Soybeans are a globally significant crop. However, a silent but devastating threat to soybean production is the soybean cyst nematode (SCN). This microscopic pest causes billions of dollars in yield losses annually, often without obvious above-ground symptoms, making its detection and management a continuous challenge for growers.
What is Soybean Cyst Nematode?
Soybean cyst nematode (SCN), Heterodera glycines, is a tiny, plant-parasitic roundworm. It primarily targets soybean roots, establishing feeding sites within the root tissue. The feeding action of SCN disrupts the plant’s ability to take up water and nutrients, directly impacting its growth and health.
SCN is the most damaging soybean pathogen globally, causing over $1.5 billion in annual U.S. losses. The nematode’s ability to survive for extended periods in the absence of soybeans complicates management.
The Nematode’s Life Cycle
The soybean cyst nematode life cycle involves six stages: egg, four juvenile stages (J1-J4), and the adult. The cycle begins when eggs, protected within hardened cysts in the soil, hatch. A second-stage juvenile (J2) emerges from the egg.
The J2 is the infective stage. It migrates through the soil to penetrate soybean roots, usually just behind the root tip. Once inside, the J2 establishes a specialized feeding site called a syncytium, drawing nutrients from the plant. The juvenile then develops through J3 and J4 stages.
During development, the female nematode swells, becoming lemon-shaped and breaking through the root surface while feeding. Adult males, which are mobile, leave the root to fertilize the females. The female then produces hundreds of eggs, mostly inside her body.
As the female dies, her body hardens into a tough, protective brown cyst, encasing the eggs. These cysts are durable, allowing eggs to survive in the soil for many years, even a decade or more, without a host plant. Under favorable conditions, SCN can complete a full life cycle in less than a month, leading to multiple generations, often three to four, within a single growing season.
Recognizing the Damage
Identifying SCN infestation is challenging because its above-ground symptoms are often vague and easily confused with other plant problems. Common signs include stunted plant growth, yellowing leaves (chlorosis), nutrient deficiencies, and reduced yield. These symptoms might be mistaken for issues like drought stress, soil compaction, or nutrient imbalances, leading to delayed or incorrect diagnoses.
Significant yield losses, sometimes exceeding 30-40%, can occur without any visible above-ground symptoms. When symptoms are present, they often appear in circular or oval patches within a field, sometimes following tillage patterns. The only definitive visual sign of SCN is the presence of tiny, white-to-yellow, lemon-shaped female nematodes or cysts directly on soybean roots. These are much smaller than beneficial nitrogen-fixing nodules.
Accurate diagnosis requires careful examination of roots by gently digging up plants and washing away soil. However, the most reliable detection method, especially in the absence of visible symptoms, is soil testing. Soil samples, typically collected in the fall, determine SCN egg population densities, guiding management decisions.
Strategies for Management
Effective management of soybean cyst nematode requires an integrated approach that combines several strategies to reduce nematode populations and minimize yield losses. Relying on a single method often proves insufficient due to SCN’s adaptability and persistence.
Resistant varieties are a primary line of defense against SCN. These soybean varieties are bred to prevent or significantly reduce nematode reproduction on their roots. However, the widespread use of varieties with the PI 88788 resistance gene has led to SCN populations adapting and overcoming this resistance in many fields. This adaptation underscores the importance of rotating resistance sources, such as incorporating varieties with the Peking resistance gene, which has shown better SCN control and higher yields in some studies.
Crop rotation with non-host crops is another effective strategy for reducing SCN populations in the soil. Non-host crops, such as corn, wheat, sorghum, and alfalfa, deprive the nematode of a food source, causing a decline in egg numbers. While continuous planting of non-host crops can lead to significant reductions in SCN populations, it is generally not possible to eliminate the nematode entirely, as some eggs can remain viable in cysts for many years.
Seed treatments offer an additional layer of protection, particularly for early-season control of SCN. These treatments, which can contain chemical nematicides or biological control agents, are applied directly to the seed before planting. They are most beneficial under low to moderate nematode pressure and are designed to protect young seedlings from initial nematode invasion.
Maintaining good soil health practices also contributes to a plant’s ability to tolerate SCN pressure. Proper drainage, balanced fertility, and effective weed control can help plants grow more vigorously, potentially compensating for some nematode damage. Some weeds can also serve as alternative hosts for SCN, making weed management important.
Sanitation practices are also important in preventing the spread of SCN. Since the nematode moves with infested soil, cleaning farm equipment, vehicles, and tools thoroughly before moving between fields can reduce the risk of spreading cysts to uninfested areas. While SCN cannot be eliminated once established, combining resistant varieties, crop rotation, seed treatments, good soil health, and sanitation provides the most robust management plan.