The most significant nematode species on agricultural crops

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nematode species

"Definition of nematodes"

Nematode was initially derived from two Greek words: nema, meaning thread, and eidos, meaning similar. 

Accordingly, these organisms were known as nematodes, but they are now known as nematodes, or for short, nematodes. Sometimes, they are called snake worms, but this name is inappropriate. 

The latter is not entirely correct because not all nematodes have serpentine movement.

Nematodes are invertebrate (primitive) cylindrical worms. They are considered mainly aquatic animals that live in salt or freshwater, or at least their bodies must be covered by a thin membrane of water in the soil to be alive and active. 

Nematodes are found in capillary water around soil grains and are widespread as they can be found in any environment where the reasons for life are available. 

They are found in dry desert lands, polar regions, hot spring waters, and ocean depths.

Opinion has finally settled on placing it in an independent division (tribe) within the animal kingdom, which is the Phylum Nematode division (tribe), due to the increased interest in studying it, especially as an agricultural pest. This phylum is considered one of the largest multicellular animal groups after the insect group, especially in density and diversity.


Morphological features of nematodes:

Its body is cylindrical and transparent.

Spindly or filamentous bodies are possible.

Some females of some sexes may swell and take different shapes (globular-lemon-pear-renal).

Its size ranges from 0.4 to 4 mm.

Parasitic species on plants are not more than 1 mm long and may reach 4 mm.

The width of the nematodes is no more than 15-20 microns, and the average length of these worms is between 3-6 mm.

It is also not visible to the naked eye, and its body is transparent, as it does not contain colored pigments.

One drop of water may contain several thousand (8000) nematodes.


The life cycle of nematodes

It takes 30-45 days in temperatures ranging from 25 to 30 degrees Celsius.

The life cycle is long in winter and reaches 60-90 days.

It comes to a halt if the temperature falls below 12 degrees Celsius.

Females lay eggs (singly or in clumps) within plant tissues or in the soil.

Egg-first age caterpillar-second age caterpillar-third age caterpillar-fourth age caterpillar-whole animal.

The infective stage in most species of nematodes is the second larval stage.

The number of generations depends on the type of nematode:

Root-knot nematodes (7–10 generations).

Renal nematodes (6–8 generations).

Citrus nematodes (4-5) generations take 30-45 days at a 25–30 m temperature.

Means of transmission and spread of nematodes

  • Transfer of agricultural soil contaminated with nematodes.
  • Use of compost contaminated with soil residues and infected roots.
  • Infected seedlings, seeds, and bulbs.
  • Humans and animals.
  • Remains and residues of infected plants (roots).
  • Agricultural machinery (plow).
  • Irrigation water, especially in the case of surface irrigation.

How do nematodes feed on plants?

Nematodes that feed on plants have needle-like spears 12–100 microns long. Nematodes feed on plants by piercing the plant cell walls with this spear, secreting enzymes, and then sucking up the contents of the cell.

The ability to absorb also occurs due to contraction in the esophageal muscle.

Totals of nematodes present in the soil:

1-Replenished nematodes.

2- Predatory nematode.- 

3- Plant parasitic nematodes: They are the ones that interest us in this field. The root-knot and citrus nematodes are two of the most well-known species in this group, found throughout Egypt. There are two types of nutrition in nematodes parasitic on plants:

  • external intrusion.
  • semi-indoor intrusive (internal static-internal roving).
  • internal snooping.
Damage caused by nematodes to crops
  1. Ulcers or tumors and knots on the roots.
  2. Yellowing, wilting, and distortion of roots and plants.
  3. Plants are stunted and, in some cases, completely collapse.
  4. It also affects the size and quality of the crop.
  5. Determine the agricultural cycle.
  6.  The presence of these nematode pests in large numbers causes crop cycle designers to be limited to a certain number of crops.
  7. Limit the use of agricultural fertilizers, such as superphosphate and urea, to increase natural enemies and decrease the number of pests.
  8. Conducting many agricultural transactions aimed at eliminating pests, such as using pesticides or farming methods that may result in land flooding or waste, adds expenses to the cost of production.
  9. When the pest population reaches an epidemic level, the land loses its productive value. Because it lacks elements, it affects plant productivity and is called "soil sick."
  10. Pests decrease soil fertility, as nematodes may eat the nitrogen-fixing bacteria in the soil.

  Factors on which nematode damage depends

The type of the crop or the plant itself (sensitive-resistant-tolerant).

Type of nematode.

Infestation size and nematode numbers.

Other environmental factors:

Soil temperature and moisture.

Soil pH.

Soil microorganisms.

The most severe nematode diseases:

  • Nematodes infect the shoots:

1-Leaf nematodes such as Aphelenchoides spp.

2-Market and bulbous nematodes such as Ditylenchus spp.

3-Seeded nematodes (for example, Anguina tritic).

  • Nematodes infect the root system.

1-External parasites such as Tylenchorhenchus

2-Endoparasites such as Tylenchulus semipenetrans - Rotylenchulus reniformis.

3-Semi-indoor parasitoids ( Meloidogyne spp., Pratylenchus).

  • The most significant nematode species on agricultural crops

1- Soybean cyst nematode (SCN)

One of the most essential soybean infections in most regions where soybeans are farmed is the soybean cyst nematode (SCN). In Minnesota, it is pervasive and has expanded into new areas and counties. 
The yield loss from SCN might be more significant than 30%. Round, tiny parasites called soybean cyst nematodes attack the roots of plants, including soybean. Even with a yield loss of 15 to 30 percent, above-ground signs are not usually obvious. As a result, SCN may exist in a field for years before it is discovered.

 When symptoms appear, they are vague and resemble those brought on by other infections or abiotic stressors. SCN infection can potentially worsen soil-transmitted illnesses affecting soybeans, such as brown stem rot, sudden infant death syndrome, and fusarium.

Symptoms

Young (microscopic) nematodes enter the plant's roots and eat. They get bigger as they age as long as they keep eating the roots. The best method for identifying SCN is to carefully remove roots from the ground and inspect them for females and cysts visible to the naked eye or with a hand lens. Beginning from the end of June and for the rest of the season, little white to yellow females with a lemon-shaped shape can be spotted on roots. In the early and middle of the summer, the females' bodies are white and relatively easy to notice; later in the season, they become brown and are more difficult to spot.

Due to the female nematodes' feeding on the infected plants, these conditions might include chlorotic discoloration, stunting, or necrosis.

Disease control

To find out if SCN is present and what the population in the soil is, soil samples should be collected and sent to a diagnostic lab. Crops other than hosts should be alternated with soybeans. It is advisable to grow resistant soybean types and to rotate between them. Cultivars with various SCN resistance sources ought to be rotated in. Weeds, moisture, and fertility should be controlled to lessen plant stress.

2-Root-knot disease

Cause: Root-knot nematode

Importance and distribution: Various species of the genus Eloidogyne M cause root-knot disease on thousands of plants worldwide. 

The root-knot nematode is the most essential plant nematode and one of the most prevalent species in many vegetable crops (tomatoes, cucumbers, potatoes, cucurbits, legumes), fruits (grapes, peaches, figs, olives, bananas, pomegranates), and field crops.

 On sandy and light lands, the loss due to infection may exceed the loss of the entire crop, and it is considered a parasitic and dormant internal nematode.

Species: The following four species are M. arenaria, M. hapla, M. javanica, and M. incognita. They are the primary and most common species in agricultural lands, constituting more than 95% of the root-knot nematodes of farmlands worldwide.


Symptoms of infection: One of the most important symptoms of this pest is the presence of knots or tumors on the roots of infected plants. Some bacterial diseases, such as coronary tuberculosis, or some protozoa, such as tumors, occur, so care must be taken in judging the infection by just looking. Therefore, it is necessary to examine the roots and soil laboratory.


How tumors form: They are formed as a result of the secretion of enzymes that break down proteins into amino acids, including the amino acid (tryptophan), which enters the vital processes within the plant tissue, forming indoleacetic acid, which pushes cells to divide and form complexes called giant cells.

3-Citrus slow decline disease

Cause: Citrus nematode

Importance and spread: 

This nematode is responsible for the slow decline in citrus diseases, one of the most dangerous and influential diseases of citrus and olives, whether in our Arab region or wherever citrus trees are grown worldwide. Symptoms of deterioration usually appear on infected trees after 3-5 years of infection and may last up to 10 years, depending on the extent of soil and seedling infection. Therefore, this disease is called "slow deterioration."

Citrus nematode is the name given to one species, T. semipentrans, but these nematodes appear to have developed strains that can reproduce on plants other than citrus, such as olives, grapes, and mangoes.

Symptoms of infection:  

Nematodes feed on the cortex area of ​​the host's root, and through continuous feeding, cells die, ulcers appear, and tissue dies on the roots. They are dark brown and stick to the soil granules firmly due to the gelatinous substance in which the female lays her eggs. In the case of severe infection, the cortex is separated from the cylinder. Symptoms of yellowing, wilting, and a decrease in the yield of the affected trees also appear.

4- Renal nematode disease

Cause: -renal nematode

Importance and spread: 

The renal and citrus nematodes constitute the two most important genera of semi-endogenous feeding nematodes. About 65 species of the genus Rotylenchulus are known, but the most famous and widespread is the type R. Grapes, jasmine, soybeans, tomatoes, and bananas.

Symptoms of infection:

Infected trees and seedlings are weak and stunted. Ulcers are noted in the epidermal layer and bark when the roots are examined. In severe infections, the crust layer separates from the vascular cylinder. Other organisms entering the infestation increase the nematode's effect and accelerate the decomposition of the affected tissue.

5- canker sore disease

Cause: -root canker nematode

Importance and spread:

The canker nematode Pratylenchus spp. It is one of the most important genera of nematodes that parasitizes and wanders inside the endo-migratory roots and is spread almost worldwide. 

With its severe canker sores on the roots, it is considered responsible for the deterioration of the root system of infected seedlings and trees, leading to a significant decrease in vegetative growth and production.

 Among the most important crops that affect it are (peach-pear-strawberry-potato-apple-banana-citrus-peanuts-olives-cotton-cowpea-peas-tomatoes-alfa-beets).

Species:

 More than 166 species belong to the Pratylenchus genus, most of which are roving endoparasites. The most critical and widespread species are P.penetrans, P.brachyurus, P.zeae, P. vulnus, and P. coffeae. Penetrans Pratylenchus is the most common species, followed by P. vulnus.

Symptoms of infection:

Infected trees show general symptoms such as weak plants, dwarfing, and yellowing of their leaves. These symptoms also appear in the field in the form of groups of plants in scattered spots, depending on the severity of the infection. 

There are also symptoms on the affected roots, such as dark brown ulcers scattered along the root caused by the oxidizing phenols produced by the parasitism nematode. These phenols cause the death of many cortex cells and may separate from the vascular cylinder.

Cause: -renal nematode

Importance and spread: The renal nematode and citrus nematode constitute the two most important genera of semi-endogenous feeding nematodes, and about 65 species of the genus Rotylenchulus are known so far, but the most famous and most widespread is the type R. Grapes, jasmine, soybeans, tomatoes, bananas.

Symptoms of infection:

Infected trees and seedlings are weak and stunted. Ulcers are noted in the epidermal layer and bark when the roots are examined. In severe infections, the crust layer separates from the vascular cylinder. Other organisms entering the infestation increase the nematode's effect and accelerate the decomposition of the affected tissue.