Written by: Karla Medina
Soil nematodes are almost invisible, yet highly important when it comes to crop health. Which nematodes matter? The good and the bad ones – because beneficial nematodes can improve the soil health and environment, while harmful plant parasitic nematodes can destroy root health and negatively impact yield.
In agricultural systems, plant parasitic nematodes can cause anywhere from 5-20% (or more) in economic yield losses worldwide. For growers, it is key to learn basic biology and life cycle of plant parasitic nematodes in order to better manage them and reduce the impact on the non plant-parasitic ones that are beneficial to soil nutrient cycles.
Simply stated, if you understand the plant parasitic nematode life cycle, you can better protect your crops, plant yield, and ultimately, your bottom line.
How Plant Parasitic Nematodes Affect Your Roots
Soil-inhabiting nematodes can be divided by what they feed on. They can either feed on bacteria, fungi, protozoa, other soil organisms (incl. nematodes), or roots in the soil. Since root-feeding nematodes are highly responsible for a portion of our global economic crop damages, we need to understand how they affect plants as they grow.
Characteristics of plant parasitic nematodes:
- Plant parasitic nematodes are microscopic, meaning the need of a microscope with magnification anywhere from 30-60x to make them visible and able to make out the variety of sizes and shapes.
- All plant parasitic nematodes will have a needle-like stylet for feeding; however, not all nematodes with a stylet are plant parasites. The presence, size and shape of the stylet and stylet knobs are fundamental characteristics.
- Another plant parasitic nematode trait is the shape of the tail-end, and how it tapers can help make a general identification at the genus level.
While identifying nematode types can be helpful, it’s most important to identify how their feeding affects your crops. The spear-shaped stylet that comes from the front end is used to pierce the thick cell wall of plant root cells and ingest internal contents of the plant cells, which harms root development, interferes with water and nutrient uptake, and causes loss of crop yield.
What is the Nematode Life Cycle?
A typical nematode life cycle is simple and comprised of six stages: the egg, four juvenile stages (J1-J4), and the adult. For many nematodes the J1 stage occurs inside the egg, and the stage that hatches from the egg is the J2. The length of the cycle will vary for each different genera and species, ranging from a few days up to a year depending on the species.
Within each species, the cycle will be dependent on the environmental conditions and host crops. If conditions are favorable the cycle shortens; vice versa, if conditions are not favorable, the cycle lengthens. For some species including most of the root-knot nematodes, warmer conditions are conducive to several generations feeding on roots per season, leading to a rapid build-up and significant crop damages.
Learning and understanding the different parasitic nematode feeding behaviors helps determine the right sample assays and management. Typically, below-ground plant parasitic nematodes are divided according to their feeding behavior and mobility around the roots into four main groups.
1. Migratory Ectoparasites: moving feeders outside of the roots
For migratory ectoparasites, all life stages are mobile except the eggs. The eggs do not feed, but all other stages will feed on the outside of roots, meaning they do not penetrate wholly into the tissue. Depending on the length of the stylet, those with short stylets will feed on the epidermal and outer cortical cells often at the root tips, and those with longer stylets feed deeper in the cortex.
Within this group, eggs are deposited in the soil, and the rest of the life cycle stages occur in the soil as they feed on suitable hosts. Reproduction can be both sexual and asexual, and soil samples will determine presence or absence of migratory ectoparasites.
Most plant-parasitic nematode species belong to this group. Examples are sting, dagger, needle, stubby root and ring nematodes.
2. Migratory Endoparasites: moving feeders within the roots
As with above, migratory endoparasites are mobile at all life stages except for the eggs.. The mobility refers to their feeding behavior and capability to move through root tissues in and out; therefore, eggs are laid singly both inside cortical tissue and also outside of roots in soil surrounding the plant.
Reproduction can be both sexual and asexual. Soil samples for diagnosis should include roots from symptomatic plants, as soil samples alone can fail to detect the presence of damaging numbers for these feeding-type of nematodes.
Migratory endoparasite nematodes can be particularly damaging because they can live and move within roots, going undetected until damage begins to show in crops as they grow. However, preventive treatments can be applied before or at planting to help diminish the harmful effects of these nematodes.
Examples of this group are lesion and burrowing nematodes.
3. Sedentary Endoparasites: stationary feeders within the roots
Some of the most damaging nematodes in the world are sedentary nematodes because they can modify the host plant’s metabolism by establishing a permanent feeding site within the plant roots. All root-knot nematodes are sedentary endoparasites, and cyst nematodes also exhibit this type of feeding behavior.
In their life cycle, eggs are deposited in a gelatinous matrix and rupture through the root cortex, and can be visible sometimes on the root surface. J2 and males are found in the soil, while females, eggs, and other juvenile stages can be extracted from the roots. The J2 is the infective stage that moves through the soil, penetrates the root, often at the root tip, and then moves inside through cortical cells to establish a permanent feeding site. The stages J3-J4 and adult develop and feed inside the site, which causes a swollen tissue (galling) typical of root-knot infection, which when severe can be detrimental to crop yield.
Examples of this group are root-knot and cyst nematodes.
4. Sedentary Semi-Endoparasites: semi-stationary feeders within the roots
Sedentary semi-endoparasites are able to partially penetrate the plant root and feed at some point in their life cycle, allowing the nematode to form a permanent feeding cell.
In this type of feeding behavior, the J2 partially penetrates the root and also develops through the life stages up to adult while having the “head-neck” portion of the body stuck inside the root. The eggs are deposited in a gelatinous matrix around the body of the female on the outside, but it can cover parts of the roots in a continuous layer. Only J2 and males can be found in the soil, but males are difficult to make out and identify.
Examples of this group are reniform and citrus nematodes.
Treatment Timing According to Nematode Life Cycles
Why does the nematode life cycle matter to growers? Because root health matters for crop yield.
To fight loss of yield from nematodes, it’s important to do soil testing regularly to determine if plant parasitic nematodes are present in the soil throughout the growing season.
Growers can also apply preventive treatments aligned with the planting time of each crop because most nematodes stay dormant between plantings if there is no crop or weeds to reproduce on. Most nematodes average 30-day life cycles, so if you apply treatments prior to planting and at regular intervals, you can protect plants from harmful nematode feeding.
MeloCon® LC is a biological nematicide that protects crops against all life cycle stages of nematodes to promote healthier roots and protect yield, featuring flexible treatments in a convenient liquid formulation.
Sikora R.A., Desaeger J., Molendijk, L.P.G. (eds). (2022) Integrated Nematode Management: State-of-the-Art and Visions for the Future. CABI Publishing, Oxforshire, UK. 498 pp. https://www.cabi.org/cabebooks/FullTextPDF/2021/20210466450.pdf
Bridge J. and Starr J.L. (eds)(2007) Plant Nematodes of Agricultural Importance: A Colour Handbook. Manson Publishing, London, UK. 152 pp.
Coyne D.L., Nicol J.M., and Claudius-Cole B. (2014) Practical plant nematology: a field and laboratory guide. 2nd edition. SP-IPM Secretariat, International Institute of Tropical Agriculture (IITA), Cotonou, Benin. https://biblio.iita.org/documents/U14BkCoynePracticalNothomDev.pdf-d663ec356760331c1acd9a16e3848f16.pdf