ABSENCE OF RESIDUAL EFFECTS OF DEFEATED RESISTANCE GENES ON PHYTOPHTHORA ROT OF SOYBEAN

Defeated resistance genes are genes conferring race-specific resistanc e that have been overcome by compatible pathogenic races. Such genes d isplay residual effects if they increase the host's level of either ra te-reducing resistance or tolerance to compatible races. The objective of our research was to investigate residual effects of defeated resis tance genes on phytophthora rot [caused by Phytophthora sojae (Kauf. a nd 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 carryin g combinations of two, three, and four resistance genes. We evaluated these genotypes for rate-reducing resistance and tolerance to phytopht hora rot by assessing extent of root lesions, plant height, and plant dry weight after 28-d growth in the greenhouse in pots with and withou t inoculum. Race 7 of P. sojae was used because this race was compatib le with all resistance genes present. In one experiment, 28 genotypes carrying zero to four resistance genes in either homozygous or heteroz ygous condition were evaluated with two replications; in a second expe riment, 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 linka ge of Rps5 with a gene or genes for susceptibility. No other significa nt effects of Rps genes were detected. These results indicate that res idual effects cannot be exploited in this pathosystem.