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Selection for Resistance to Four Races of Phytophthora megasperma var. sojae in Soybean¹

Phytophthora rot is a serious disease of soybean [Glycine max (L.) Merr.] incited by the fungus Phytophthora megasperma Drechs var. sojae Hildeb. Screening for resistance to this disease is complex because of the genetic variability for virulence in the pathogen. The cultivar ‘Tracy’ was used as the resistant parent in crosses with a very susceptible breeding line, D55-1492, and the cultivar ‘Forrest’. These crosses were used to study the genetics of resistance to physiologic races 1, 2, 3, and 4 of the pathogen in order to find more efficient screening methods. F₃ lines from the crosses Tracy x D55-1492 and Forrest x Tracy and F₃ populations from 50 backcross families of Forrest (2) x Tracy were evaluated. The plants were grown hydroponically and were infested with race 1, 2, 3, or 4 of the pathogen. Data from the Tracy x D55-1492 and Forrest x Tracy crosses indicated a two-gene segregation ratio when challenged by races 1, 3, and 4 and a one-gene ratio with race 2. The reaction of the backcross families suggests that testing with race 2 and any one of the other three races would have been sufficient to select the 22 families that were segregating for all four races and would thus be an efficient screening method.

Key Words: Glycine max (L.) Merr. • Segregation ratios

² Geneticist. USDA-SEA-AR, Stoneville, MS 38776.

Received for publication October 2, 1978.