NPIPM:Soybean cyst nematode
Compiled by: Buyung Hadi, from the materials by: Greg Tylka, Loren Giesler, Dean Malvick and Lawrence Osborne
Soybean cyst nematode (SCN) was first found in the United States in 1954 although its introduction to North America likely occurred long before it was first identified. Soybean cyst nematode is currently perceived as the most serious threat on soybean production in the US.
Causal Organism
Soybean cyst nematode, Heterodera glycines, is a microscopic roundworm that attacks the roots of soybean and several other leguminous crops, such as cowpea, edible (dry and succulent) beans and crimson clover. A short table of known hosts and non-hosts of SCN is given below.
| Host crops | Weed hosts | Non-hosts |
|---|---|---|
| Birdsfoot trefoil | Common chickweed | Alfalfa |
| Edible beans | Common mullen | canola |
| Clover (Aliske, Crimson, Sweet) | Field Pennycress | Clover (Red, White, Ladino) |
| Cowpea | Henbit | Corn |
| Lespedezas | Pokeweed | Forage grasses |
| Lupine (White, Yellow) | Purslane | Small grains (barley, oats, rye, wheat) |
| Soybeans | Sericea lespedeza | Sorghum (grain and forage) |
| Vetch (Common, Crown, Hairy) | Wild mustard | Sugar beets |
Genetic variability occurs within the SCN populations in the United States. In the past, a scheme classifying the SCN population into different races based on the population's response towards four known sources of resistance (PI 88788, PI 90763, Pickett and Peking) was used.
The race designation did not take into account newer sources of resistance. Thus, a new classification scheme that will allow broader variety recommendation was proposed in 2002 and has been widely used. In this new scheme, a population of SCN is inoculated to several indicator lines associated with different sources of resistance (Table 2) and a susceptible check. The population is then classified to different HG(Heterodera glycines)-types upon which the pathogen is virulent. Virulence is measured with a Female index (FI), calculated as the percentage of the number of females found on a particular indicator line compared to the number of females found on the susceptible control. If the FI is greater than 10, the population is considered virulent for the particular indicator line. An example of HG Type classification of some SCN populations is illustrated in Table 3. In the HG-type scheme, more indicator lines can be added as new sources of resistance are developed. Furthermore, a designation of a population HG Type do not have to take into account all possible indicator lines. The "-" following some the HG Type designation in Table 3 indicates that the test did not take into account all possible indicator lines.
| Number | Indicator line |
|---|---|
| 1 | PI 548402 (Peking) |
| 2 | PI 88788 |
| 3 | PI 90763 |
| 4 | PI 437654 |
| 5 | PI 209332 |
| 6 | PI 89772 |
| 7 | PI 548316 (Cloud) |
| Sample number | Female indices on PI 548402 | Female indices on PI 88788 | Female indices on PI 437654 | HG Type |
|---|---|---|---|---|
| #14 | 7 | 32 | 0 | 2- |
| #57 | 7 | 8 | 0 | 0- |
| #59 | 19 | 13 | 0 | 1,2- |
Symptoms and Signs
Above ground symptoms of SCN infection are not always visible. A soybean crop may lose as much as 30% of yield due to SCN without showing above ground symptoms. Consequentally, cases are commonly encounterd where soybean yield losses occur for a several years before the presence of SCN is suspected. Grain yield below expectation may indicate the presence of SCN in the field, especially in isolated locations within the field. Yield maps can be very helpful in identifying areas to intensify scouting for SCN.
When symptoms are present, they are first manifested as a slight variation in height of otherwise healthy-looking dark-green plants, sometimes described as rolercoastering of row height. High levels of SCN may induce leaf yellowing, plant stunting and even plant death. These symptoms are easily confused with potassium deficiency, but are also somewhat similar to, nitrogen deficiency, iron chlorosis, herbicide injury, soil compaction, and other plant diseases. Fields with high SCN infection may have elliptic or circular areas with severe above-ground symptoms. Tillage patterns may help explain the ovoid shape of the affected areas.
Diagnostic signs of SCN infection can be found by digging roots from the soil and searching for females and cysts on the root surface. Female SCN appears as pearly white or yellow lemon-shape bodies on the root surface, visible to the unaided eye. The cysts, being a dead body of the females encapsulating viable eggs, are of a similar shape with the females but brownish in color. It takes several weeks from planting to actually observe these diagnostic signs on infected roots. Swollen females are common on smaller roots and much less common on the tap root. The females and cysts are readily lost when digging plants in very dry soil as the smaller roots that are most commonly infested may be left behind when digging the plants.
Disease Impacts
Soybean cyst nematode infection of soybean roots have stunted roots and fewer nitrogen-fixing bacterial nodules, resulting in the disruption of water and soil nutrient intake. Soybean cyst nematode also feeds directly from the roots, taking away plant nutrients. Infected plants develop fewer pods, which in turn results in lower yield. Feeding damage by SCN also predisposes the plants to infection by other root infecting pathogens, especially the sudden death syndrome and brown stem rot pathogens.
Life Cycle and Epidemiology
Soybean cyst nematodes undergo several phases in their life cycle. The eggs serve as the survival means for SCN, a portion of which are retained within the cyst. The cyst acts as an extra protective layer to these eggs. The juveniles hatching from these eggs move freely and actively enters the roots of soybean plants by means of digestive enzyme. Within the roots, the nematodes induce the development of a specialized feeding site. Feeding on these specialized sites, the nematodes grow in size and develop into adult males or females. The SCN females eventually become large enough that they rupture out of the roots and are visible on the root surface. The SCN males retain a vermiform (worm-like) shape in its adulthood, move out of the roots and mate with the females. The mated SCN females lay some of their eggs in gelatinous masses deposited on the rear of the body and retain some of the eggs within their body. The females eventually die with the viable eggs still stored within their body cavities. The nematode body wall forms a cyst that protect the eggs.
Under a constant temperature of 25˚C (77˚F), the life cycle of SCN can be completed in 21 days. In the field, temperatures and soil moisture levels conducive for soybean growth are also optimal for SCN population development. The SCN population density in the beginning of a soybean season is a critical factor determining yield loss. Thus, management strategy for SCN should be developed and implemented as soon as SCN presence within a field is confirmed.
Management Approaches
Scouting
Since above-ground symptoms of SCN infection are not always apparent, field testing for the presence of SCN in areas with SCN history is crucial. To test a field, soil core samples need to be collected. Divide the field to blocks of 10-acre-sections and sample each section separately. Within a section, walk a zig-zag or an M pattern path and collect 10-20 soil core samples (6-8 inches or 15-20 cm deep). Use a cylindrical soil probe to collect the soil core samples. Bulk the core samples from a section and mix it thoroughly in a bucket. Place 1 pint (0.55 L) of mixed soil in a plastic bag, label the bag with the field information and date of collection, and keep it in a cool and dark environment until shipping. Send the soil samples to your state diagnostic clinic for SCN confirmation. Soil samples can be collected at any time, but the optimal time to sample for SCN is as close to soybean harvest as possible since this is the peak period for SCN population. Use the sampling locations and testing data to construct an infestation map for your fields. This information can help you track the progression of the problem over time.
Cultural Methods
Soybean cyst nematode is an obligate parasite that, once hatched, requires the presence of susceptible hosts in the field to mature and multiply. Thus, incorporation of non-host crops, such as corn, wheat, grain sorghum and other crops (Table 1) in a rotational scheme can significantly reduce SCN population in an infected field. Short-term use of non-host crop will not eliminate the SCN population from an infected field since the nematode eggs may survive for years in the absence of host crops. The amount of SCN population reduction due to non-host crop rotation varies with geographic location. In the northern plains, rotation scheme that include two or more years of non-host crop before planting soybean is recommended. Unfortunately, the economic value of the alternative non-host crop are, at times, limited. In this case, a rotation scheme should incorporate resistant and tolerant soybean varieties. Care should be taken to use different SCN resistant soybean varieties or ideally soybean varieties with different sources of resistance in a rotation to avoid the buildup of SCN population that can break a certain mode of resistance. Avoiding such buildup of SCN population can also be achieved by planting tolerant susceptible varieties as a part of the rotation scheme. Even though tolerant varieties do not limit the growth of SCN population feeding on their roots, SCN infection on tolerant soybean varieties incurs lower yield loss than infection on non-tolerant varieties.
Because SCN is moved with soil, sanitation practices that limit movement of soil from one field to another should be practiced. Such practices include cleaning tillage and harvest equipment between fields to remove excess sol that may cling to the equipment.
Host Plant Resistance
Soybean varieties with resistance to SCN are widely available. Even though SCN can still survive and multiply on resistant soybean varieties, its population growth is more limited and the population of the pest is often suppressed. Consequently, soybean varieties with SCN resistancetypically produce higher grain yield than susceptible varieties when planted in SCN-infested fields. There are a number of sources of SCN resistance used in public and private breeding programs. The soybean accession PI 88788 is a common source of resistance. Other sources of SCN resistance include PI 90763, PI 209332, PI 437654, Peking, and Hartwig. As noted in the cultural method section above, it is advisable to use soybean varieties from different sources of resistance in a rotation.
Chemical Control
Nematicides are available for SCN management. However, there is little evidence to show that their use increases the yield enough to justify the cost.
Online Resources
Video: Scouting for SCN during the soybean growing season
