Nematodes: the invisible killers of the greenhouse world

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December 11, 2023
min read
Everyone who runs a greenhouse should be aware of these silent but deadly pests that can have a very large impact on your production. Let’s dive into the world of these microscopic plant-parasitic nematodes, their life cycle, how to recognise them, and most importantly how to prevent them in the first place.

Life Cycle

The nematode life cycle consists of six stages: egg, four juvenile stages (J1-J4), and the adult stage. In many cases, the J1 stage occurs inside the egg, with J2 nematodes hatching from the egg. The cycle duration varies among genera and species, ranging from days to a year depending on environmental conditions and host crops.

The growth cycle length of a species is influenced by environmental factors. Favourable conditions shorten the cycle, while unfavourable conditions extend it. Root-knot nematodes, for instance, thrive in warmer conditions, allowing for multiple generations per season and resulting in rapid crop damage.


Nematode identification and their impact on plants

Traditionally, identification relies on physical characteristics like body length, morphology of sexual organs, and mouth and tail parts. However, this approach is difficult due to limited variation among closely related taxa and a decline in skilled taxonomists. Molecular methods, including DNA and protein sequence analyses, have been employed to complement or overcome these issues with varying success. Image analysis and computer vision have also played a role in enhancing identification accuracy.

Plant parasitic nematodes feed on all plant parts, including roots, stems, leaves, flowers, and seeds. They employ their stylet for feeding, with a specialisation based on their feeding habits. Root feeders are the most common type, residing in the soil and causing general damage to the above-ground plant, primarily linked to root damage. Consequences of nematode infestations include nutrient deficiency, wilt, stunting, yield depression, and potential plant death. Key diagnostic signs include the presence of root cysts or galls and the appearance of "nematode wool" on bulbs and corms.

Understanding parasitic nematode feeding behaviours is crucial for effective sample assays and management. Below-ground plant parasitic nematodes are categorised into four main groups based on their feeding behaviour and mobility around roots;

  1. Migratory Ectoparasites: moving feeders outside of the roots;
  2. Migratory Endoparasites: moving feeders within the roots;
  3. Sedentary Endoparasites: stationary feeders within the roots;
  4. Sedentary Semi-Endoparasites: semi-stationary feeders within the roots.


Preventative Strategies

The management of nematode diseases presents a formidable challenge, given the often subtle nature of their symptoms, which can lead to inadvertent neglect. The accurate identification and differentiation of nematode species play a critical role in facilitating informed decisions for the targeted control of these plant parasites, while also preserving the diversity of non-parasitic nematodes.

Plant parasitic nematodes can be managed through four primary strategies: biological, cultural, chemical, and genetic control methods.

  • Biological Control: Beneficial nematodes and nematophagous fungi can be introduced to combat harmful nematode species (yes there are also helpful nematodes)
  • Chemical control: Soil sterilisation with heat or chemical methods. Many fumigants have faced EPA bans due to environmental concerns, limiting their availability. However, a handful of alternatives still remain accessible. In contrast, non-fumigant nematicides operate in a manner similar to insecticides. These products have the ability to decrease nematode populations, although their efficiency falls short of that achieved by fumigants. Due to the relatively high cost associated with nematicide application, their practicality is limited to high-value crops
  • Cultural control: One way to do this is crop rotation, however, managing certain nematodes, like root-lesion nematodes, presents a challenge when relying on crop rotation. This difficulty arises from their extensive host ranges
  • Genetic control: Using resistant varieties, choosing nematode-resistant rootstocks for susceptible crops can halt infestations


Important note: not all nematodes are detrimental to your greenhouse, actually, most are harmless or even beneficial freeing nutrients for the plants to consume. These are called free-living nematodes and are beneficial and sometimes essential for plant health and growth.

Nematodes: A general history

Nematodes, the unassuming giants of the animal kingdom, constitute a staggering four-fifths of all life on Earth. The vast majority of nematodes lead unassuming lives, some even rendering vital contributions to the environment. These unsung heroes, the free-living nematodes, labor as nature's foremost decomposers, cycling minerals and nutrients from bacteria, fungi, and other organic matter back into the soil. They are particularly adept at mineralising nitrogen, a pivotal component for photosynthesis. If free-living nematodes vanished, our fields would bear witness to desolate crops, deprived of the ability to harness sunlight for energy. This makes discerning the beneficial from the detrimental nematodes in soil ecosystems all the more important.

Beneath the Earth's surface, plant-parasitic nematodes adopt a different strategy. Instead of preying on microorganisms, they turn to plant roots, employing their needle-like stylets to pierce plant cells and extract vital plant fluids. This unwholesome practice, unsurprisingly, is detrimental to plant well-being, rendering affected plants frail, yellowed, and stunted, eventually leading to plant death.

The root-knot nematode is one of the most common and problematic plant-parasitic nematodes. Unlike free-living nematodes, it invades plant roots and manipulates them from within, leading to the formation of galls or knots in the roots. Beneath the soil, they plunder the roots, while above the ground, the affected plant dies. The problems created by plant-parasitic nematodes, like the root-knot nematode, is responsible for annual losses ranging from $80 billion to $118 billion in damaged crops. There are even parasitic nematodes that live in the human intestinal system, which are responsible for 125 thousand deaths every year.

On a global scale, we coexist with nematodes rather than the other way around. Among the trillions of nematodes on our planet, the majority assume roles which are vital to sustaining plant life. Nonetheless, the spectrum of nematodes also includes those that kill crops and afflict humanity.

Written by:
Dr. Mohanna Mollavali

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