New Sources of Resistance to Phytophthora sojae in the Soybean Plant Introductions.

Single dominant Rps genes have been highly effective in managing Phytophthora 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 (PI273483 to PI427107) from the United States Department of Agriculture Soybean Germplasm collection were evaluated for resistance to P. sojae using the hypocotyl inoculation technique for Rps genes and the layer test for partial resistance. Of the 1,015 accessions tested, 159 accessions were susceptible 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 resistant to these three races and 32 accessions were resistant to an additional five races chosen specifically to elicit a susceptible interaction with two and three Rps gene combinations. In addition, 55.5% of the 887 accessions tested had high levels of partial resistance or tolerance (scores ≤4.0) to P. sojae. The majority of the accessions that were resistant to all of the races tested and those that had very high levels of partial 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.