Pratylencus (lesion nematode) – Morphology, Life cycle, Control, Importance

What is lesion nematode?

• Lesion nematodes, scientifically referred to as Pratylenchus species, are a type of plant-parasitic nematode that cause damage primarily to roots, creating necrotic lesions. These lesions, which are dead areas of tissue, can significantly impair plant health by reducing water and nutrient uptake. The common name, “lesion nematode,” originates from the visible damage they cause on the roots, while the term “meadow nematodes” was historically used due to their initial discovery in meadow habitats.
• These nematodes are classified as migratory endoparasites, meaning they move freely within and outside of the root tissue without becoming sedentary, unlike other nematodes such as root-knot or cyst nematodes. Their feeding is mainly focused on the cortex of the roots, causing internal damage as they move and feed within the plant tissue.
• Lesion nematodes are particularly problematic for perennial plants, both woody and herbaceous. While they may be present in row crops, they typically do not reach high enough population levels in a single season to cause severe yield loss. However, crops like corn and soybeans can suffer significant damage, particularly in less fertile soils or where the same crop has been grown repeatedly without rotation or the use of nematicides. The damage is especially noticeable in years with unusually warm early-season soil temperatures.
• These nematodes are found across the globe, with more than 40 species described. In Illinois alone, five species are common: Pratylenchus penetrans, P. alleni, P. hexincisus, P. neglectus, and P. scribneri. Some locations may have a single species, while others might host multiple species simultaneously.
• Of these species, P. penetrans is particularly noteworthy due to its significant impact on nurseries, orchards, and strawberry fields, often causing severe declines in tree health and replant failure, especially in cherry, apple, and peach orchards. This species has been extensively studied due to its economic importance, particularly in the eastern United States and Canada. In contrast, the other four species are more commonly found in row, forage, and small-grain crops and are believed to be native to Illinois. Occasionally, other species of lesion nematodes are introduced into new areas through infected plants.

Symptoms of lesion nematode

Lesion nematode infestations can cause a range of symptoms in both herbaceous and woody plants, primarily affecting root systems and leading to noticeable aboveground effects. The severity of symptoms depends on nematode population levels, environmental conditions, and plant susceptibility. The following list outlines key symptoms and contributing factors:

• Aboveground Symptoms on Herbaceous Plants:
  Infected plants typically display patches of stunted and yellowish (chlorotic) growth. These patches are often round or oval and give the field an uneven or ragged appearance. The most intense damage is found at the center of these areas, while symptoms decrease towards the outer edges. Affected plants exhibit smaller leaves and produce fewer of them, which significantly reduces overall yield.
• Impact of High Nematode Populations:
  When lesion nematode populations are particularly high, they can interact with other soilborne pathogens to weaken or kill plants, creating barren spots in fields. As the season progresses, these barren areas may expand. However, if growing conditions are optimal, moderately infected plants might recover somewhat by developing deeper root systems, though they often still yield poorly. Lightly infected plants may not show obvious growth abnormalities but may still experience minor yield reductions under unfavorable conditions.
• Influence of Environmental Stress:
  Lesion nematode damage is exacerbated when plants experience additional stresses such as high early-season temperatures, inadequate rainfall, low soil fertility, or attacks from root-rot organisms. Under these conditions, nematode damage becomes more pronounced, and yield loss increases.
• Symptoms in Woody Plants (Trees and Shrubs):
  Trees and shrubs infected by lesion nematodes often show a gradual decline over several years as nematode populations increase. Foliage may appear pale green or chlorotic, and seasonal growth is reduced. Flowers and fruit production are diminished in both quantity and quality. Infected plants become less vigorous and are more susceptible to winter injury and other diseases. Simply replacing a dying plant without treating the soil may result in poor growth or death of the new plant.
• Root Symptoms:
  Lesion nematodes cause small, brown lesions on roots early in the growing season. As the season progresses, these lesions expand, merge, and darken, giving the roots a discolored appearance. Nematode feeding damages cortical tissue, causing it to disintegrate, leading to root girdling and sloughing off of the epidermis. Severely infected plants may experience almost complete root destruction, which makes it easy to pull them from the soil. Moderately infected plants have short, brownish roots, with most remaining root systems located near the soil surface.
• Lesion Formation Mechanism:
  Lesions form as a result of the interaction between plant glycosides and enzymes released by the nematode during feeding. These chemicals are toxic to both invaded and adjacent host cells. The extent of damage depends on the concentration of glycosides in the plant tissue and the amount of enzyme released by the nematodes. Additionally, the wounds created by the nematodes allow soilborne pathogens to enter, further contributing to lesion formation and worsening plant health.
• Variability in Plant Tolerance:
  Plant species vary in their tolerance to lesion nematodes. Some plants are severely affected by low nematode populations, while others can support high populations without showing visible damage. Economic damage thresholds—nematode population levels that lead to significant yield loss—have been established for only a few plants, and they can vary depending on weather conditions and soil characteristics.
• Environmental Influence on Damage Severity:
  Lesion nematode injury is most severe in soils that are light in texture and low in essential nutrients like nitrogen, potassium, or calcium. Soils with low organic matter content also exacerbate the damage. Plants experiencing moisture stress or exposed to high temperatures are more likely to suffer serious harm. However, in optimal growing conditions, plants may not show any symptoms, even when harboring high nematode populations.

Epidemiology of lesion nematode

Lesion nematodes (Pratylenchus species) are highly adaptive plant parasites that follow a complex epidemiological pattern influenced by their biology and environmental conditions. They infect a wide range of plants, including economically important crops, and can cause significant damage to roots, leading to yield losses. Understanding the epidemiology of lesion nematodes requires examining their life cycle, infection process, and interactions with the environment and other pathogens. Their ability to persist and spread in the soil makes them formidable pests in agriculture.

• Life Cycle and Development:
  Lesion nematodes undergo six life stages: egg, four juvenile stages (J1-J4), and the adult stage. The life cycle can be completed in 4-8 weeks, though this duration is highly dependent on environmental conditions like temperature and moisture. After the first-stage juvenile (J1) develops inside the egg, the nematode molts into the second-stage juvenile (J2), which hatches from the egg and is capable of infecting plants. Lesion nematodes are motile at all life stages, except for the egg and J1, and can penetrate plant tissues to begin feeding. They are primarily root parasites, although they may occasionally invade other plant parts like tubers and rhizomes.
• Penetration and Feeding:
  Lesion nematodes penetrate plant roots at various points but tend to prefer the root hair zone. Once inside the root, they migrate intracellularly through the cortex, feeding on plant cells with their stylet (a needle-like feeding organ). Their feeding activity causes cell lysis, creating cavities in the root tissue. The nematodes move continuously within the root, feeding on healthy cells while leaving behind necrotic lesions. These lesions are the hallmark of Pratylenchus infection and can result in reduced plant vigor and impaired nutrient uptake. Unlike sedentary plant-parasitic nematodes, lesion nematodes do not induce specialized feeding structures, as they remain motile throughout their parasitic phase.
• Reproduction and Population Dynamics:
  Lesion nematodes can reproduce sexually (amphimixis) or asexually (parthenogenesis), depending on the species and environmental factors. Populations can increase rapidly within root tissues, sometimes reaching densities of 1,000 to 3,000 nematodes per gram of root. They can overwinter in the soil or plant debris at any life stage, though the fourth-stage juveniles (J4) are particularly well-suited for survival during adverse conditions. Lesion nematodes can leave the root and return to the soil, making their life cycle highly flexible and capable of continuing within or outside of plant roots.
• Interaction with Other Pathogens:
  The damage caused by lesion nematodes serves as entry points for other soilborne pathogens, particularly fungi like Fusarium and Verticillium. These interactions can create disease complexes, where the combined infection of nematodes and fungi leads to more severe plant damage than either pathogen alone. For instance, Pratylenchus penetrans and Verticillium dahliae together cause a syndrome known as “potato early dying.” This synergistic effect exacerbates plant decline, especially when nematode populations are high. Conversely, if fungal infection precedes nematode invasion, nematode populations may remain low, as lesion nematodes prefer living cells for feeding.
• Spread and Transmission:
  Lesion nematodes typically move short distances, usually no more than 1-2 meters from the root zone they initially infect. However, in situations where plant roots graft, such as in fruit trees, nematodes can move from plant to plant. In fields, the spread of lesion nematodes tends to be “contagious,” with small foci of infestation gradually enlarging. The spread is accelerated by environmental stresses like drought or nutrient deficiencies, which weaken plants and make them more susceptible to nematode attack. Cultural practices such as soil cultivation, along with the use of contaminated planting material or unsanitized farm equipment, can also facilitate the spread of nematodes to non-infested areas.
• Environmental and Cultural Influences:
  Lesion nematode populations are influenced by environmental factors, especially soil moisture and temperature. Warm, moist conditions are conducive to their reproduction and migration within the soil. Moreover, farming practices, such as crop rotation and soil cultivation, play a critical role in nematode management. Poorly sanitized farm machinery and the movement of infected plant material, such as tubers and seedlings, are common ways the nematodes are introduced into new areas. In many locations, Pratylenchus species are endemic, meaning they are naturally present in native vegetation and may already infest soil before cultivation.

Morphology of lesion nematode

Lesion nematodes (Pratylenchus species) are small, vermiform (worm-like) nematodes that exhibit specific morphological characteristics distinguishing them from other nematode genera. The morphology of lesion nematodes is crucial for taxonomic identification and is typically based on adult structures. Though juveniles share many characteristics with adults, they lack fully developed reproductive organs. The external and internal features of these nematodes reflect their adaptations to parasitism, enabling them to penetrate and feed on plant tissues. Understanding their morphology aids in the recognition and study of these economically significant pests.

• Overall Body Shape and Cuticle:
  The body of a lesion nematode is cylindrical, elongated, and tapers at both ends. Its shape is maintained by the internal pressure of body fluids, pressing against the flexible but sturdy cuticle, a structure akin to a water balloon under pressure. The cuticle, a crucial external feature, is composed primarily of collagen and is molted four times during development. The cuticle is annulated, with fine rings allowing flexibility and movement, similar to an earthworm. These annulations facilitate the nematode’s ability to navigate through soil and plant tissues. Adult lesion nematodes range in length from 300 to 900 µm, with a body length-to-width ratio of 20:1 to 30:1, depending on the species.
• Head Region and Feeding Structures:
  The anterior (head) region of lesion nematodes is a critical morphological zone due to the presence of their feeding apparatus. The nematode’s head is identifiable by a short, hollow spear-like structure called the stylet, located just inside the tip. The stylet, equipped with basal knobs, functions like a hypodermic needle, allowing the nematode to penetrate plant tissues and feed on cell contents. The stylet is a defining feature of the genus Pratylenchus and is essential for the nematode’s parasitic lifestyle. Above the stylet lies the lip region, which is characteristically flat and darkened, a trait unique to this genus.
• Esophagus and Metacorpus:
  Behind the stylet is the metacorpus, a muscular pumping organ that facilitates the movement of food and secretions through the nematode’s esophagus. The metacorpus, along with its rhythmic contractions, enables the ingestion of plant sap and other cellular materials. Below the metacorpus lies the esophageal glands, which overlap the intestine on the ventral (stomach) side of the nematode’s body. These glands secrete enzymes that aid in breaking down plant cell walls during feeding. The intestine, a long dark structure, extends from the esophageal glands to the nematode’s tail.
• Female Morphology:
  Adult female lesion nematodes are identifiable by the presence of a vulva located 70-85% down the length of the body from the head, depending on the species. The vulva is an external opening through which the female reproductive system releases eggs. A row of progressively larger cells, positioned over the dark intestine, represents the female gonad responsible for egg production. Near the tail, the anus is visible, marking the termination of the digestive tract. The female tail is conoid in shape, tapering to a rounded tip, a characteristic trait of lesion nematodes.
• Male Morphology:
  Males of the Pratylenchus genus are generally smaller and more slender than females. While the absence of a vulva distinguishes males, they possess a row of cells that form the testis, which is analogous to the female’s gonad but terminates at the anal opening. A key feature of male morphology is the presence of two dark spicules near the tail. These spicules are used during copulation and are easily recognizable under a microscope. The tail of the male is more pointed than the female’s, often featuring two cuticular flaps known as alae or a bursa, which serve as additional mating structures.

Disease Cycle of lesion nematode

Lesion nematodes (Pratylenchus species) are small, worm-like roundworms that cause significant damage to plant roots by feeding on root cortical cells. The life cycle of lesion nematodes plays a crucial role in understanding their impact on plants, as well as in developing effective management strategies. Below is a detailed overview of the disease cycle of lesion nematodes:

• Reproduction and Egg-Laying:
  Lesion nematodes, such as P. penetrans and P. alleni, reproduce sexually. After mating, females lay their eggs either singly or in small groups, typically within host roots or in the soil near the root surface. Some species, like P. hexincisus, P. neglectus, and P. scribneri, reproduce through parthenogenesis (without males). Despite the difference in reproduction methods, the life cycle in these species remains consistent.
• Egg Development and Hatching:
  Once laid, the eggs undergo the first larval stage and molt while still inside the egg. Depending on soil temperature, the eggs hatch within 1 to 3 weeks. The second-stage larvae, which emerge from the eggs, are immediately capable of infection. These larvae will go through three more molts before becoming adult nematodes, ready to continue the cycle.
• Infective Stages:
  All post-egg life stages of lesion nematodes are infective. They are drawn to host plant roots, particularly near the root hair zone and root tips. Penetration into the root often occurs behind the root’s elongation region. Initially, the nematodes may feed ectoparasitically (from the root surface) before entering the root by forcing their way between or through epidermal cells. This entrance is aided by both the mechanical force of their stylet (a specialized feeding structure) and the secretion of cell wall-degrading enzymes from their esophageal glands.
• Feeding and Tissue Damage:
  Once inside the roots, lesion nematodes feed on the cortical cells. As they consume the cells, they leave behind cavities, which disrupt the plant’s ability to transport water and nutrients, leading to root damage and eventual necrosis. When the root tissue becomes too necrotic or otherwise unfavorable, the nematode will migrate to healthier parts of the root or even leave the root entirely, moving through the soil to infect another root system.
• Overwintering and Environmental Effects:
  Lesion nematodes overwinter in the soil or inside host roots as eggs, larvae, or adults. The duration of their life cycle depends on the species and environmental conditions, particularly soil temperature. For example, the optimal temperature for population growth in species like P. alleni, P. neglectus, and P. scribneri on soybeans is around 86°F (30°C). In contrast, P. penetrans prefers temperatures around 77°F (25°C). Soil temperatures below 59°F (15°C) cause most lesion nematodes to become inactive, except for P. penetrans, which can still function, though with reduced activity, at temperatures above 68°F (20°C).
• Temperature and Life Cycle Duration:
  Temperature plays a significant role in the speed of lesion nematode development. For instance, P. penetrans can complete its life cycle in about 30 days at 86°F (30°C), 35 days at 76°F (24°C), but the cycle extends to 86 days at 59°F (15°C). Other species likely follow a similar developmental pattern, though their precise life cycles have not been as thoroughly studied. In regions where soil temperatures are cooler, such as Illinois, lesion nematode populations tend to increase more slowly than in warmer southern areas of the United States.

Interactions of lesion nematode

Lesion nematodes (Pratylenchus species) play a significant role in plant health not only through direct root damage but also via their interactions with other soilborne pathogens. These interactions can exacerbate plant diseases and lead to more severe outcomes than when either pathogen acts alone. The complexity of these interactions provides insight into how nematodes contribute to disease complexes that weaken plant defense mechanisms and increase susceptibility to a range of other pathogens.

• Synergistic Interactions with Fungi:
  Lesion nematodes commonly interact with soilborne fungi, particularly Verticillium and Fusarium species, which are responsible for causing wilt diseases in a variety of crops such as pepper, eggplant, tomato, and potato. These fungi invade plant vascular tissues, disrupting water and nutrient transport. The damage caused by lesion nematodes to the root cortex allows easier fungal entry into plant tissues, creating a synergistic effect. As a result, plants that might have resisted fungal infections on their own become more vulnerable due to the weakening of their root systems by nematodes. This interaction intensifies symptoms, such as wilting and reduced plant vigor, far beyond what would be seen if the nematodes or fungi were acting independently.
• Interactions with Trichoderma viride:
  Lesion nematodes also interact with Trichoderma viride, a soilborne fungus, particularly on crops like alfalfa and celery. While Trichoderma viride can sometimes act as a biological control agent by competing with other harmful fungi, its interaction with lesion nematodes can become detrimental under certain conditions. The combined presence of Pratylenchus nematodes and Trichoderma viride in the same root zone can lead to severe plant damage. This interaction may compromise plant defenses to the extent that it leads to plant death, an outcome less likely if only nematodes or the fungus were present.
• Additive Effects with Other Nematodes:
  Lesion nematodes can also interact with other nematode species, compounding damage to the host plant. For instance, the simultaneous presence of Pratylenchus with other endoparasitic nematodes, such as root-knot nematodes (Meloidogyne species), can overwhelm the plant’s root system. While lesion nematodes primarily affect cortical cells, root-knot nematodes form galls and disrupt nutrient uptake. The combined feeding and migration of these nematodes create more extensive damage, often resulting in a greater reduction in plant growth and yield than would be caused by either nematode species alone.
• Lowering Plant Resistance:
  One of the key effects of lesion nematode interaction is their ability to lower a plant’s natural resistance to other pathogens. By feeding on the root cortex, lesion nematodes damage cellular structures and open pathways for other soilborne pathogens to invade. This weakened state of the plant reduces its ability to mount an effective defense against further infections, thereby increasing the likelihood of secondary diseases. In particular, lesion nematode damage facilitates the establishment of vascular wilt pathogens, which may not have been able to successfully infect a healthy, uninjured plant.
• Disease Complexes and Plant Death:
  While lesion nematodes alone rarely kill plants outright, their interactions with other pathogens often result in severe disease complexes that can lead to plant death. For example, when lesion nematodes weaken the root system, plants become more susceptible to both fungal infections and environmental stressors, such as drought or poor soil fertility. The combined effects of these stressors and pathogen interactions can ultimately overwhelm the plant’s ability to recover. This phenomenon is especially prominent in crops grown under suboptimal conditions, where lesion nematodes act as a catalyst for more serious plant health issues.

Control of lesion nematode

The control of lesion nematodes (Pratylenchus species) involves an integrated approach to reduce nematode populations and limit the damage they cause to crops. Lesion nematodes are notorious for their wide host range and ability to inflict damage on a variety of plants, including both crops and weeds. Control strategies must therefore be multifaceted, combining cultural practices, biological controls, and, where appropriate, chemical methods. Implementing these strategies helps protect plant health and optimize crop yields while minimizing the impact of nematode-induced stress.

1. Maintain Optimum Growing Conditions:
   Lesion nematodes inflict the most damage on plants already under stress. Ensuring plants are grown under ideal conditions can significantly reduce nematode impact. Providing adequate moisture, proper nutrition, and ensuring good soil aeration are key factors in keeping plants resilient. Additionally, controlling other pests and diseases can help mitigate the compounded stress that increases susceptibility to nematode damage. Therefore, maintaining optimal growing conditions allows plants to better withstand nematode presence without suffering significant losses.
2. Crop Rotation:
   Although lesion nematodes have a broad host range, crop rotation remains an effective control measure in some cases. The success of rotation depends on the specific nematode species and their host preferences. For instance, Pratylenchus hexincisus and Pratylenchus scribneri, which commonly infest corn and soybeans in regions like Illinois, respond differently to various crops. By rotating with poor host plants such as alfalfa or red clover, nematode populations can be reduced. Besides, eliminating weeds and volunteer plants that serve as alternative hosts is critical in preventing nematode survival between crop cycles.
3. Heat Treatment of Propagation Material:
   Hot water treatment is a reliable method for eradicating lesion nematodes from infected propagation materials, particularly in transplants. The temperature and exposure time must be carefully controlled to avoid damaging the plants while ensuring nematode mortality. Typically, temperatures between 113°F and 131°F (45°C to 55°C) sustained for 10 to 30 minutes are effective. Testing the treatment on a small number of plants prior to large-scale applications ensures no heat damage occurs, especially for sensitive varieties.
4. Soil Heat Treatment:
   Heating soil is a common practice in greenhouse and home garden settings to control nematodes and other soilborne pathogens. This method is highly effective for small quantities of soil. Lesion nematodes can be killed by exposure to temperatures ranging from 105°F to 126°F (40°C to 52°C), depending on the species. Methods like aerated steam are particularly efficient in sterilizing the soil. For homeowners, baking soil in an oven at 180°F (82°C) for 30 minutes or 160°F (71°C) for 60 minutes offers an accessible solution for eliminating nematodes in smaller plots.
5. Use of Nematicides:
   Chemical control of lesion nematodes using nematicides can be effective, especially for high-value crops such as orchards, nurseries, and strawberries. Preplant fumigation may be necessary to manage nematode populations before planting, especially in areas prone to heavy infestations. However, the cost of chemical fumigants is often prohibitive for field crops, and they are generally not used in home gardens unless applied by certified professionals. Besides fumigants, certain granular nematicides and insecticides show promise for early-season control in row crops, vegetables, and turfgrass, though many are still awaiting broader registration and approval for use.
6. Resistant Varieties:
   Although some varietal differences in susceptibility to lesion nematodes have been observed, particularly among corn, soybean, and wheat cultivars, this area requires further research. Population development of P. scribneri and P. hexincisus can vary based on the plant variety, but resistant cultivars specifically bred to combat lesion nematodes are not widely available. In crops like tomato and potato, similar varietal differences exist with other nematode species. The development of resistant varieties remains a promising avenue for long-term nematode management but is not yet a fully realized option.

Importance of lesion nematode

Here are several key aspects highlighting the importance of lesion nematodes:

• Agricultural Damage:
  Lesion nematodes are known to cause substantial damage to a wide variety of crops, including potatoes, soybeans, corn, and peanuts. They invade root systems, leading to necrotic lesions that impair the plant’s ability to absorb water and nutrients. This damage can reduce crop yields significantly, leading to economic losses for farmers and affecting food supply.
• Disease Complexes:
  Lesion nematodes can interact synergistically with other pathogens, particularly soilborne fungi like Fusarium and Verticillium. These interactions can result in more severe disease syndromes, such as “potato early dying,” where the combined effects of nematodes and fungi lead to enhanced plant decline. Understanding these complexes is crucial for developing effective management strategies, as controlling one pathogen may not be sufficient to mitigate the overall disease impact.
• Soil Health and Ecosystem Impact:
  The presence of lesion nematodes can influence soil health. Their feeding activity leads to root damage, which can affect the overall plant health and the microbial community in the rhizosphere. Additionally, the wounds they create can serve as entry points for other pathogens, further disrupting the balance of soil ecosystems.
• Economic Implications:
  The economic burden imposed by lesion nematodes is significant. Crop losses due to their presence necessitate the use of management practices such as nematicides, crop rotation, and resistant varieties, which can increase production costs. For many farmers, especially those cultivating high-value crops, the financial impact of lesion nematode infestations can be considerable.
• Global Distribution and Host Range:
  Lesion nematodes have a wide host range and are found in diverse agricultural environments worldwide. Their ability to infect various plant species allows them to thrive in different climates and soil types, making them a global agricultural concern. Their ubiquitous presence necessitates continuous monitoring and research to manage their populations effectively.
• Resilience and Survival:
  Lesion nematodes are capable of surviving adverse environmental conditions, including extreme temperatures and drought, by entering a state of dormancy. This resilience means that once introduced into an area, they can persist for long periods, complicating eradication efforts and making them a long-term challenge for growers.
• Research and Management:
  Ongoing research into lesion nematodes focuses on understanding their biology, ecology, and interactions with other organisms. Developing resistant crop varieties, exploring biological control methods, and improving cultural practices are essential components of integrated pest management (IPM) strategies aimed at mitigating the impact of these nematodes.