STRATEGY A featured segment designed to share business- critical information to seed-selling professionals. Visit SeedWorld.com to download this department and other tools. The $1 Billion Pest Meets Its Match Soybean cyst nematode is one of the world’s most destructive soybean pests. How can we begin to combat it? Joe Funk jfunk@issuesink.com WIDELY DESCRIBED AS the world’s most destructive soybean pest, soybean cyst nematode (SCN) is estimated to cause losses exceeding $1 billion each year in the United States and Canada alone. What makes this pest so hard to manage? Cysts containing eggs can survive dormant in the soil for 10 or more years. Once a field is infected with SCN, there’s no way to rid them; growers must manage them. Plants infected with high numbers of SCN have poorly developed root systems that cannot effectively use nutri- ents and water. Plants that show visible SCN damage might appear slightly yellow and stunted. More frequently, however, infected plants have no apparent symptoms except for yield loss. Easy Answers Breeders identified PI 88788 as a resistance line in 1962, but it was not available to farmers until the 1980s. For 20-plus years, more than 95% of all SCN-resistant soybean varieties in the Midwest included resistance from the PI 88788 gene. PI 88788 is still effective; however, researchers and farmers are seeing resistance build. All it takes is a few resistant individuals to create a problem. “Some may think that the resistant varieties cause SCN to mutate, but that’s not the case,” explains Greg Tylka, an Iowa State University professor of plant pathology and micro- biology. “There is always the possibility, but overwhelmingly, SCN resistance is the result of natural selection driven by PI 88788. “When using the same herbicide- or nematode- resistant gene or antibiotic over and over, the small minor- ity in a population will eventu- ally become the majority.” As the proportion of resistant nematodes in the soil increases, soybean yields decline. Yield data from 15 years of Iowa variety trials, summarized by the SCN Coalition, indicate as much as a 14 bushel-per-acre average yield loss for varieties with PI 88788 resistance grown in SCN fields infested. The fatal flaw is that virtu- ally all SCN-resistant varieties have the same resistance genes. Tylka says there’s a 97% chance, at least in Iowa and likely throughout the Midwest, that the resistance in newly released varieties is from PI 88788.” He says the remaining 3% is nearly all from Peking, another SCN-resistant line. “It’s surprising that the PI 88788 resistance has done as well as it has for as long as it has,” says Brian Diers, a University of Illinois profes- sor and soybean breeder. “However, when we look at SCN samples from fields, we generally find that SCN is infecting PI 88788 more read- ily than in the past. “It is not that resistance from PI 88788 has completely broken down and it is not working. When nematode samples from farmer’s fields are tested, we often see that the roots of PI 88788 have about 30-40% as many nema- todes as a susceptible check. On the one hand, this means that resistance is breaking down but on the other hand, it means resistance is still 60-70% effective.” Still, Tylka, like others is worried about how we will deal with the decrease in the effectiveness of the resistance currently deployed in varieties. “If nothing changes, there’s only one outcome: more yield 56 / SEEDWORLD.COM JUNE 2019 With a 24-day lifecycle, juvenile soybean cyst nematodes burrow into soybean roots to feed and develop.