Written by: Karla Medina
Soil nematodes are almost invisible, yet highly important when it comes to crop health. There are good and bad nematodes, and both matter. 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% to more than 20% of economic yield losses worldwide. For growers, it is key to learn basic biology and life cycle of plant parasitic nematodes to better manage them – and to reduce their impact on the nematodes that are beneficial to soil nutrient cycles.
How Plant Parasitic Nematodes Affect Your Roots
Soil-inhabiting nematodes can be categorized by what they feed on. They can either feed on bacteria, fungi, protozoa, other soil organisms (including nematodes) or roots in the soil. Since root-feeding nematodes are responsible for a significant portion of global economic crop damages, it is important to understand this pest and how they affect plants as they grow.
Characteristics of plant parasitic nematodes:
- Plant parasitic nematodes are microscopic in size. A microscope with 30-60x magnification is necessary to see and determine their variety of shapes and sizes.
- All plant parasitic nematodes 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.
- All plant parasitic nematodes have a taper-shaped tail end. This trait is key for 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 reduces yield.
What is the Nematode Life Cycle?
A typical nematode life cycle is comprised of six stages: the egg stage, four juvenile stages (J1-J4) and the adult stage. For many nematodes, the J1 stage occurs inside the egg, and the stage that hatches from the egg is the J2 stage. The length of the cycle will vary for each different genera and species, ranging from a few days to twelve months.
Within each species, the cycle depends on environmental conditions and host crops. If conditions are favorable, the cycle shortens, and 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. This leads to rapid nematode 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 into four main groups according to their feeding behavior and mobility around the roots.
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 outside of the roots, meaning they do not penetrate wholly into the tissue. Those with short stylets will feed on the epidermal and outer cortical cells often at the root tips, while longer stylets feed deeper in the cortex.
Eggs are deposited in the soil, where the rest of the life cycle stages occur as the nematodes feed on suitable hosts.
Most plant-parasitic nematode species belong to this group, including sting, dagger, needle, stubby root and ring nematodes. Reproduction can be both sexual and asexual, and soil samples determine their presence or absence.
2. Migratory Endoparasites: moving feeders within the roots
Migratory endoparasites, including lesion and burrowing nematodes, are also mobile at all life stages except for the eggs. They will feed and move through the inside and outside of root tissues. As a result, eggs are laid singly both inside cortical tissue and in soil surrounding the plant.
These nematodes can be particularly damaging because they can remain undetected until damage begins to show as crops grow. However, preventive treatments can be applied before or at planting to help address harmful effects.
Reproduction can be both sexual and asexual. Samples for diagnosis should include roots from symptomatic plants, as soil samples alone can fail to detect the presence of damaging numbers for these moving feeders.
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, sometimes becoming visible on the root surface. J2 stage nematodes and males are found in the soil, while females, eggs and other juvenile stages can be extracted from the roots. The J2 stage is ineffective, as the juvenile 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. J3, J4 and adult stage nematodes develop and feed inside the site, which causes a swollen tissue, or galling. This is a typical symptom of root-knot infection and can be detrimental to crop yield when severe.
4. Sedentary Semi-Endoparasites: semi-stationary feeders within the roots
Sedentary semi-endoparasites can partially penetrate the plant root to feed, forming a permanent feeding cell.
In this type of feeding behavior, the J2 stage nematode partially penetrates the root and develops through the life stages up to adult with the “head-neck” portion of their body inside. 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 stage nematodes and males can be found in the soil, but males are difficult to identify.
Examples of this group are reniform and citrus nematodes.
Treatment Timing According to Nematode Life Cycles
The nematode life cycle matters because root health matters when it comes to crop yield. That’s why understanding the root of an existing or potential yield problem helps inform better protection for your crops and your bottom line.
Regular soil testing can help fight loss of yield from plant-parasitic nematodes by determining if they are present in the soil throughout the growing season.
Since most nematodes stay dormant between plantings if there is no crop or weeds to reproduce on, growers can also apply preventive treatments aligned with the planting time of each crop. 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 feeders. NemaCleanTM is one biological nematicide that protects crops against all life cycle stages of all types of plant parasitic nematodes.
This type of product allows flexibility and convenience to apply whenever control is needed. Plus, with higher efficacy at lower rates, NemaClean is more cost-effective than other biological nematicides.
For healthier crops starting at the roots, learn more about how you can control yield-robbing feeders without harming beneficial nematodes.