Plant Parasitic Nematodes: Species, Diagnosis and Control

Underground Marvels: Delving into the Enigmatic Realm of Plant Parasitic Nematodes!

What are Plant Parasitic Nematodes (PPNs)?

Plant-parasitic nematodes are microscopic worms (usually less than 1 mm long) armed with a spear-like device called “stylet” for feeding. They are a biologically diverse group of organisms that can be found almost everywhere. Their ability to adapt to a wide variety of habitats, including soil and marine, provides an evolutionary advantage for their prosperous survival and species longevity.

Economic Impact of PPNs: Why do they matter for agriculture? 

Plant-parasitic nematodes pose a significant threat to crop production due to the extensive damage and substantial yield losses they cause to a wide range of crops (Aderoju & Ajayi, 2022; Bridge et al., 2005). Damage caused by the collective effort of plant-parasitic nematodes has been estimated to be 80 – 118 billion dollars annually (Nicol et al., 2011). The direct and indirect damage they cause could result in stunted growth, delayed crop maturity, toppling, reduced yields and quality of crop produce, high production costs, and consequently income loss. 

Top 7 Most Important PPNs You Should Know!

• Root‐knot Nematodes (Meloidogyne spp.)
• Cyst Nematodes (Heterodera and Globodera spp.)
• Root Lesion Nematodes (Pratylenchus spp.)
• The Burrowing Nematode (Radopholus similis)
• Stem and Bulb Nematode (Ditylenchus dipsaci)
• The Pine Wilt Nematode (Bursaphelenchus xylophilus)
• The Reniform Nematode (Rotylenchulus reniformis) (Jones et al. 2013)

General Symptoms of Plant Parasitic Nematodes

The damage caused by nematodes is often non-specific and easily confused with other abiotic or biotic stress symptoms. Symptoms of nematode damage are found both above and below ground (Coyne et al., 2007).

Above-ground symptoms include; 

• gall formation or abnormal swelling of seeds or leaves, 
• leaf stripe, bleaching and discoloration of leaves,
• swollen, crinkled and disorganized tissue growth, 
• internal stem necrosis, signified with a red ring, 
• chlorosis/browning of leaves). 
• Reduced fruit and seed size leading to low yields and crop productivity.

Patchy distribution symptoms (right) and seedlings with curly chlorotic leaves (left)

Below-ground symptoms mostly require uprooting plants or excavating roots to observe symptoms. Symptoms include;

• root galling (most common),
• shortened/stubby or abbreviated roots, 
• root lesions, tuber necrosis, and tuber cracking, 
• cysts or ‘pearly’ roots, deformed or altered root architecture.

 Cysts or ‘pearly’ roots (right) and root galling (left)

Management of Plant-parasitic Nematodes in Agricultural Fields.

1. Sampling (Important precautionary measure)

Accurate diagnosis is essential for successful management programs. For this reason, soil sampling can help the farmer define the problem on time and recognize the nematode species infesting his/her plants and fields. It is highly recommended that growers apply this practice before buying a new field or/and planting/sowing their crop. This is particularly important in tropical and subtropical regions where precise damage thresholds may not be established. For diagnostic purposes, samples can be collected from crop fields or plant groups after at least 2-3 weeks of active growth. This enables informed management decisions based on the types and numbers of nematodes identified.

2. Cultural Control Methods

Crop rotation and cover cropping can be used in integrated pest management protocols to reduce plant-parasitic nematode incidence and replenish soil nutrient levels. Planting corn as a rotational and cover crop has been shown to reduce the incidence of root-knot nematodes (Osei et al., 2010). Other cultural approaches include proper field sanitation (solarization), using planting materials (seeds, etc.) free from nematodes, and alternating planting times.

3. Biological Control Methods

Alternative means of pest management, such as the use of biological controls, are of great interest for crop producers. The efficacy of nematophagous bacteria and fungi in the control of some nematode pests has been well-documented. Common microorganisms, such as Pasteuria spp. and Bacillus spp., have been explored for their bio-control efficacy on a good number of plant-parasitic nematodes (Kokalis, 2015; Gao et al., 2016) 

4. Physical Control Methods

Plant parasitic nematodes can be easily killed in the laboratory by the application of heat, irradiation, or osmotic pressure. Solarization is also widely used for controlling nematodes. Hot baths of the propagation material can also be used to kill the nematodes before planting. 

Figure 3: Solarization (heat treatment) of nematode-infested fields. Source: Ramesh Pokharel, Colorado State University Extension 

5. Use of Plant Extracts

Plant extracts often contain a myriad of compounds that demonstrate nematode-suppressive properties. Ethanolic extracts of Neem, Marigold, and Eucalyptus have been shown to exhibit nematicidal activity against many PPNs.

Plant with nematicidal activity against PPNs (Neem1, Eucalyptus2 and Marigold3)

6. Use resistant plant varieties (Host resistance)

Incorporating plant varieties that harbor multiple resistance to various plant pathogens is an attractive and practical approach. Nematode-resistant genes found in gene pools of a variety of plant species can be introgressed into the genomes of economically important crops with natural susceptibility through transgenic technologies; this helps the plants to synthesize and release volatile organic compounds, which help to combat or mitigate the effect of infestation by nematodes (Ali et al., 2017).

7. Chemical Control Methods

Nematicides are chemically synthesized products that kill or adversely affect nematodes. They are grouped into fumigant and non-fumigant products based on their volatility in soil.

♦  Fumigant nematicides

Fumigant nematicides are toxic compounds with narrow- to broad-spectrum effects. In addition to killing plant-parasitic nematodes, they may also be effective against numerous soil-borne pests and pathogens. Examples include Telone, Vapam, and Paladin nematicides.

♦  Non-fumigant nematicides

Non-fumigant nematicides are nonvolatile toxic chemicals that can be applied prior to planting, at planting, or after planting through soil drenching, drip irrigation, or spraying onto the crop foliage to reduce population densities of nematodes and protect crops from damage. In contrast to fumigant nematicides, non-fumigants’ efficacy does not depend on soil temperature, and they can also double as insecticides. 

Examples include Vydate, Movento, Velum Prime, and Mocap nematicides.

Author Reference: Aderoju, M. S., and Ajayi, A. M. 2022. Occurrence, diversity and relative abundance of plant-parasitic nematodes: A survey of selected Crop fields in Akure, southwestern Nigeria. African Journal of Biological Sciences. 4:71–80.

Additional References

1. Ali, M. A., Azeem, F., Abbas, A., Joyia, F. A., Li, H., and Dababat, A. A. 2017. Transgenic strategies for enhancement of nematode resistance in plants. Frontiers in Plant Science. 8.
2. Bridge, J., Plowright, R. and Peng, D. Nematode parasites of rice. In: Luc M, Sikora R, Bridge J, editors. Plant Parasitic Nematodes in Subtropical and Tropical Agriculture. 2nd ed. Egham: CABI Bioscience; 2005. pp. 87-130.
3. Coyne, D. L., Nicol, J. M., and Claudius-Cole, B. 2007. Practical plant nematology: A field and laboratory guide. International Institute of Tropical Agriculture.
4. Crow, W.T. and Han, H. 2005. Sting nematode. The Plant Health Instructor
5. Gao, H., Qi, G., Yin, R., Zhang, H., Li, C., and Zhao, X. 2016. Bacillus cereus strain S2 shows high nematicidal activity against Meloidogyne incognita by producing sphingosine. Scientific Reports.;6:28756.
6. Jones, J. T., Haegeman. A., Danchin. E. G., Gaur, H. S., Helder, J., Jones, M. G., Kikuchi, T., Manzanilla-López, R., Palomares-Rius, J. E., Wesemael, W. M., and Perry, R. N. 2013 Top 10 plant-parasitic nematodes in molecular plant pathology. Mol Plant Pathol. 14:946-61.
7. Kokalis-Burelle, N. 2015. Pasteuria penetrans for control of Meloidogyne incognita on tomato and cucumber and arenaria on snapdragon. J. Nematol. 47:207-213
8. Nicol, J., Turner, D., Coyne, L., den Nijs L., Hockland, S., and Maafi, Z. 2011.Current nematode threats to world agriculture. In: Jones J, Gheysen G, Fenoll C, editors. Genomics and Molecular Genetics of Plant-Nematode Interactions. Berlin: Springer Science Business Media. pp. 21-43.
9. Osei, K., Gowen, S., Pembroke, B., Brandenburg, R., and Jordan, D. 2010. Potential of leguminous cover crops in management of a mixed population of root-knot nematodes (Meloidogyne ). J. Nematol. 42:173-178
10. Ramesh Pokharel, Colorado State University, Western Colorado Research Center. 2/2011.