Symptoms

Depending on the level of infestation, soybean cyst nematode (SCN) can cause yield losses ranging from 0 to 80 percent. Yield damage, even when severe, is usually not associated with any visible symptoms. Farmers cannot tell that SCN is reducing yields until crop harvest. At that time, low yields are frequently blamed on problems other than SCN. Consistent, moderate damage by SCN may not even be noticed by producers, since acceptable yields may be obtained even though higher yields may be possible.

Occasionally, nonspecific aboveground symptoms are associated with SCN. These symptoms are most common in sandy soil types and are frequently associated with other plant stresses. Aboveground symptoms include plant yellowing, stunting, wilting in the heat of the day, and, rarely, plant death. A potassium deficiency symptom, evident as a bright yellowing of the margins of leaves, is the most common aboveground symptom of SCN infection. When symptoms do occur, they are usually evident as groups of plants that follow oblong, circular, or streaked patterns.

SCN-affected plants (left) vs. unaffected plants (right)

Root symptoms in plants heavily damaged by SCN are more common than aboveground symptoms. These symptoms include reduced root growth, a "bottle brush" appearance of the root systems, root browning, necrosis, and poor nodulation. Cysts (bodies of female nematodes)(indicated by black arrow) are visible on diseased roots four weeks after planting through the rest of the season. Cyst numbers are greatest on small roots, so plants must be dug, not pulled, when looking for cysts. Each cyst will be about the size of a period at the end of a sentence and 25 to 50 times smaller than nitrogen fixing nodules.

The soybean cyst nematode is a microscopic roundworm that feeds on the roots of soybean plants. SCN survives the winter as eggs in a cyst. Each cyst contains 50 to 500 eggs. A wormlike juvenile, approximately 1/64 inch in length and the diameter of a human hair, is inside each egg. The cyst protects the eggs from adverse soil conditions. Eggs hatch in the spring about the same time soybeans are planted. Not all eggs hatch and some remain dormant in cysts for several years. Juveniles that do hatch move through the soil until they encounter soybean roots and penetrate them: this process is aided by soybean root exudates that attract the juveniles. The majority of juveniles fail to find a soybean root and die of starvation. Juveniles that find a root enter it and begin feeding using a hypodermic needle-like structure called a stylet. Juveniles develop into either adult males or females. Males mate with females, leave the root and die. Fertilized females remain in the root, and their bodies begin to swell. About 28 day hatching, the swollen bodies of SCN females break through the root surface. These small white bodies are visible with the naked eye but can be seen more easily using a hand lens. As the white female ages, her body begins to turn yellow and eventually a golden brown. The brown color indicates the death of the female, which is now referred to as a cyst.

• Fields should be observed for SCN every four weeks from vegetative stage until harvest. Observe all areas of the field with and without suspected symptoms.

• Sample all soybean fields for SCN in the fall, winter or early spring prior to planting. Cyst numbers are greatest on small roots, so plants must be dug, not pulled, when looking for cysts. Each cyst will be about the size of a period at the end of a sentence and 25 to 50 times smaller than nitrogen fixing nodules. Samples from fields in Kentucky may be sent to the Plant Pathology Diagnostic Lab for confirmation.

• If an SCN analysis indicates a field has moderate to high nematode levels, plant a non-host crop, such as corn or grain sorghum, or a resistant soybean cultivar.

• Use a resistant cultivar that has resistance to multiple SCN races. Do not plant resistant cultivars more frequently than once every three years. For more information on recommended crop rotations or preventing soybean race shifts, see the listed sources of additional information at the end of this factsheet.

• Maintain balanced soil fertility and fertilize fields according to soil test recommendations. Use production practices that enhance overall crop health.

• SCN can be spread by moving soil from one field to another. When using equipment in a field know to have SCN, wash the equipment to remove the infested soil before entering another field. SCN can also be spread by footwear and transportation vehicles. Do not drive vehicles in fields and avoid walking in muddy fields. If you must drive or walk in an infested field, if possible, remove soil by washing.

• IPM-3 Kentucky IPM Manual for Soybeans

• PPA-3 Soybean Cyst Nematode in Kentucky by Don E. Hershman, Extension Plant Pathologist, University of Kentucky

• PPFS -AG-S-8 Value of Wheat Residue in Soybean Cyst Nematode Management by Don E. Hershman, Department of Plant Pathology, University of Kentucky

• PPFS-AG-S-9 Sampling Soybean Fields For Soybean Cyst Nematode Analysis by Don E. Hershman, Department of Plant Pathology, University of Kentucky

• PPFS-AG-S-10 Use of Soybean Blends in Soybean Cyst Nematode Management Programs by Don E. Hershman, Department of Plant Pathology, University of Kentucky

• PPA-10b Kentucky Plant Disease Management Guide for Soybeans by D.E. Hershman, Extension Plant Pathologist

• Compendium of Soybean Diseases, J.B. Sinclair and P.A. Backman (The American Phytopathological Society Press)

• Annual Kentucky Soybean Performance Tests, Progress Report 348