Soybean Cyst Nematode is a disease caused by a nematode first identified in Japan in 1915 and first reported in the United States in 1954. The disease is most likely in countries like Japan, Java, North America, Columbia, and Brazil. The nematode penetrates and feeds within the soybean roots, disrupting their ability to take up water and minerals thus leading to extreme economic losses. The disease is prevalent in Sub-tropical regions and is more severe in areas having light, sandy soils.
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Causal organism of Soybean Cyst
Soybean cyst nematode Heterodera glycine is an obligate parasite that belongs to the family Heteroderidae under the Order Rhabditida under the Class Chromadorea under the phylum Nematoda. Along with Soybean, some other leguminous plants are also susceptible to disease.
Morphology of Heterodera glycine
Heterodera glycines is a microscopic roundworm which are sexually dimorphic i.e. males and females have different body shapes.
The females are swollen and sedentary which are 500µm-900µm in length and 200µm-700µm in width. The female stage of SCN can only be seen through the naked eye. A cone-shaped projection is formed from the rear of the female’s body which is called a vulval cone that gives a characteristic “lemon” shape. The adult females are usually white and their heads are embedded into the root of the host plant. As the female matures, it lays eggs eventually hardens, and turns brown which is known as a cyst. This cyst protects the eggs.
Adult males are worm-shaped, motile, and elongated which are 1200µm-1500µm long and 26µm-30µm wide. The adult male has a blunt tail with a pair of two dark hooks called “spicules”. Adult males are similar to the second juvenile stage J2.
Life Cycle of Soybean Cyst Nematode Heterodera glycine
The life cycle can be completed within 24-30 days under optimum conditions.
SCN has six life stages which are as follows-
Embryonic stage/ egg stage and J1 stage– Mostly the eggs are produced within the female’s body, however, some are on the outside. The egg on the outside hatch to re-infect the soybean roots.
The first stage J1 molts within the egg to reach the J2 stage, whereas the egg inside the female’s body is modified as a cyst.
J2 stage– The J2 stage is the stage that infects soybean roots. The J2 migrates in soil, penetrates into the root, and migrates intra-cellularly to reach vascular tissue. The J2 inserts its stylet-releasing cellulose enzyme and modifies the host cell into a multinucleate feeding site which is known as Syncytium.
J3&J4- As the nematode continues to feed, the nematode molts into J3 and J4 stages. Almost half of the nematodes become swollen to form adult females whereas males become coiled structures and they emerge as vermiform, adult nematodes.
Adult stage and Fertilization- Males mate with females to fertilize the egg. After fertilization, the eggs that are retained within the female body are called cysts that dislodge and exist freely in the roots which later cause infection.
Symptoms of Soybean Cyst Nematode
There are no specific aboveground symptoms that can diagnose the disease, however, some aboveground symptoms include, yellowing of leaves, stunted growth, premature senescence, and even plant death. In the case of secondary infections, necrotic lesions are also seen.
The underground symptoms include pearly white-colored depositions and cysts on the root surface. The roots are dark-colored and poorly developed. Stressed plants are more susceptible to infection.
Disease Cycle of Soybean Cyst Nematode
The J2 is the infective stage during which the pathogen migrates into the soil and penetrates the plant root just beneath the root cap region.
The pathogen migrates towards the vascular bundle and releases cellulase enzymes to degrade the cell walls of the plant. The pathogen causes enlargement of plant cells due to secretions from its stylet. These enlarged cells are called syncytia.
These syncytia serve as feeder cells for the nematode. When the pathogen reaches J3 or J4, syncytia starts to degenerate. The syncytia inhibits the secondary growth of vascular bundles and reduces the function of conductive tissues thus causing stunted growth.
The subsequent juvenile stages molts and as the nematode feeds, they gradually transform into swollen females and vermiform males.
The swollen female lays an egg and after fertilization, it transforms itself into a hard brown-colored structure called cysts. These cysts enclose viable eggs and are responsible for the next cycle of infection.
Diagnosis and Detection of Soybean Cyst
Soybean cysts are difficult to detect because there are no specific above-ground symptoms and can only be diagnosed by analyzing underground symptoms-
Soil sampling– Soils are collected from fields and can be tested in the laboratory to detect the presence of nematodes and their population density.
Root inspection- Yellowish-white lemon-shaped adult females and brown cysts are visible to the naked eye.
Disease Management of Soybean Cyst
Soybean cyst nematode can be managed by following practices-
Cultural practices
Crop rotation– Soybean crops can be rotated with non-host crops such as cotton, tobacco, peanuts, sorghum, etc. It is one of the most effective ways to reduce nematode populations. Proper sanitation of infected fields and equipment should be done before planting new crops.
Planting cover crops such as oats, rye, etc can suppress the nematode populations and improve soil structure.
Scouting– Scouting can be done by collecting soil samples to test the presence of nematode from the laboratory. This helps in the early detection of disease before it destroys the overall crop field.
Resistant varieties
Planting resistant varieties like PI 548402, PI88788, and Peking can be effective in controlling the disease. Soybean varieties with SCN resistance produce higher yields than the susceptible varieties in the infested fields.
Chemical control
There are a number of chemical products to treat soybean seeds. Abamectin is one of the pesticides that are used to control soybean cysts. Nematicides such as Fluropyram, are also used to control the growth of pathogens. Some fumigants such as 1,3-dichloropropene are also used in heavily infested fields. Excessive use of chemicals may be hazardous to the environment.
Biological control
Bacterial parasite such as Pasteuria nishizawae consumes SCN eggs, so introducing such bacteria can be effective in controlling the disease. Fungi such as Trichoderma and Gliocladium are effective in suppressing nematode populations.
Soil amendments
Adding organic matter such as composted leaves, manure, grass, etc. helps boost beneficial soil microbes, improving soil structure and moisture retention.
Technological innovations
Using technologies such as remote sensing, and some mobile apps can make it easier to monitor disease progression.
Besides, awareness programmes, campaigns can also be conducted to provide knowledge about the disease.
References
- Soybean cyst nematode disease. (n.d.). Soybean Cyst Nematode Disease. https://www.apsnet.org/edcenter/resources/archive/Pages/SoyCystNema.aspx
- Arjoune, Y., Sugunaraj, N., Peri, S., Nair, S. V., Skurdal, A., Ranganathan, P., & Johnson, B. (2022). Soybean cyst nematode detection and management: a review. Plant Methods, 18(1). https://doi.org/10.1186/s13007-022-00933-8
- Agrios, G. N. (2005). PLANT DISEASES CAUSED BY NEMATODES. In Elsevier eBooks (pp. 825–874). https://doi.org/10.1016/b978-0-08-047378-9.50021-x
- Soybean cyst nematode (SCN). (n.d.). UMN Extension. https://extension.umn.edu/soybean-pest-management/soybean-cyst-nematode-scn
- Entomology, P. E. (n.d.). Soybean cyst nematode. https://extension.entm.purdue.edu/publications/E-210/E-210.html
- Soybean diseases. (n.d.). https://www.ndsu.edu/pubweb/~bernelso/soydiseases/cyst.shtml
- Soybean Cyst Nematode | Integrated Crop Management. (n.d.). https://crops.extension.iastate.edu/soybean/diseases_SCNbiology.html
- Soybean cyst nematode. (n.d.). https://nematode.unl.edu/scn/scnisu.htm
- Soybean cyst nematode. (n.d.). Ohioline. https://ohioline.osu.edu/factsheet/plpath-soy-5
- Thapa, A., Chowdhury, I. A., Yan, G., & Markell, S. (2022). Soybean cyst nematode. ResearchGate. https://www.researchgate.net/publication/373580186_Soybean_Cyst_Nematode
