Soybean Genotypes Resistant to Phytophthora sojae and Compensation for Yield Losses of Susceptible Isolines

Two isolines, with different alleles for resistance to Phytophthora sojae, of the soybean cultivars, Beeson, Century, and Williams were grown in replicated tests to assess yield losses attributable to this pathogen. Isolines susceptible to prevalent races of the pathogen had black seed coats as a marker trait; isolines resistant to prevalent races of the pathogen had yellow seed coats. Included in the tests were blends composed of equal numbers of seed of the two isolines for each cultivar. Tests were conducted at three locations in Indiana and one location in Ohio for three years. In six environments, where Phytophthora root and stem rot damaged soybean, isolines susceptible to prevalent races of the pathogen produced seed yields from 65 to 93% of the yields of isolines resistant to these races. In four of these environments, isoline blends produced yields equal to those of the resistant isoline. In two environments, where susceptible isolines averaged 65 and 69% of the yield of the resistant isoline; the blends averaged 89 and 83% of the yield of the resistant isoline. Where the pathogen reduced yields of susceptible isolines, yellow seeds of the isoline resistant to prevalent races of the pathogen contributed from 10 to 33% more seed than would be expected if resistant and susceptible isolines contributed equally to seed yield. The data demonstrate that plants of the resistant isoline were compensating for reduced productivity of the susceptible plants in the blend.