New sources of resistance to Phytophthora sojae in the soybean plant introductions

Single dominant Rps genes have been highly effective in managing Phytophtho ra sojae. However, numerous physiological races of P. sojae have developed in response to deploying single gene resistance. New sources of resistance with potentially novel Rps genes are needed. A selection of accessions (PI2 73483 to PI427107) from the United States Department of Agriculture Soybean Germplasm collection were evaluated for resistance to Fl sojae using the h ypocotyl inoculation technique for Rps genes and the layer test for partial resistance. Of the 1,015 accessions tested, 159 accessions were susceptibl e to races 7 (vir 1a, 2, 3a, 3c, 4, 5, 6, 7), 17 (1b, 1d, 2, 3a, 3b, 3c, 4, 5, 6, 7), and 25 (1a, 1b, 1c, 1k, 7). However, 162 accessions were resista nt to these three races and 32 accessions were resistant to an additional f ive races chosen specifically to elicit a susceptible interaction with two and three Rps gene combinations. In addition, 55.5% of the 887 accessions t ested had high levels of partial resistance or tolerance (scores less than or equal to4.0) to P. sojae. The majority of the accessions that were resis tant to all of the races tested and those that had very high levels of part ial resistance originated in the Republic of Korea. These results indicate that this region is an area with many sources of resistance to P. sojae for both specific Rps genes and partial resistance.