological effects or components of a disease complex.

• Ectoparasitic nematodes remain outside the root and insert their stylets into epidermal or cortical cells. Their feeding commonly causes root-tip injury, reduced elongation, swelling, and excessive lateral-root formation. Examples include Xiphinema, Longidorus, and Trichodorus.
• Migratory endoparasitic nematodes enter roots and move through tissues while feeding. Cell destruction during migration produces cavities, necrotic lesions, and predisposition to root-rotting organisms. Examples include Pratylenchus and Radopholus.
• Sedentary endoparasitic nematodes establish permanent feeding sites after entering roots. Root-knot nematodes induce giant cells, whereas cyst nematodes induce syncytia. These feeding structures act as strong nutrient sinks and cause galling, tissue distortion, reduced root efficiency, and plant stunting.

Therefore, symptom type is closely related to whether the nematode feeds externally, migrates through tissues, or establishes a permanent feeding site.

Plant-parasitic nematodes contribute to disease through several mechanisms:

• Mechanical injury: Stylet penetration and migration create wounds in plant tissues.
• Cell destruction: Migratory nematodes kill cortical and vascular-associated cells during feeding and movement.
• Physiological modification: Sedentary nematodes alter host gene expression and induce giant cells or syncytia.
• Nutrient diversion: Feeding sites become metabolic sinks that divert photosynthates and nutrients from normal growth.
• Reduced uptake: Root injury lowers the absorption of water and minerals.
• Altered defence: Nematode secretions and prolonged feeding may suppress or modify plant immune responses.
• Predisposition to pathogens: Wounds and weakened tissues facilitate infection by fungi and bacteria.
• Vector activity: Certain ectoparasitic nematodes transmit plant viruses.

Consequently, nematodes may act as primary pathogens, predisposing agents, synergistic partners, or vectors in plant disease complexes.

• Predisposition is a condition in which prior nematode infection makes a plant more susceptible to another pathogen. This may result from wounds, tissue necrosis, nutrient leakage, or weakened defence mechanisms.
• Synergism occurs when the combined disease severity caused by a nematode and another pathogen is greater than the sum expected from their separate effects. For example, root-knot nematodes may greatly increase Fusarium wilt severity.
• Disease complex refers to a disease condition produced through the interaction of two or more causal agents, often including a nematode and a fungus, bacterium, or virus, under a favourable environment.

A typical sequence is:

1. The nematode invades and modifies or damages root tissue.
2. The second pathogen enters or colonizes the affected tissue.
3. Host defence and vascular function are further disrupted.
4. Disease develops earlier or becomes more severe than either infection alone.

The severity of a disease complex depends on interactions among the host, nematode, associated pathogen, and environment.

• Host factors: Cultivar susceptibility, plant age, nutritional status, and resistance genes influence disease expression.
• Nematode factors: Species, race or pathotype, initial population density, feeding habit, and timing of infection are important.
• Associated pathogen factors: Virulence, inoculum density, infection route, and ability to colonize nematode-damaged tissues affect severity.
• Sequence of infection: Nematode infection before fungal or bacterial inoculation often produces stronger synergism than simultaneous or later infection.
• Time interval: An interval may be required for wounds, feeding sites, or physiological changes to develop.
• Environment: Soil temperature, moisture, pH, aeration, and nutrient availability influence both organisms.
• Microbial community: Antagonistic or beneficial microorganisms may suppress one or both pathogens.

Therefore, the presence of two pathogens does not automatically produce synergism; the outcome may be synergistic, additive, neutral, or antagonistic.

Nematode-fungus interactions frequently result in severe root rots and vascular wilts.

• Nematode penetration and migration create wounds that serve as entry points for fungal hyphae.
• Damaged cells release nutrients that stimulate fungal spore germination and growth.
• Root lesions provide dead or weakened tissues that are readily colonized by fungi.
• Nematode feeding alters host metabolism and may reduce structural or biochemical defence.
• Root-knot and cyst nematodes modify vascular-associated tissues and may increase the effect of wilt fungi.
• Destruction of feeder roots by both organisms intensifies water and nutrient stress.
• Fungal colonization may enlarge nematode-induced lesions and accelerate root decay.

Important complexes include Meloidogyne with Fusarium or Verticillium, Pratylenchus with Fusarium or Rhizoctonia, and Radopholus similis with root-rotting fungi.

Root-knot nematodes and Fusarium wilt fungi form an important disease complex in crops such as tomato, cotton, tobacco, and legumes.

• Juveniles of Meloidogyne penetrate roots and induce giant cells and surrounding gall tissues.
• Penetration sites, cracks near enlarged females, and damaged tissues may facilitate fungal entry.
• Nematode infection alters root physiology and may reduce the effectiveness of host resistance to Fusarium wilt.
• Fusarium colonizes vascular tissues and restricts water movement, producing yellowing, wilting, and vascular discoloration.
• Combined infection often causes earlier wilting, greater stunting, and higher mortality than either pathogen alone.
• The strongest interaction commonly occurs when nematode infection precedes fungal infection, allowing sufficient time for host modification.

Management must target both organisms through resistant cultivars, clean planting material, crop rotation, biological control, soil treatment, and sanitation.

Root-knot nematodes:

• Establish sedentary feeding sites and induce giant cells.
• Produce galls, tissue distortion, and cracks associated with enlarged females.
• Cause long-term physiological changes and nutrient diversion.
• Commonly interact with vascular wilt fungi such as Fusarium and Verticillium.
• May compromise resistance mechanisms in some host cultivars.

Lesion nematodes:

• Migrate continuously through the root cortex.
• Kill cells and produce extensive necrotic lesions.
• Create numerous infection courts and nutrient-rich dead tissues.
• Commonly interact with root-rotting fungi such as Fusarium, Rhizoctonia, and related soilborne fungi.
• Increase lesion size and accelerate cortical decay.

Both groups predispose roots to fungi, but root-knot nematodes primarily act through permanent feeding-site formation and physiological alteration, whereas lesion nematodes act mainly through migration, wounding, and cortical necrosis.

The sequence of infection strongly affects the outcome of a nematode-fungus interaction.

• Nematode before fungus: This often produces maximum disease because the nematode has time to create wounds, induce feeding sites, alter host metabolism, and weaken defence before fungal exposure.
• Simultaneous infection: Synergism may occur, but it can be weaker if suitable infection courts or physiological changes have not yet developed.
• Fungus before nematode: Severe fungal infection may reduce living root tissue available to nematodes. In some cases, fungal colonization may also interfere with nematode penetration or development.
• Length of interval: A short or moderate interval may favour predisposition, whereas a long interval may cause extensive tissue decay and reduce nematode reproduction.
• Pathogen biology: The optimum sequence differs according to whether the fungus is a vascular wilt pathogen, cortical root pathogen, or opportunistic colonizer.

Thus, controlled interaction experiments should include each organism alone, simultaneous inoculation, alternative inoculation sequences, different time intervals, and uninoculated controls.

Effective management must suppress both the nematode and the fungal pathogen.

• Use certified, pathogen-free seed, seedlings, tubers, or other planting material.
• Grow cultivars resistant or tolerant to the nematode, fungus, or both.
• Rotate with non-host or poor-host crops after identifying both pathogens.
• Remove infected residues and prevent movement of contaminated soil and irrigation water.
• Improve drainage, soil structure, fertility, and organic matter to support root health.
• Apply suitable biological control agents, including antagonistic fungi or bacteria, where validated.
• Use soil solarization, organic amendments, or approved nematicides and fungicides when economically justified.
• Control weeds that maintain either pathogen.
• Monitor nematode population density and fungal inoculum before planting.

Treating only the fungus may fail if nematodes continue to predispose roots, while nematode control alone may be insufficient where fungal inoculum is already abundant.

Nematodes can enhance bacterial diseases through physical and physiological effects.

• Stylet punctures and nematode penetration create wounds through which bacteria enter.
• Migrating nematodes damage cortical and vascular-associated tissues, producing additional infection courts.
• Leakage of sugars, amino acids, and other nutrients from injured cells supports bacterial multiplication.
• Nematode-induced galls and tissue cracks may retain moisture and provide protected sites for bacterial colonization.
• Feeding can alter plant defence responses, making tissues more susceptible.
• Combined root damage and bacterial colonization intensify wilting, rotting, and plant decline.
• Some nematodes may carry bacteria externally or internally, although mechanical transmission is not universal and must be demonstrated for each association.

Examples include interactions of root-knot nematodes with bacterial wilt caused by Ralstonia solanacearum and bacterial diseases associated with Anguina species.

The root-knot nematode-bacterial wilt complex commonly involves Meloidogyne species and Ralstonia solanacearum.

• Root-knot juveniles penetrate roots and establish feeding sites, causing gall formation and tissue disruption.
• Wounds and cracks associated with penetration and gall development can facilitate bacterial invasion.
• Nematode-induced physiological stress may weaken root defence and accelerate bacterial colonization.
• The bacterium multiplies in xylem vessels and produces vascular blockage and dysfunction.
• Combined infection causes rapid wilting, severe stunting, poor root development, and plant death.
• Disease may occur earlier and become more severe than infection by the bacterium alone.
• Nematode infection can sometimes reduce the practical effectiveness of resistance expressed against bacterial wilt.

The complex is agriculturally significant because both organisms persist in soil or host plants, have broad host ranges in many production systems, and are difficult to control after establishment.

Species of Anguina are seed- and aerial-part-infesting nematodes that may interact closely with bacteria.

A well-known example is the association of Anguina tritici with Rathayibacter tritici in tundu or yellow ear rot of wheat.

• Infective nematode juveniles migrate in surface moisture to young wheat tissues.
• The nematode invades floral primordia and normally induces seed galls.
• Bacterial cells associated with the nematode reach susceptible floral tissues.
• The bacterium multiplies and produces yellow, sticky slime on affected ears.
• Diseased heads may become distorted, and grain development is reduced.
• Dry bacterial slime and nematode-containing galls can contaminate harvested seed and contribute to disease carryover.

This association demonstrates that a nematode may do more than create wounds; it can assist the movement and establishment of a bacterial pathogen in susceptible plant tissue.

Certain soil-inhabiting ectoparasitic nematodes transmit plant viruses while feeding on root cells.

The principal vector groups are:

• Xiphinema and Longidorus: Transmit several nepoviruses.
• Trichodorus and Paratrichodorus: Transmit tobraviruses.

The transmission process involves:

1. A vector nematode feeds on a virus-infected plant and acquires virus particles.
2. Virus particles are retained at specific sites in the lining of the feeding apparatus, especially the odontostyle-oesophageal region or oesophageal lumen, depending on the vector group.
3. The nematode moves through soil and begins feeding on a healthy root.
4. Retained virus particles are released or introduced into wounded root cells during feeding.
5. The virus replicates in the plant and may move systemically.

These are highly specific associations: a nematode species generally transmits only particular viruses or virus strains.

Nepovirus transmission:

• Vectors mainly belong to Xiphinema and Longidorus.
• These nematodes possess a long odontostyle and feed on root tissues.
• Examples include transmission of grapevine fanleaf virus by Xiphinema index and arabis mosaic virus by certain longidorids.
• Virus particles are retained at specific sites within the feeding apparatus.

Tobravirus transmission:

• Vectors belong to Trichodorus and Paratrichodorus.
• These nematodes have a curved onchiostyle and feed mainly on epidermal or cortical root cells.
• Important examples include tobacco rattle virus and pea early-browning virus.
• Vector-virus specificity is associated with interactions between viral coat proteins and retention sites in the nematode feeding system.

In both cases, the nematodes acquire virus during feeding and inoculate healthy plants during subsequent feeding. The virus does not normally multiply within the vector, so transmission is described as non-propagative.

Biological features:

• Transmission involves specific recognition between the virus and retention sites in the nematode feeding apparatus.
• The virus is acquired from infected roots and inoculated into healthy roots during feeding.
• Virus particles generally do not replicate within the nematode.
• Retention may persist for extended periods in adult or juvenile vectors, but virus is generally lost when a juvenile molts because the relevant lining is shed.

Epidemiological importance:

• Vector nematodes survive in soil and move viruses between roots.
• Their uneven soil distribution produces expanding patches of infected plants.
• Perennial hosts and weeds act as reservoirs for both virus and vector.
• Viruses may spread locally through vector movement and over longer distances through infected planting material or contaminated soil.

Management implications:

• Use virus-free planting material and resistant cultivars.
• Test soil for vector nematodes before establishing susceptible crops.
• Control reservoir weeds and volunteer hosts.
• Prevent movement of contaminated soil and infected propagating material.
• Use rotation, fallow, soil treatment, or other locally effective measures to reduce vector populations.

Successful control requires simultaneous management of the virus, its nematode vector, and alternative host plants.