Soybean Cyst Nematode and Actions to Reduce Damage

• Soybean cyst nematode (SCN) may reduce soybean yields without farmers knowing it. 
• The goal of managing SCN is to achieve improved, sustainable soybean yield over time through the proper use of all available management tools.  
• A purposeful, comprehensive SCN management program can reduce SCN impacts on the bottom line and improve soybean production.  

How Serious is Soybean Cyst Nematode? 
Based on a survey from the University of Missouri, SCN can lead to an estimated loss of more than 125 million bushels in total U.S. soybean production annually1. As the number 1 pest in soybeans, nematologists and plant pathologists estimate that SCN robs more yield per year than the next 5 soybean pathogens combined, with an estimated $1.5 billion in annual soybean yield losses.¹ 

According to the University of Illinois, SCN can lead to losses up to 80%. However, the more common losses of up to 40% are not obvious enough to be visible from above-ground symptoms², meaning SCN can reduce soybean yields without farmers realizing it. Once SCN is introduced into a field, it can never be eradicated; it will be there forever. Because of that, it is a pest that must be managed; otherwise, it will eventually become a significant problem. Losses associated with SCN in any given year will be directly dependent on environmental factors, such as drought or other natural events. However, planning and use of SCN management strategies can reduce the impact of these SCN-related losses.³ 

Identification and Life Cycle 
SCN are microscopic roundworms that invade and infest soybean roots. Multiple generations of SCN occur each year in the U.S. within a single growing season, with as few as 2 generations in the north and as many as 6 in the southern U.S. There are 3 major life cycle stages of SCN: egg, juvenile and adult. The egg is the overwintering SCN stage that hatches as a juvenile roundworm and is attracted to young developing roots early in the season (see Figure 1). 

SCN juveniles enter the soybean root and move toward vascular tissue – the tissue that transports moisture and nutrients throughout the plant. The juveniles modify plant cells and begin to feed, robbing nutrients and damaging their host. SCN females continue to feed inside the root but eventually grow large enough to burst outside the root, while the males leave the root to mate with exposed females. The female SCN continue to feed, with the largest portion of the developing body exposed on the root exterior (see Figure 2).⁴ 

The young, exposed, developing female is initially white in color but becomes yellow to brown with age. Following fertilization, the female produces to 200-500 eggs. As her life cycle is completed, the female dies and changes from yellow to brown. Some of the maturing eggs will immediately develop and hatch, starting the life cycle over again (see Figure 3).⁴ The remaining female’s body becomes the familiar “cyst” structure, which can act as a long-term, resilient casing, helping some eggs to survive for years. SCN’s ability to overcome management practices is largely due to extended egg hatch timing, increasing the chances of successful life cycle completion across years.² 

SCN commonly complete 3-5 generations per growing season in the U.S. based primarily on the following factors (in no particular order)³:

• Planting date 
• Soil temperature 
• Host suitability 
• Geographic location 
• Presence of alternative hosts 
• Length of growing season 

During the soybean growing season, the most typical SCN life cycle can be completed in 24-30 days, based largely on environmental conditions such as temperature and moisture levels.

SCN Impact on Soybeans 
SCN reduces soybean performance and yield in several ways. The greatest impact is caused by SCN juveniles establishing themselves within the root and causing vascular plant tissue disruption. As the juveniles develop into full-grown adults, the efficiency of moisture and nutrient transport within the infected plant is drastically affected. Secondary effects of SCN infection include: 

• Stunting and damage of developing soybean root system 
• Reduction of nitrogen-fixing Rhizobium bacteria root nodules 
• Stress interactions with any number of pests which flare within stressed soybean plants  
• Disease introduction through SCN entry points within the root 

A common pest introduced through SCN feeding is Fusarium virguliforme, the causal organism of sudden death syndrome (SDS). This disease is often closely associated with SCN. Other diseases associated with SCN are brown stem rot, Pythium, Phytophthora and iron deficiency chlorosis (IDC). 

Management 
Although SCN can have drastic effects on soybean yield, there are management strategies that have predictably positive results over time. Below are some strategies farmers should consider: 

Identify field presence: Soil sampling is reported to be the most reliable means of confirming and monitoring SCN levels.⁵ Initially, SCN soil sampling is recommended to provide a baseline. Then, a regular soil sampling program once every 3-5 years will provide a picture of whether management practices are producing the desired result. Due to the irregular distribution of SCN within fields, it's best to use soil sampling only as a means to confirm presence of SCN and monitor changes in SCN pressure over years. 

Weed management: Soybeans are not the only host for SCN. An Indiana agricultural field survey determined that known SCN-host winter weeds were present in 93%of surveyed fields.⁶ According to Purdue University Extension, there are 6 known winter weeds that allow various levels of successful SCN reproduction⁷, and management of these weeds should be an important goal: 

• Purple deadnettle (strong host) 
• Henbit (strong host) 
• Field pennycress (moderate host) 
• Shepherd’s purse (weak host) 
• Small-flowered bittercrest (weak host) 
• Common chickweed (weak host) 

Crop rotation: Non-host crop rotation is a foundational principle in managing SCN. Table 1 shows several commonly grown U.S. crops that are not SCN hosts. Use of non-host crops provides the unique opportunity to reduce fieldwide SCN numbers by disrupting the SCN life cycle. Although reductions are possible, several consecutive rotations with non-host crops are needed for significant population decreases, and total elimination will not be feasible. It is possible to see greater reduction with rotation in longer growing season regions as result of hatch events extending out over longer time frames. However, rotating out of soybeans for more than 3 years has been found to offer little value in further reduction of SCN egg numbers. All the susceptible SCN eggs that will hatch without a host present have hatched and the overwintering egg numbers stabilize. 

SCN-resistant varieties: If SCN presence is confirmed in fields planned for soybeans, SCN-resistant varieties are strongly recommended. SCN-resistant varieties reduce the ability of SCN to successfully colonize the soybean root, leading to a reduction of the SCN reproduction rate. Considering SCN can have 3-5 generations per year where each female in each generation produces 200-500 eggs, any reduction in their reproduction rate can result in impactful reductions in end-of-season SCN egg numbers. Planting varieties without SCN resistance may not always result in noticeable yield loss, however repeated use will enable higher SCN reproduction rates, increasing the risk of SCN exploding into a significant yield-limiting pest in later years.  

Alternate source of SCN resistance: There are 7 different sources of SCN resistance that have been identified and utilized by soybean breeders for SCN management. Sources of resistance are often referenced by a Plant Introduction (PI) number. Although 7 sources of resistance have been identified, only two are frequently utilized by breeders. The most utilized source is PI 88788, representing more than 90% of commercial varieties sold today. PI 58402, also known as Peking, is utilized in a limited number of varieties sold. 

SCN-resistant varieties limit SCN egg laying capacity within soybean roots, but do not completely prevent reproduction. Up to 10% of normal reproduction can still occur on SCN-resistant varieties. Due to long-term use of predominantly 1 source of resistance, SCN populations have slowly adapted to PI 88788, and it’s not uncommon to observe reproduction rates greater than 10% with some populations. SCN populations will likely slowly increase due to continued adaptation to PI 88788. 

Golden Harvest has introduced a new source of SCN resistance, PI 89772, and is working to provide new commercially available varieties with it. Continued use of crop rotation to non-host crops will remain critical. If farmers are unable to rotate sources of resistance, they should rotate to a soybean variety that does not utilize PI 88788, as reproductions can vary between varieties.⁴ 

Seed-applied nematicide: The last element of a comprehensive SCN management program is considering the use of a seed-applied nematicide. In combination with all the management tools outlined, a seed-applied nematicide can offer additional protection against nematodes. Since healthy root development is vital to establishing the highest yield potential, nematicides have been one of the most anticipated seed-applied technologies offered in recent years. 

Golden Harvest recommends 2 seed-applied nematicide options: 

1. Clariva® Complete Beans seed treatment, a combination of separately registered products, for season-long SCN protection. 
2. Saltro® seed treatment, which is available to add to existing fungicide/insecticide seed treatment options. Saltro provides protection against SDS in addition to providing robust activity against SCN, root knot, reniform, lesion and lance nematodes.  

For more insight into SCN management, contact your local Golden Harvest Seed Advisor. 

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References: 

1. Wrather, J.A., T.R. Anderson, D.M. Arsyad, J. Gai, L.D. Ploper, A. Porta-puglia, H.H. Ram and J.T. Yorinori. 1997. Soybean disease loss estimates for the top 10 soybean producing countries in 1994. Plant Dis. 81:107-110. 
2. Schmitt, D.P., J.A. Wrather and R.D. Riggs. 2004. Biology and management of soybean cyst nematode. 2nd Edition. Schmitt & Associates of Marceline. 
3. Niblack, T.L. and G.L. Tylka. SCN management guide 5th Edition. Plant Health Initiative – A North Central Soybean Research Program. 
4. Tylka, G. L. 2012. Soybean cyst nematode field guide 2nd Edition – Iowa State University Extension and Outreach. 
5. Faghihi, J., and V. R. Ferris. 2006. Soybean cyst nematode. Department of Entomology. Purdue University. 
6. Creech, J. E., and W. G. Johnson. 2006. Survey of broadleaf winter weeds in Indiana production fields infested with soybean cyst nematode (Heterodera glycines). Weed Technol. 20:1066-1075. 
7. Mock V.A., J.E. Creech, B. Johnson, J. Faghihi, V.R. Ferris, A. Westphal and K. Bradley. 2007. Winter annual weeds and soybean cyst nematode management. Purdue University Extension bulletin WS-36.