PLANT PARASTIC NEMATODE (PPN) FACT SHEET
Nematodes are worm-shaped non segmented, nearly microscopic animals; many are virtually invisible to the unaided eye. Of the known nematode species, approximately 50%, 25%, 15%, and 10% are free-living, marine, animal parasites, and plant parasitic nematodes (PPNs), respectively.
Types of nematodes
There are three functional groups of nematodes:
Saprophytic nematodes are also known as decomposers because they break down organic matter in the soil, release nutrients for plant use, and improve soil structure, water holding capacity and drainage. They are usually the most abundant type of nematode in the soil.
These nematodes feed on other nematodes, so can be useful in controlling pest species. They eat larger nematodes by attaching themselves to their cuticle and scrape away until the prey’s internal body parts can be extracted. They also eat bacteria, fungi, and small single celled organisms (protozoa). The digested pests are then added to the soil organic matter reserves. Some have become specialized predators of insects, known as entomopathogenic nematodes.
For the sake of simplicity; Crop Nutritional Laboratories (CNL) combines the two together in the reports as these don’t cause any harm to the crops.
Plant Parasitic nematodes (PPNs)
Parasitic nematodes cause problems in agricultural production because they feed on plant roots and slow plant growth. In some cases they also allow the entry of fungal rots that destroy the roots. Agricultural cultivation tends to encourage an increase in parasitic nematodes over other species
They feed on plant cells using a stylet which is a spear-like structure. It’s so distinct in most PPNs. They use this small, sharp, hollow structure to withdraw plant cell juice by sucking and thereby weaken the host plant.
PPNs may be endoparasitic (feeding inside the plant cell), semi-endo or semi-ecto parasitic (body half inside and half outside the plant surface), and/or ectoparasitic (feeding from outside the plant cell).This is based on the feeding habits. They can also be classified based on the plant part attacked e.g. Root nematodes (Meloidogyne spp, pratylenchus spp), Foliar nematodes (Anguina, Aphelenchoides etc.) and Bulb and stem nematodes (Ditylenchus spp).
The common PPN species associated with crops are: root-knot ( Meloidogyne spp.), root lesion (Pratylenchus spp.), spiral (Helicotylenchus and Scutellonema spp.), bulb and stem (Ditylenchus spp.), foliar (Aphelenchoides spp.), dagger (Xiphinema spp.), lance (Hoplolaimus spp.), citrus nematodes (Tylenchulus spp.),ring nematodes (Criconemoides spp) and the stunt nematodes (Tylenchorhynchus spp).
Symptoms caused by most of the plant parasitic nematodes are difficult to distinguish from those caused by other soil-related problems. Thus, nematode identification based on symptomology alone is difficult. Laboratory diagnostics is compulsory to help in identification hence making proper management decisions.
General symptoms include;
- Non-uniform plant growth — mostly in patches and poor plant establishment giving unhealthy appearance.
- Plants weakened over time and are killed when combined with other problems.
- Chlorosis and stunting (symptoms resembling those of some virus infections and nutrition deficiencies)
- Root knots (galls), lesions on roots, stubby roots or splitting of roots.
- Excessive branching of roots and hairy root symptoms.
- Poor root health, growth and establishment.
- Premature leaf drop and an increased tendency to wilt or dieback during dry periods.
Knowing the number of PPNs present helps to determine the control strategies. If the population level is high enough to cause economic damage (i.e., at or above the ‘economic density threshold’ for that species), then application of control strategies is recommended.
Important Beneficial Nematodes
1) Saprophytic (Saprofagic) Nematodes (Free living)
These are also known as decomposers because they break down organic matter in the soil, release nutrients for plant use, and improve soil structure, water holding capacity and drainage. They are usually the most abundant type of nematode in the soil. They are either fungal or bacterial feeders. Both mineralize N from their prey groups. Bacterial-feeding nematodes are more important in bacterial-dominated soils while fungal-feeding nematodes are more important in fungal dominated soils. The presence and numbers of bacterial- and fungal-feeding nematodes is extremely important for productive soils. However, in agricultural systems, bacterial-feeding nematodes typically release more inorganic N than fungal-feeding nematodes.
These are the most abundant nematode group in agricultural soils. Their abundance closely
follows that of bacterial populations, which also tend to increase when soil disturbances, such as tillage, increase the availability of readily-decomposable organic matter.
Have ornate lip structures that help in sucking the bacteria. Bacteria feeders release plant-available nitrogen when they consume bacteria.
Fungal-feeding nematodes are relatively more abundant in less-disturbed (e.g. notill systems) and perennial systems, where conditions for fungal growth are promoted, than in disturbed systems.
They havesmall, narrow stylets, or spears, in their stoma (mouth) which they use to puncture the cell walls of fungal hyphae and withdraw the cell fluid. This interaction releases plant-available nitrogen from fungal biomass.
These nematodes feed on other nematodes and other small soil organisms. They eat smaller organisms or attach themselves to their cuticle and scrape it away until the prey’s internal body parts can be extracted. They help moderate population growth of bacterial- and fungal-feeding nematodes and protozoa, and help regulate populations of plant-parasitic nematodes. Some have become specialized predators of insects, known as entomopathogenic nematodes.
NB: Have teeth within their mouths.
A predatory nematode feeding on a parasitic nematode
Important Plant Parasitic Nematode Genera in Crops
The Root Knot nematodes (Meloidogyne spp)
This is sedentary endoparasite. Males and juveniles are filiform while the females are globular. Root-knot nematodes can go from egg to reproducing adult (complete life cycle) in as little as 3 to 4 weeks. Plant root cells around the infective female larvae react by dividing many times and enlarging to produce a gall.
Depending on the particular root-knot nematode species, the susceptibility of the host plant and the numbers of infecting nematodes root galls may vary in size from 1/8 of an inch to 1 or more inches in diameter. At high population levels most of the roots become galled. As the female nematode feeds inside the root, the gall enlarges, becomes spherical, and 15 to 30 days after infection the female lays 200-500 eggs in a mass attached to their body. Root galls are easily seen by our naked eyes and root-knot infected plants can be recognized easily by uprooting plants and looking at the roots.
Known to be parasitic to hundreds of species, including fruits, grasses, vegetables, and numerous weeds.
As a result of feeding, infested roots show
- Knot-like galls on roots.
- Stunted growth and plants have chlorotic leaves.
The affected roots exhibit severe galling. NB: Galling is the result of the proliferation of cells of the affected roots.
Root-knot nematode galls on roots
Lesion nematodes (Pratylenchus spp)
These are migratory nematodes. They enter the root and burrow tunnels through the root cortex. All life stages except the egg stage, are motile and can invade roots. Eggs are laid inside root tissues or in the soil. They hatch, and the juveniles enter the roots and contribute to root injury. Since they are migratory they are capable of repeatedly entering and exiting from root tissue, although several generations can occur inside the roots without the nematodes migrating into the surrounding soil.
Known to parasitize over 400 plant species including potato, peanut, monocots, and fruits.
These nematodes cause small brown lesions on the white lateral roots and kill the fine
feeder roots. The entire root system appears discolored when these lesions merge. Such root lesions are frequently invaded by other root-rotting fungi and bacteria. Severely affected plants may lose all feeder roots. Infected plants often exhibit stunting, chlorosis, and twig dieback. They can cause a decline in vigor of plants.
Adult root lesion (Pratylenchus sp. ), short body and stout stylet.
Burrowing nematodes (Radopholus spp)
All nematode stages are vermiform (wormlike), colorless and less than 1 mm in length. Adult males and females are different in appearance.
They are migratory endoparasites, completing its life cycle as it tunnels through the root cortex. Females and juvenile stages are infective but males with their weak stylets do not feed. The nematode usually penetrates roots near the tip, but can invade along the entire length of the root. They move between cells of the root cortex, feeding on them until the cells collapse and form necrotic passages.
Radopholus similis female head region (A) and full body with vulva [v] near midbody (B)
Over 350 plant hosts in tropical and subtropical regions including banana, citrus, black pepper, ginger, tea, coconut and other tropical palms.
The nematode causes a slow decline of many plant species, but symptoms are distinctive in banana and plantain ( Musa hybrids and cultivars.), citrus (Citrus spp.) and black pepper (Piper nigrum).We will highlight the damage on banana and plaintain alone.
Banana and plantain
The most dramatic disease symptom in banana plantations is the uprooting (toppling) of plants. Burrowing nematode feeding destroys anchor roots and makes plants susceptible to toppling, especially when fruiting or during strong winds. Additional aboveground symptoms in bananas and plantains caused by root damage include slow sucker formation, delayed fruiting, smaller fruit and reduced bunch weight, and a shortened plant life. It also kills feeder roots and creates reddish-brown lesions on larger root surfaces, both at the point of entry and throughout the cortex. Eventually, burrowing nematodes can migrate from roots into the rhizome causing black, circular lesions, causing the blackhead disease. Emerging roots may be attacked as they grow out of the infected rhizome.
Lodging banana plants suffering from Toppling Disease caused by the burrowing nematode.
Spiral nematodes (Helicotylenchus spp)
Spiral nematodes generally are ectoparasites, but some are semi-endoparasites and a few are endoparasites. Ecto and semi-endoparasites
lay their eggs in soil near to the roots whereas endoparasites lay eggs inside the roots.
They form a C-shape when resting.
The ectoparasites feed at or near the root tips causing de-vitalization, the most severe type of root injury. They also induce stubbiness to primary and lateral roots and coarse root systems lacking feeder roots.
They cause lesions which are restricted to the outer cortex.
Banana roots with necrosis confined to outer cortex due to attack by the spiral nematode(Helicotylenchus spp).
Adult showing the characteristic spiral body shape, stout tail and stylet. (Note the terminal projection on the tail-arrow).
Spiral nematodes ( Scutellonema spp)
This is a migratory endoparasite of mainly roots and tubers. These include yams and potatoes. It will also be present in soils around the host plants. It has also been reported in legumes and grass family crops.
Morphologically similar to Helicotylenchus spp but they have a scutellum which is characteristic of this spp.
Posterior body portion of Scutellonema spp. Arrow indicates the scutellum, characteristic of this species. Its tail is more rounded with no terminal projection.
It invades the young, developing tubers through the tissues of the tuber growing point, alongside emerging roots and shoots, through roots and also through cracks or damaged areas in the tuber skin. Their feeding leads to a condition called dry rot in the tubers and especially the yams. Severely infested tubers will have surface cracking or flaking of the epidermis exposing the black dry rot tissues underneath. In potatoes it causes tuber rots.
The nematodes feed intracellularly in tuber tissues resulting in rupture of cell walls, loss of cell contents and the formation of cavities .They are mainly confined to the sub-dermal, peridermal and underlying parenchymatous tissues in the outer 1-2 cm of tuber. The S. bradys (yam nematode) continues to feed and reproduce in yams stored after harvesting. In tubers with partial dry rot, more nematodes are found in the oldest, apical portions, adjacent to the stems.
Nematode-infested (left) and healthy (right) yam tubers
Ectoparasites of plant roots, root hairs, algae, etc. It’s a plant feeders (algal, lichen (algal or fungal component), or moss feeders that feed by piercing), or hyphal feeders. They are also associated with plants. Relatively slight, small stylets penetrating only thin cell walls.
Studies show that the nematode fed and reproduced as a migratory ectoparasite on the roots of seven species of conifer.
Damage: The nematode causes “slow decline” and the affected trees show reduced terminal growth, chlorosis and shedding of terminal leaves, dieback of branches and considerable reduction in number and size of fruit. Roots of infested trees show brownish discoloration and ultimately decay resulting in reduced volume.
Citrus nematode (Tylenchulus spp.)
It was first reported to be attacking citrus but has been reported to attack other crops that do well infinely textured soils or in sandy soils with high organic matter content.
The Tylenchulus spp juveniles (Larvae)appear similar to those of Meloidogyne (root-knot nematodes) because of their narrow and tapered posterior body portion. But the Citrus nematodes are differentiated by the presence of a stronger stylet, more posterior excretory pore and absence of pharyngeal overlap.
Mature females are found attached to roots and are typically covered by soil particles and debris that stick to the gelatinous matrix, helping to protect the exposed portion of their body and eggs. The female posterior portion of the body protruding from the root surface is swollen and enlarged and ends in a finger-like projection, while the elongate and not swollen anterior portion of the body remains hidden and embedded in the cortical parenchyma.
The juvenile of Tylenchulus spp
Swollen female nematodes
Damage and symptoms
In Citrus, symptoms of slow decline have been reported and these can vary depending on the level of nematodeinfestation, age of trees and time of infection. Young citrus orchards will not demonstrate symptoms until nematode populations increase to high. Symptoms are more prominent in established orchards as trees are stressed, either by suboptimal growing conditions, drought, or root stunting and decay induced by nematode infection. Typically, reduced leaf and fruit size, canopy thinning, and exposure of bare crown limbs are the most conspicuous symptoms of slow decline and result in yield.
Citrus trees exhibiting symptoms of citrus decline caused by the citrus nematode
The stunt nematodes (Tylenchorhynchus spp)
The nematode spp are primarily considered to be migratory ectoparasitic feeders, feeding along the root surface penetrating epidermal cells of roots and root hairs. Occasionally, they feed endoparasitically on some hosts, confined to the outer cortical layers of the root
Damage: They have been reported to feed in large aggregations at the root tip, causing mechanical breakdown of epidermal, cortical and undifferentiated vascular tissue at this site. While this causes a reduction in main root growth, it is compensated for by increased lateral root growth.
The spp is often found associated with other nematode plant parasites. They have a widespread distribution mainly due to their wide host range, lack of specialization and plant tolerance to infection
Stem and bulb nematodes (Ditylenchus spp)
It is amigratory endoparasite. At the beginning of the crop season, 4th-stage juvenile enters young tissues, especially seedlings when below the soil surface. Feeding breaks down middle lamellae; nematode probably secretes a pectinase enzyme; plant parts become “crisp” and are easily broken. Migration on plant parts above ground requires free water, and may occur after rain or sprinkler irrigation. Nematode enters through stomata or by direct penetration of the plant.
Ditylenchus spp adult nematode
It is known to attack over 450 different plant species, including many weeds. However, it occurs in more than ten biological “races” some of which have a limited host-range. The principal hosts are faba beans, garlic, Hyacinthus orientalis, leeks, lucerne, maize, Narcissus pseudonarcissus, oats, onions, peas, Phlox drummondii, P. paniculata, potatoes, rye, strawberries, sugarbeet, tobacco, Trifolium pratense, T. repens, tulips. It has also been reported on carnations, celery, Hydrangea, lentils, rape, parsley, sunflowers, and wheat.
In general, this nematode causes swellings and distortion of aerial plant parts and necrosis or rotting of stem bases, bulbs, tubers and rhizomes. Symptoms are host depended.
On Allium spp. (onions, garlic, leeks, etc.)
Penetration of onion leaves by D. dipsaci causes leaf deformation and leaf swellings or blister-like areas on the surface. The leaves grow in a disorderly fashion, often hang as if wilted and become chlorotic. Young plants can be killed by high infestations. The inner scales of the bulb are usually more severely attacked than the outer scales. As the season advances the bulbs become soft and when cut open show browning of the scales in concentric circles. Conversely, D. dipsaci on garlic does not induce deformation or swellings, but causes leaf yellowing and death.
Fungal feeding nematodes (Aphelenchus spp)
Aphelenchus avenaeis a common soil inhabiting nematode which feeds on fungi. Cases of Aphelenchus in healthy plant tissue have been recorded; they are not known to cause any damage on higher plants.
Fungal feeding nematodes (Aphelenchoides spp)
The genus Aphelenchoidesis composed of about 150 speciesAphelenchoides-species are found inevery soil where they feed on soil fungi. Three species feed on the aboveground parts of higher plants and cause considerable damage.
They live in the buds where they damage the developing leaves.Infection by Aphelenchoides species in ornamental and strawberry plants often results in the killing of either a flower or leaf bud, or a growing point that produces a “blind” plant. When an infected growing point is not killed and continues to grow, the stems, foliage, and other structures that develop from it are likely to be crinkled, distorted, or twisted. The penetration of and feeding on leaf tissue will cause leaves to crinkle, curl, and discolor in the interveinal regions. Strawberry fruits may also become malformed and discolored from foliar nematode infection.
Aphelenchoides mouth part
Damage symptoms on straw berry and ornamental respectively
Dagger nematodes (Xiphinema spp)
This is the nematode with the longest stylet having a guiding ring located near its base. The nematode causes direct damage to a wide variety of plants by their feeding activities, but indirectly; several dagger species also transmit soil borne plant viruses. Females lay eggs singly in the soil near plants, this hatch to produce first stage juveniles which invade plant roots.
They are ectoparasites and the nematodes insert their long stylet deep into root tips where they feed on root tip cells.
Dagger nematode feeding causes some necrosis, stunting and swelling of root tips. Several lateral roots may appear above the damaged root tips. Root-tip swelling may be confused with the galls of root-knot nematodes.
Swelling of root tips due to the dagger nematode feeding damage.
Sweet cherry leaves with symptoms of cherry rasp leaf virus infection;
Needle nematodes ( Longidorous spp)
Two factors make needle nematode damage relatively easy to diagnose. The first factor is that needle nematodes are very large (4 to 5 mm long) .Second, because they are so large, they are confined to sandier soils (with larger pore spaces) and do not have to be considered as the cause of problems in heavier soils.
These nematodes are ectoparasites that damage host roots by feeding on root tips causing some root tip galling. They stunt the lateral roots and essentially destroy the fibrous root system.
Symptoms of damage include depressed, discolored plants with short, weak root systems that may occur in patches in fields.
It is a vector of some plant viruses.
Corn root damage due to needle nematode feeding.
Pin nematodes (Paratylenchus spp)
These are migratory endoparasites. They form a C-or spiral shape while at rest.They pierce root cells from the soil outside of the plant and remain motile throughout their lives. At times, some may also imbed their anterior ends in the roots and establish longer-term feeding sites. Low numbers may appear inconsequential, but in high enough numbers, pin nematodes can damage crops.
Symptoms of Paratylenchus damage consist of general decline, poor root systems, brown, necrotic areas on roots of some hosts, and “bad patches” in fields. Paratylenchus species have been found to be associated with roots of peppermint, wheat, potato, beans, alfalfa, carrots, garlic, and onion, as well as in pastures
Poor root system due to the feeding of the Paratylenchus spp
General Management of nematodes
Most of the time, management must focus on reducing nematode numbers to levels below the damage threshold. However, management of nematodes is important as they are a predisposing factor to soil borne pathogen infection and environmental stresses such as salt injury and/or micronutrient deficiency. Below are some of the management strategies to be employed to manage nematode population.
- Provide enough organic matter to increase the free living nematodes as they control many different PPNs in the soil. If your soil does not have enough free living nematodes, add soil with higher number of free living nematode and provide food (organic matter) to these nematodes.
- Cover crops can be grown outside of the normal agricultural growing season, and some are antagonistic to nematodes. Cover crops such as sudan grass and marigolds actually produce chemicals that are toxic to nematodes. Cover crops have the added benefits of stabilizing topsoil and improving soil quality.Additionally; Marigold, sudan grass and Brassica spp. can be used as green manure crops to control PPNs and boost free living nematode populations in the soil. Glucosinilate or isocthiocyanate content in many Brassica species is known to control many PPNs.
- Crop rotation is important in avoiding the buildup of the various nematodes. It’s important however to remember that some nematodes have a wide host range on crops from different families hence during rotation, incorporate the other methods of management and carefully plan it.
- Soil solarization is very effective to control many nematode and other soil borne pathogens. For soil solarization: plow field to ensure looseness, ensure adequate moisture, cover with plastic, seal the plastic to make it air tight and maintain the seal for at least 45 days. Target the hottest months for this process.
- In areas with plenty of water and the area can be kept empty of crops for some time, flooding can be done to eliminate the nematodes.
- Host resistance: Use varieties which are known to be resistance to nematodes.
- Exclusion and sanitation: Use of clean planting materials and ensuring field /greenhouse sanitation can help in managing the nematodes. Areas with old crops should be worked on last and the tools and equipment cleaned between fields/greenhouses. All the footwear need to be cleaned between fields.
- Hot water treatment: All crops that use tubers and rhizomes as planting materials can be dipped into hot water for 5 minutes before planting. Examples include the bananas against the Radopholus similis and the yams against the Scutellonema bradys.
- Use of biological nematicides readily available in the market. This are extracts mainly from the neem (Azadirachtin products) and garlic. These should be used as per the manufactures instructions.
NB: Inorganic nematicides are very toxic both to the nematodes and the user hence should be used only when it’s absolutely necessary and all the above measures have failed.