POD INOCULATION TECHNIQUE WITH PHYTOPHTHORA-SOJAE TO EVALUATE SOYBEANPOPULATIONS FOR RPS ALLELES IN-FIELD PLANTINGS

The pathogen Phytophthora sojae (M.J. Kaufmann and J.W. Gerdemann) can be injected into soybean pods [Glycine max (L.) Merr.] to determine p resence of phytophthora root and stem rot resistance. The objective of this study was to use pod inoculation in the field to screen soybean F-2 populations segregating for Rps alleles. The potential exists to i nject different races of the pathogen into pods at different nodes of a single plant, allowing for multiple screenings at a time. Controllin g the amount of Phytophthora sojae pathogen injected into each pod was necessary for accurate scaring of disease progression. Injecting 25 m u L of Race 1 or Race 3 mycelium inoculum gave optimal symptom differe nces. The amount of disease progression up the pad to the point of att achment or the complete lack of rotting was determined to distinguish between susceptible and resistant plants, respectively. The data obtai ned from scoring pods conformed wed to ratios of expected models. This technique provides a rapid and inexpensive field screening for identi fying resistant or susceptible progeny early in the selection process.