Soil Infestation Density Affects the Results of Heterodera glycines Race Tests

Production of females by stock populations of races 1, 2, 3, 4, 5, 6, 9, and 14 of Heterodera glycines on 'Lee 74', 'Pickett', 'Peking,' PI 88788, and PI 90763 soybean cultivars and lines at inoculum densities of 100, 1,000, 4,000, 5,000, and 10,000 eggs and second-stage juveniles/pot (2.2, 21.8, 87.1, 108.9, and 217.9/cm(3) soil) was tested to determine the effects of soil population density on race identification using the 16-race scheme. Tests of all races were repeated 1 to 3 times during the year, except for the race 9 test, which was conducted once. Only races 3 and 9 had the same race designation at all infestation densities in repeated tests. Races 2 and 4 consistently were identified at all except the lowest infestation densities. Race 5 was identified as race 15 at the 100 infestation density in one test, and as race 1 at the 1,000 density in another test. Race 6 had significant numbers of females on Peking and P190763 that resulted in female indices of 34 on Peking and 14 on P190763 when the infestation density was 10,000. Race 14 was consistently identified between infestation densities of 4,000 and 10,000 but was identified as race 6 or 9 at the 100 infestation density and as race 4 or 14 at the 1,000 infestation density. Race 1 was identified as race 5 in a few instances because Pickett was not resistant to this population. The results of this series of experiments suggest that an infestation density of 4,000 eggs and second-stage juveniles/pot is best for race identification. Races were very poorly differentiated at the lowest density, differentiation was inconsistent at the 1,000 infestation density, and densities higher than 4,000 had reduced numbers of females on Lee 74 and relatively high numbers on the differentials which resulted in poor race differentiation with some races.