Conference Proceedings
SOYBEAN CYST NEMATODE:
WHAT DOES IT MEAN TO THE SEED INDUSTRY?
Gregory L. Tylka
Plant Nematologist
Associate Professor, Department of Plant Pathology
Iowa State University
{Paper presented at the 18th Annual Iowa State University Seed Technology Conference,
Ames, Iowa, February 13, 1996}
Soybean cyst nematode, Heterodera glycines, was first detected in the United States in North Carolina in 1954 (8). The nematode was discovered in Iowa and other parts of the upper Midwest in the late 1970s and early 1980s (5). Since then, soybean cyst nematode has spread quickly throughout the region and has become a major factor limiting soybean yields throughout the Midwest. Soybean yield losses due to soybean cyst nematode in the Midwest have been estimated to exceed 48 million bushels per year from 1989 to 1991, far greater than loss estimates for any other disease (1). Although the exact magnitude of the yield loss estimates may not be accurate because the figures were not based on any standard quantitative method, the relative rankings of importance of the various diseases likely are correct.
In order to discuss the effects of soybean cyst nematode on the seed industry, one must first understand the basic biology of the nematode. Soybean cyst nematode is a microscopic, unsegmented roundworm that has three main stages in the life cycle: egg, juvenile, and adult. The life cycle can be completed in approximately 30 days under optimum conditions. Consequently, there is the potential for two or more generations to occur per growing season in the Midwest. Worm-shaped soybean cyst nematode juveniles hatch from eggs in the soil when temperature and moisture levels become satisfactory in the spring (6). The hatched juvenile is the only stage of the nematode capable of infecting roots, and juveniles that do not penetrate host roots and begin feeding will die from starvation, predation, or parasitism within days to weeks.
After penetrating the soybean roots, juveniles migrate to the vascular tissue in the center of the root. Upon reaching the vascular tissue, the juveniles cease moving, lose most of their body muscles, and begin to feed. The juvenile nematodes inject secretions into the plant cells which modify and transform the cells into specialized feeding sites called syncytia. The juveniles swell as they feed, and eventually, female nematodes become so swollen that they break out through the root tissue and are exposed on the surface of the root. Male nematodes are not swollen as adults; instead, the worm-shaped males migrate back out of the roots into the surrounding soil and fertilize the lemon-shaped adult females on the roots. After fertilization, males eventually die whereas females remain attached to the roots and continue to feed. The fertilized swollen females produce eggs, initially in a mass or egg sac outside the body and later within the body cavity of the female. Eventually, the entire body cavity of the adult female becomes filled with eggs, and the female dies. It is the egg-filled body of the dead female that is referred to as the cyst. Cysts will eventually dislodge from the roots and become free in the soil. The walls of the cyst become hardened to provide excellent protection for the 200 to 400 eggs contained within. Soybean cyst nematode eggs survive within the cyst until conditions become proper for hatching. Although numerous eggs will hatch within the first year, many also can survive dormant within the protective cysts for several
years (2).
Effects on the Seed Industry
A. Dissemination of Soybean Cyst Nematode in Seed
Soybean cyst nematode juveniles are mobile, but the small size of the juveniles and the short length of time that they are active in the soil render the nematode incapable of moving more than an inch on its own. The nematode is moved greater distances by means of anything that moves soil. Major means of spread of soybean cyst nematode include movement of infested soil on farm equipment and via wind erosion. However, local and long-distance spread of soybean cyst nematode within soil particles in harvested seed also is possible. Consequently, growers are strongly advised not to use seed harvested from infested fields for planting in noninfested fields (7). Growers are encouraged to have seed cleaned very thoroughly if they must use harvested seed to establish a new crop in subsequent years.
Although there exists the potential for soybean cyst nematode to be introduced to previously noninfested fields in soil particles in commercial seed, the likelihood is very slight. The acceptable level of contamination of seed with soil particles tolerated by seed companies is such that the probability of introducing SCN to a noninfested field is considerably less likely via soil particles in commercial seed than by other means of spread. However, it has been a concern and challenge for seed companies to maintain their standards for numbers of soil particles in seed following cool, wet growing seasons when soybean plants were short and pods were formed relatively low on the plant. Such conditions occurred in Iowa in 1993, and several seed companies were faced with the decision to either temporarily increase tolerance levels for soil particles in seed or to purchase additional equipment to maintain their existing standards.
B. Soybean Cyst Nematode-Resistant Soybean Cultivars
Soybean cyst nematode-resistant soybean varieties are a very effective means of managing the nematode. The soybean cyst nematode juveniles that penetrate the roots of resistant soybean varieties are unable to successfully establish feeding sites needed to sustain nematode development and reproduction. In the southeastern United States, resistant varieties were developed not long after the first soybean cyst nematode infestations were identified in the late 1950s and early 1960s. When the nematode became established in the upper Midwest, however, there were no resistant soybean varieties available that were adapted for the northern environments. Consequently, use of resistant soybean varieties was not a practical management option to many growers in Iowa and Minnesota until the 1990s.
Currently, numerous public and private soybean varieties resistant to soybean cyst nematode are available in all maturity groups grown in the United States. Considerable effort and funds have been, and will continue to be, expended by the soybean seed industry to develop high-yielding varieties that are resistant to nematode. Resistant soybean varieties produce acceptable yields and suppress reproduction of the nematode in infested fields. Additionally, many resistant varieties yield similar to non-resistant (susceptible) varieties when grown in noninfested fields. To illustrate the utility of resistance as a management strategy, numerous resistant and susceptible soybean varieties are grown in small field plots in soybean cyst nematode-infested and nearby noninfested fields at various research sites throughout Iowa each year. In 1994 in north central Iowa, the yield of the highest-yielding resistant variety, Bell, was equivalent to that of the highest-yielding susceptible variety, Sturdy, in the noninfested field. However, Bell produced 15 bushels per acre more than Sturdy in an adjacent field infested with soybean cyst nematode. Such results are typical of the performance obtained from many currently available public and private soybean cyst nematode-resistant soybean varieties.
There are two main sources for soybean cyst nematode resistance genes used in soybean varieties, Peking and PI 88788. A few commercial varieties with resistance genes from PI 90763, PI 209332, and, most recently, PI 437654 also are available. All of these sources of resistance, with the possible exception of PI 437654, allow limited soybean cyst nematode reproduction. Unfortunately, reproduction of even a few individuals on a resistant soybean variety creates the potential for selection and eventual increase of that nematode subpopulation (3, 4, 9). Thus, it is somewhat risky to rely on resistance as the only strategy for soybean cyst nematode management because of the potential for development of races of soybean cyst nematode capable of reproducing on resistant soybean varieties and the limited availability of effective, alternative tactics for management of the nematode. To maintain acceptable soybean yields, prevent rapid increases in nematode population densities, and discourage the selection of new nematode races, Iowa State University recommends alternating use of resistant soybean varieties with different sources of resistance in conjunction with growing nonhost crops and susceptible soybeans for integrated management of soybean cyst nematode (7).
Summary
As with most other aspects of soybean production, soybean cyst nematode has had a significant effect on the soybean seed industry and will continue to do so in the future. The need to maintain strict tolerance levels for contamination of seed by soil particles is critical to continue to prevent the spread of the nematode in commercially purchased soybean seed. Furthermore, continued and increased efforts by the seed industry are needed to develop new soybean cyst nematode-resistant soybean varieties with greater yield potential, new sources of soybean cyst nematode resistance, and additional resistance to other pathogens.
Literature Cited
1. Doupnik, B., Jr. 1993. Soybean production and disease loss estimates for north central United States from 1989 to 1991. Plant Disease 77:1170-1171.
2. Inagaki, H., and M. Tsutsumi. 1971. Survival of the soybean cyst nematode Heterodera glycines Ichinohe (Tylenchida: Heteroderidae) under certain storage conditions. Applied Entomology and Zoology 6:156-162.
3. Luedders, V.D., and V.H. Dropkin. 1983. Effect of secondary selection on cyst nematode reproduction on soybeans. Crop Science 23:263-264.
4. McCann, J., V.D. Luedders, and V.H. Dropkin. 1982. Selection and reproduction of soybean cyst nematodes on resistant soybeans. Crop Science 22:78-80.
5. Noel, G.R. 1992. History, distribution, and economics. Pp. 1-13 in R.D. Riggs and J.A. Wrather, eds. Biology and management of soybean cyst nematode. St. Paul: APS Press.
6. Schmitt, D.P., and R.D. Riggs. 1989. Population dynamics and management of Heterodera glycines. Agricultural Zoology Reviews 3:253-269.
7. Tylka, G.L. 1995. Soybean cyst nematode. Iowa State University Cooperative Extension, Publication Pm-879. 5 pp.
8. Winstead, N.N., C.B. Skotland, and J.N. Sasser. 1955. Soybean cyst nematode in North Carolina. Plant Disease Reporter 39:9-11.
9. Young, L.D. 1982. Reproduction of differentially selected soybean cyst nematode populations on soybeans. Crop Science 22:385-388.