Absence of Residual Effects of Defeated Resistance Genes on Phytophthora Rot of Soybean

Defeated resistance genes are genes conferring race-specific resistance that have been overcome by compatible pathogenic races. Such genes display residual effects if they increase the host's level of either rate-reducing resistance or tolerance to compatible races. The objective of our research was to investigate residual effects of defeated resistance genes on phytophthora rot [caused by Phytophthora sojae (Kauf. and Gerde.)] of soybean [Glycine max L. (Merr.)]. We used near-isogenic lines of soybean (cv. ‘Harosoy’) carrying resistance genes Rps1, Rps3, Rps4, Rps5, and Rps6 to develop additional soybean genotypes carrying combinations of two, three, and four resistance genes. We evaluated these genotypes for rate-reducing resistance and tolerance to phytophthora rot by assessing extent of root lesions, plant height, and plant dry weight after 28-d growth in the greenhouse in pots with and without inoculum. Race 7 of P. sojae was used because this race was compatible with all resistance genes present. In one experiment, 28 genotypes carrying zero to four resistance genes in either homozygous or heterozygous condition were evaluated with two replications; in a second experiment, 13 genotypes homozygous for zero, one, or two resistance genes were evaluated with six replications. Genotypes carrying Rps5 showed significantly more root rot than rps5 genotypes, possibly due to linkage of Rps5 with a gene or genes for susceptibility. No other significant effects of Rps genes were detected. These results indicate that residual effects cannot be exploited in this pathosystem. Salaries and research support provided in part by State and Federal Funds appropriated to the Ohio Agric. Res. and Development Ctr., The Ohio State Univ. Manuscript no. 96-93, Ohio Agric. Res. and Development Ctr.