Beware of your Enemy within your Soils – Soil Nematodes

By Jack Wekesa

Nematodes which are typically most abundant in upper soil layers where organic matter, plant roots and other resources are most abundant can be very devastating to the farmer. Some species can damage plant roots, stems, foliage and flowers by puncturing the cell walls using their sharply pointed mouths. It is estimated in flowers that 5 % of the Pesticides budget is for nematodes control.

There are over 4,100 species of plant-parasitic nematodes. However, the most economically important nematodes are rootknot nematodes (Meloidogyne spp.), cyst nematodes (Heterodera and Globodera spp.), root lesion nematodes (Pratylenchus spp.) and the burrowing nematode Radopholus similis. Nematodes are structurally simple organisms, covered by a protein cuticle which acts as a sturdy support and protects them from toxic compounds. It also allows them to increase or decrease in body size. They have cuticle forms that act as sensory organs or participate in the locomotion of nematodes. They are elongated, slender, filiform circular sections, with complete absence of epithelium. The root knot and cyst nematodes are biotrophic and induce complex feeding structures in the roots of their hosts which supply the nematode with a rich and long-lasting food source.

1. Root-knot Nematodes (Meloidogyne spp.)

Root-knot nematodes are obligate plant parasites. The genus contains 98 species and they parasitize almost every species of vascular plants.

2. Cyst Nematodes
The cyst nematodes are obligate biotrophs and are of great economic importance throughout the world. The most damaging species include soybean cyst nematodes (SCNs) (Heterodera glycines), potato cyst nematodes (PCNs) (Globodera pallida and G. rostochiensis) and cereal cyst nematodes (CCNs) (including Heterodera avenae and H. filipjevi).

3. Root Lesion Nematodes
There are over 60 named species of root lesion nematodes (Pratylenchus spp.) which are distributed worldwide. Pratylenchus species rank third only to root-knot and cyst nematodes as having the greatest impact on crops worldwide.

Life Cycle
The life cycle of most plant-parasitic nematodes are similar and start with an egg. The egg undergoes embryonic development resulting in a first-stage (J1) juvenile nematode. Depending on the nematode species, the J1 may hatch from the egg or molt within the egg, forming a second-stage juvenile (J2). The majority of plant-parasitic nematode species will hatch at the J2 stage.

There are four juvenile developmental life stages (J1, J2, J3 and J4) that are separated by molting and conclude with an adult nematode. The complete life cycle from egg to egg requires 3-4weeks depending on the nematode species, the soil temperatures and soil moisture. The reproductive potential of plant-parasitic nematodes in field crops is exponential, with multiple generations during the long growing season. Depending on the species, each nematode female can lay dozens to hundreds of eggs in her life span.

The root-knot and cyst nematodes infect crops at the J2 stage while the female reniform nematode does so at the young adult stage. The J2 hatches from the egg and swim in the moisture layer surrounding soil particles in search of a root. No juveniles or adult males of the reniform nematode have been observed feeding on a plant’s root system. The root-knot nematode enters roots just behind the root cap while the reniform and cyst nematodes enter at any point on the root system. After entering the root, all three species migrate through the root system to the vascular tissue. There, the nematode becomes sedentary and forms specialized feeding sites referred to as giant cells. The giant cells are created by the feeding activity of the nematodes, and they act as a nutrient sink and feed the nematode throughout its life cycle.

Damages of Nematodes
Nematode symptoms can be difficult to detect and may be confused with symptoms of nutrient deficiency. Meloidogyne sp. – a type of root- knot nematode – is a serious pest on roses. The galls it induces on the roots constrict the vascular system blocking the transport of water and nutrients through the plant to the extent that heavily infested plants are often dwarfed with smaller leaves and appear paler than normal. They may wilt and die when exposed to water and nutrient stresses.

Growers are advised to inspect the roots of planting material, even though they originate from a reputable breeder. By doing this, a grower will prevent further infestation since successive growth of plants that host the nematodes will lead to an increase in their population.

Low production, poor quality of stems and reduced stem length are realized due to the damage caused by nematodes if not eradicated early enough.

Management of Nematodes
A proactive and holistic approach should be considered in sustainable management of this economic significant pest.

• Monitoring
  This is key to the management of plant parasitic nematodes by carrying out soil/media and water analysis on a regular basis. Population pressure and crop damage will guide growers on the most sustainable measure to be taken at any given situation.
• Cultural Controls
  Sites with histories of root knot nematode problems should be kept nematode free. Non-host crops should be grown to reduce population densities. Weed control is also important because many weeds serve as hosts for the root knot nematode. In addition, the source of water should be treated to avoid re-introduction of new populations once you treat the area.

Chemical Controls
Different Greenhouse and Sites should be routinely sampled for plant-parasitic nematodes, at least every month.

Nematodes management programs are developed based on;

• Threshold levels
• Development stage and lifecycle of the Nematodes
• Natural enemies including the Saprofagic nematodes
• Three Mode of action program Below is a table of our Nematicides in Amiran Kenya.

Below is a table of our Nematicides in Amiran Kenya.