Temperature effects on race determination in Heterodera glycines

Currently there are 16 possible races for Heterodera glycines, and these ar e differentiated based on ability of a nematode population to develop on a set of four differential soybean genotypes. Because results are based on nu mbers of nematode females that develop to a specific stage rather than on t he reproductive capability of these females, race determinations based on f emale indices may not represent results obtained after several reproductive cycles of H. glycines. Counting numbers of eggs and juveniles, and then de veloping corresponding indices, would allow reproduction to be considered i n making race determinations. Our objectives were to compare the present ra ce identification scheme for H. glycines based on female indices with those using egg and juvenile indices and to examine the effect of temperature on race designations using female, egg, and juvenile indices. Race designatio ns for H. glycines populations from mio locations in Illinois were determin ed at 20, 27, and 30 degreesC in a water bath. The numbers of females, eggs , and juveniles (at 19 days) were recorded, and an index based on each life stage was calculated. Race determinations based on female, egg, or juvenil e indices were inconsistent when conducted at 20 degreesC, which demonstrat es that this temperature is not suitable for identifying races of H, glycin es. However race designations at 27 and 30 degreesC were consistent for all three indices. This indicates that counting females, eggs, or juveniles sh ould be equally reliable when race determinations are conducted at these tw o temperatures, and choice of method would depend on investigator preferenc e or research objective.