Insects and Diseases

Figure 1. The susceptible variety shown on the left is stunted, and this will eventually lead to yield loss. Compare this to the resistant variety on the right.

Figure 2. The soybean roots shown on the right are from a susceptible SCN variety compared with the roots on the left from a resistant variety.

Go to the following pages to study:

Life cycle and biology of SCN

Key plant symptoms of SCN

Soybean Cyst Nematode

The number of SCN in a field can be greatly reduced through proper management, but it is impossible to eliminate SCN from your field once it has become established. Therefore, you must choose appropriate management practices so that you can continue profitable soybean production. The goals for soybean management in the presence of SCN are to improve soybean health and yield, reduce SCN numbers despite continuing planting soybean, and preserve the yield potential of resistant soybean varieties. Since no single management practice will meet all three goals, you must use an integrated system combining several components. The use of resistant varieties and a properly designed crop rotation are among the top practices to consider.

Management

Resistant Varieties. Resistant varieties are successfully being used to manage SCN. Unlike susceptible varieties, resistant varieties reduce the ability of SCN to develop and complete its life cycle at a reduced rate. The use of resistant varieties allows you to grow soybean with high yield while managing SCN numbers so that soybean can be grown profitably in the future. In the past, farmers may have been reluctant to use resistant varieties because there was a yield drag between resistant and susceptible varieties. This is not the case anymore. Combined efforts of soybean breeders and nematologists, have brought along high-yielding SCN-resistant varieties.

Selecting SCN-resistant varieties will help manage SCN in your fields.

Several different sources of resistance exist and these sources have been incorporated into various soybean varieties. The three sources of SCN resistance genes that are incorporated into soybean varieties suitable for growth in Iowa are PI 88788, PI 548402 (Peking), and PI 437654 (Hartwig – ex. CystX). The main source of SCN resistance today is PI 88788. Repeated use of the same resistant variety or the continuous use of varieties with the same resistance may eventually result in a reduction in the ability of those varieties to manage the nematode and produce maximum yields. Resistant varieties are not resistant to all SCN populations, nor are they immune. Most resistant varieties contain only one source of resistance. This allows you to rotate sources of SCN resistance to help prevent the development of more damaging SCN populations. Unfortunately, SCN-resistant varieties that yield comparably do not necessarily control the nematode equally. SCN-resistant varieties can vary considerably in how well they control nematode population densities, even top varieties. Greater SCN reproduction will result in a higher SCN egg population in the soil the next time that soybean is grown in that field. Consequently, growers must consider how SCN-resistant soybean varieties affect SCN populations in addition to how well the varieties yield to maintain the long-term productivity of the land for soybean production. Selecting SCN-resistant varieties based solely on yield data is risky because some relatively high-yielding soybean varieties allow substantial amounts of SCN reproduction. Keep this point in mind when evaluating soybean variety trials. For more information on high yielding varieties that also reduce nematode populations visit the ISU SCN variety trial web page.

It is not uncommon for other soybean nematodes and diseases to be present in SCN-infested fields, and for interactions among the pathogens to occur. Infection by SCN juveniles or the eruption from roots by the maturing females may create openings in the root surface that serve as entry points for other soilborne soybean pathogens such as Pythium, Rhizoctonia, Phytophthora, Sudden Death Syndrome (SDS), and Charcoal Rot. In northern states, SCN is one of the partners involved in the development and spread of SDS. The fungus that causes SDS (Fusarium solani) is fully capable of acting on its own, but research in the past 10 years has shown that SCN speeds up the development of SDS symptoms and increases their severity, leading to greater yield loss. Recent research also reports that SCN infection increases the incidence and severity of Brown Stem Rot (BSR), another important soybean disease in northern states. Soybean varieties have been developed with resistance to several pathogens. Important examples in northern varieties are those with resistance to both SCN and Phytophthora root rot, while several southern varieties are resistant to both SCN and root-knot nematodes. So what do you do if you have both SCN and another soybean disease in your field? Since SCN is the single most damaging pest in soybean and it will occur every year, unlike fungal diseases, you should take care of the SCN problems first.

Crop Rotation. Crop rotation produces many benefits and should be a part of your management program whether you have SCN or not. If you have SCN, your rotation should include non-host crops (usually corn) and resistant soybean varieties. Once you successfully reduce SCN numbers, you should consider including susceptible soybean varieties in your rotation. Your rotation should not include other hosts for SCN. In a field planted to a non-host crop, SCN numbers will not increase and should decrease, the amount of decrease varies in relation to geographical area. SCN numbers may decrease by as much as 90% in the southern United States, but only 10-40% in the north (some of the difference is due to poor winter survival in the south). Once your field is infested with SCN, resistant varieties must be used in a rotation if you want to grow soybean profitably over the long term, but they must be used appropriately. Even on these resistant varieties, some nematodes will grow and reproduce. Therefore, continued use of the same resistant varieties will result in a nematode population in the field that can damage plants and reduce yield. Each time you grow an SCN-resistant variety you should select a different variety. If possible, select a variety with a different source of resistance. One of the goals of crop rotation is the reduction of SCN numbers. If you are successful in reducing SCN, you may want to plant SCN-susceptible varieties. Susceptible varieties are used in the rotation to slow the SCN population changes that may occur with continued use of resistant varieties. Some susceptible varieties are less sensitive to SCN feeding; these tolerant varieties should be considered in your rotation.

Other Cultural Practices. Maintaining adequate soil fertility and controlling weeds, diseases and insects improve soybean plant health. These practices help plants compensate for damage by SCN, but do not decrease SCN numbers. These practices should be a part of your management to maintain your yield potential. No-tillage practices reduce water and wind erosion, and should slow SCN movement. In addition, research has show that population densities of SCN is lower under no-tillage practices. Soil that remains on tillage and harvest equipment can also move SCN and should be removed when they are moved from an infested to a non-infested field. Last, always purchase certified seed since soil particles infested with SCN may be in non-certified seed.

Keys to Reducing SCN

Since there are many options in reducing the number of nematodes in your field, you can not focus on just one to get rid of the problem. You need to monitor SCN populations through periodic soil sampling, and note the change in numbers in response to your management practices. Another important key concept is rotating. You need to rotate with non-host crops, rotate with resistant soybean varieties, and also rotate with tolerant or susceptible soybean varieties when SCN numbers are low, to slow down the adaptation of SCN to resistant varieties. If you have good management of weeds, water, and fertility you will avoid compounding damage due to SCN. By combining all of these suggestions, you should be successful in reducing yield loss from SCN in your fields.

More Information

Research shows that soil pH may be used to predict the yield-robbing potential of soybean cyst nematode (SCN) and BSR and to guide soybean growers in management decisions to minimize yield loss caused by both pathogens. The following publication from the University of Wisconsin and Iowa State University describes these situations. Soil pH Influences Soybean Disease Potential (pdf)

Links

ISU SCN Management Recommendations (pdf)
SCN-resistant soybean varieties for Iowa (updated November 2006 pdf)
So many choices, so much potential: SCN-resistant soybean varieties (PDF)
Recommended management practices of SCN (pdf)