Lack of Predictable Race Shift in Heterodera glycines-Infested Field Plots.

Soybean cultivars with different sources of resistance to Heterodera glycines were grown at three locations initially infested with races 2, 3, and 6 in order to investigate H. glycines race shift in field populations. Each spring and fall, soil samples were taken from each plot and race tests were conducted to evaluate effects of cultivar and time of sampling. Field experiments were paired field plots rotated annually with corn since 1991. Cultivars included at the northern and central Missouri sites were Williams 82 (susceptible to H. glycines), Linford (PI 88788 source of resistance), MFA 9043 (Peking) replaced by Morsoy 9345 (Peking and PI 88788) from 1995 to 1997, and Jackson II (Peking + PI 88788) replaced by Asgrow 3431 (Peking and PI 88788) in 1996-97. Cultivars at the southern Missouri site were Essex or Hutcheson (susceptible to H. glycines), Forrest (Peking), Hartwig (PI 437654), and Rhodes (PI 88788 + Peking). In 1995, race tests were performed at four temperature regimes to determine temperature effects on race designations. Race shifts were not predictable based on the source of resistance of the soybean cultivar planted. Variability in female numbers on Lee 74 among tests caused changes in female indices (FI). Furthermore, race designations were influenced by the time of sampling and temperature at which the race tests were conducted. The variability of H. glycines populations in both field and greenhouse situations diminishes the value of race test results when making cultivar recommendations.