The use of resistant cultivars is the most common practice in managing the soybean cyst nematode (SCN), Heterodera glycines. Currently, most commercial SCN-resistant soybean cultivars were developed from a single source of resistance, plant introduction (PI) 88788. The effect of crop sequences including rotations of SCN-susceptible soybean ‘sturdy’ with SCN-resistant soybean ‘freeborn’ carrying resistance derived from PI 88788, soybean ‘Pioneer 9234’ carrying resistance derived from PI 548402 (peking), and nonhost corn was studied at two field sites in southern Minnesota, the United States. Parasitic ability of SCN measured as a female index (FI) on PI 88788 and freeborn increased with the number of years freeborn was planted. After more than 5 years of freeborn, either in monoculture or rotation with other soybean cultivars and corn, the SCN population changed from the original race 3 (HG type 0 or 7) to race 1 (HG type 2.7 or 2.5.7). After 10 years of freeborn, the changed nematode population reproduced freely on the once resistant cultivar (FI > 60). There was no selection pressure from the use of PI 88788-resistance on SCN populations that can overcome peking resistance. Planting 3 or fewer years of Pioneer 9234 had no noticeable effect on the virulence phenotype of the SCN population. This study suggests that more cultivars from resistance sources other than PI 88788 are urgently needed for effective management of the nematode in Minnesota and other regions in the world.
Key words: Heterodera glycines, HG type, host-parasite relationship, race, soybean cyst nematode, virulence phenotype.
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