Rotating Sources of SCN Resistance
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Summary
- Soybean cyst nematode (SCN) damages soybeans in all growing areas of the US, likely causing losses of over one billion dollars per year.
- SCN cannot be eradicated, but producers can manage infected fields to maintain soybean yield, reduce SCN numbers and preserve the effectiveness of resistant varieties.
- These goals can be accomplished by appropriate rotation of resistant varieties, rotation of non-host crops, and monitoring of SCN population levels. Resistant varieties may increase yield by up to 50% in heavily infested fields.
- The source of resistance and genetic background play a role in a variety's ability to control SCN. Rotating resistant varieties is often the best strategy for managing SCN numbers.
- Pioneer soybean breeders continue to develop more, improved SCN-resistant varieties by using their patented marker-assisted selection (MAS) procedure to track resistance genes and combine yield with SCN resistance.
- Using MAS and lab and field screening, Pioneer researchers have stacked disease resistance and other agronomic traits into SCN varieties to create stable, high yielding, "best in class" Pioneer® brand soybean varieties.
The Soybean Cyst Nematode Problem
Soybean cyst nematode (SCN) is the single most damaging pest of soybeans in the United States. Over the last 30 years, SCN has moved increasingly northward from the southern US, and is now damaging tens of millions of soybean acres. All major soybean-producing states are affected, and the economic impact is likely over a billion dollars per year.
SCN is neither a disease nor an insect, but a tiny worm-like parasite with a three-stage life cycle - egg, juvenile and adult. The juveniles burrow into the roots of host plants and feed on young root cells. This feeding damages soybean plants in several ways:
- Extracts valuable nutrients from the plant
- Stunts and retards root growth
- Disrupts water and nutrient flow through the roots
- Suppresses nodulation by nitrogen-fixing bacteria
- Creates an entry port for root-rotting organisms
The extent of above-ground damage resulting from root feeding is variable. Under conditions of moisture stress, low soil fertility, compaction, disease pressure or other environmental stresses, above-ground damage is usually dramatic. But if soil moisture is plentiful and soil fertility is high, there may be no visible above-ground injury, even though yields are reduced.
It takes a period of years for SCN to build up in fields to damaging levels. Preventing this buildup requires early detection and management. However, SCN may decrease yields substantially without inducing obvious symptoms, so many fields may be infested without the knowledge of the grower. Sampling fields with no symptoms is the only way to detect SCN before it becomes an economic problem.
In addition to soybean, SCN can survive on a variety of crops and weeds (Table 1.)
Table 1 . Some SCN host plants in the Midwest US.
| SCN Hosts | |
| Beans (adzuki, bush, dry, green, lima, mung, red, snap) | Henbit |
| Birdsfoot-trefoil | Hop clover |
| Chickweed (common, mouse-ear) | Lespedeza |
| Clover (alsike, crimson, scarlet, sweet) | Lupine (white, yellow) |
| Common mullein | Pokeweed |
| Cowpeas | Purslane |
| Garden peas | Soybean |
| Ground cherry | Vetch (common, crown, hairy, winter) |
| Hemp sesbania | Winged pigweed |
(Iowa State Univ. Soybean Extension and Research Website) (http://extension.agron.iastate.edu/soybean/diseases_SCNbiology.html)
Corn, alfalfa and small grains are the most common crop choices for reducing SCN numbers. However, because SCN persists in the soil for many years, it cannot be totally controlled by crop rotation or any other practice. Rather, growers must try to keep SCN numbers as low as possible while maintaining soybean yield. This Crop Insights will explain SCN management using resistant soybean varieties.
Figure 1. This field shows a combination of symptoms from SCN and brown spot. Diseases often interact with SCN to compound yield losses.
Genetic Resistance to SCN
Researchers have identified a number of soybean lines that have the ability to resist nematode reproduction on their roots. Currently, there are three main sources for genetic resistance to SCN ? PI 88788, PI 548402 (Peking), and PI 437654 (Hartwig and CystX ®). The PI 88788 source is used in over 90% of existing SCN-resistant varieties marketed in the US. Only a small number of varieties currently use the PI 548402 source, and even fewer use the PI 437654 source.
SCN populations are genetically diverse and have historically been placed into population classes or races by their ability to reproduce on soybean tester lines. The PI 88788 source is resistant to races 3 and 14, while the Peking source is resistant to races 1, 3, and 5. The Hartwig source is resistant to almost all races, but this resistance is not complete immunity. The three sources differ in their ability to control SCN reproduction. PI 88788 reduces nematode reproduction by around 80 to 90%, while Peking and Hartwig sources reduce nematode reproduction by nearly 100% for the races they control.
In addition to the source of resistance, the genetic background also plays a role in a variety's ability to control SCN. For this reason, rotating resistant varieties is often the best strategy for controlling an SCN population.
Variation in SCN Populations
Race 3 is the most common SCN type in the US. Therefore, varieties with the PI 88788 source of resistance will provide good protection in the majority of SCN environments. If an alternative race exists or the SCN population is quite high, however, this source may not provide adequate protection and the use of a Peking or Hartwig source may be warranted. Appropriate resistant varieties can increase soybean yields by more than 50% in heavily infested fields. A combination of resistant varieties, crop rotations, and soil sampling can help manage SCN race variability and population levels.
Managing SCN Populations
Figure 2. SCN cysts on soybean roots.
SCN cannot be eradicated from a field, even when soybeans are removed from the rotation. And for economic reasons, producers need to continue to plant soybeans and maintain high yields over the long term. Thus, the goal for soybean producers in the presence of SCN should be to maintain soybean yield, reduce SCN numbers and preserve the effectiveness of resistant varieties. These goals can be accomplished by appropriate rotation of resistant varieties, rotation of non-host crops, and monitoring of SCN population levels.
Managing SCN Population Numbers
How much SCN numbers decrease in response to a non-host crop varies by geographical area. SCN numbers may decrease by as much as 90% in the southern US, but only 10 to 40% in northern and central states. Some of the difference is due to poor winter survival in the South. SCN-resistant varieties can also vary in how well they control nematode population densities, even when yields are the same (Doerge, 2005). This is important, because greater SCN reproduction will result in higher SCN populations in the soil the next time soybeans are grown in that field. (This is why growing an SCN susceptible variety in a field with a history of SCN, which increases the density of the pest, is usually not recommended.) So in addition to comparing yields, growers should also compare how resistant soybean varieties affect SCN population densities. Regular soil sampling to monitor SCN populations and determine how numbers change in response to management practices is an integral part of effective SCN management. Yield loss can occur with SCN egg counts of just 100 per 100 cc of soil (100 cc is less than one-half cup). At 1000 eggs per 100 cc of soil, economic losses for most susceptible soybean varieties are significant. Pioneer on-farm research indicates that SCN resistant varieties are almost always the best choice when SCN egg counts reach 4000 per 100 cc of soil. But in many fields, egg counts can reach 10,000 per 100 cc of soil, which can cause damage even to resistant varieties. In some geographies, this level of SCN pressure would be considered only moderate. Management practices and population thresholds vary by state, but all depend on egg counts determined by soil sampling at appropriate times. Egg counts above a certain (high) threshold result in a recommendation for a non-host crop such as alfalfa, corn, oats, sorghum or wheat to be grown. Growers wanting to keep soybeans in their rotation should evaluate the Peking or Hartwig sources as a tool to manage high SCN numbers. Growers should see their state extension website or bulletin for local SCN thresholds and management recommendations.
Breeding for Resistant Soybean Varieties
Pioneer soybean breeders are intensifying their efforts to develop more, improved SCN-resistant varieties. One of the challenges in the past has been that screening for resistant plants was quite tedious, time-consuming and variable. However, Pioneer has patented a new selection procedure which uses genetic markers in place of traditional screening. This marker-assisted selection (MAS) allows breeders to track the genes associated with resistance and increases their effectiveness in combining yield with SCN genes. Using MAS in the breeding process greatly improves the efficiency of selection and increases the rate of new product development. Pioneer soybean breeders have utilized MAS to combine high yield with SCN resistance in varieties with PI 88788 and Peking sources of resistance.
Pioneer breeders have also combined the power of MAS with lab and field screening to stack other important agronomic traits into new SCN varieties. These include tolerance to brown stem rot, sudden death syndrome, white mold, iron deficiency, cercospora leaf spot and phytophthora root rot. This has resulted in stable, high yielding, "best in class" Pioneer® brand soybean varieties.
Other Management Practices
Maintaining adequate soil fertility, reducing compaction, and controlling weeds, diseases and insects all improve soybean growth and plant health. These practices help plants compensate for damage by SCN, or prevent interactions with SCN that result in cumulative yield loss. However, they do not decrease SCN numbers and cannot substitute for practices like rotating crops and selecting the right SCN resistant varieties for your environment.
Figure 3. Note genetic differences between soybean varieties in this research plot grown under SCN pressure.
References
Doerge, T. 2005. New Soybean Varieties Moderately Resistant to SCN. Crop Insights Vol. 15, #8. Pioneer Hi-Bred Int'l., Inc., Johnston, IA.
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