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Additional Sources of Stem Canker Resistance in Soybean Plant Introductions

USDA-ARS, Soybean Production Research Unit, P.O. Box 196, Stoneville, MS 38776

^* Corresponding author (sprstone{at}attmail.com).

Stem canker caused by Diaporthe phaseolorum (Cooke & Ellis) Sacc. f. sp. meridionalis Morgan-Jones can cause significant yield losses in soybean [Glycine max (L.) Merr.] in the southern USA. Many genotypes are known to be resistant to the disease; however, little is known about the inheritance of resistance and allelic relationships of genes conferring resistance. The objective of this study was to determine the inheritance of resistance in two germplasm accessions, PI 230976 and PI 398469, and define allelic relationships with the four described genes. Each PI was crossed with a susceptible genotype, J77-339, and the four fines carrying Rdcl, Rdc2, Rdc3, or Rdc4. Crosses were made in the field at Stoneville, MS, in 1992, and F₁ backcrosses to J77-339 were made in 1993. Plants of parents and 10 F₂ populations were toothpick inoculated in the field in 1993. The F_2:3 and backcross populations were later evaluated in a greenhouse. The F₂ populations derived from J77-339 x PI 230976 and J77-339 x PI 398469 approached a 3 resistant/1 susceptible ratio indicating that a single dominant gene for resistance is carried by each plant introduction. The F₃ data confirmed this result. Backcross data substantiated complete dominance. Response of all F₂ populations derived from crosses with genotypes carrying known genes suggested a 15:1 ratio indicating nonallelism. It is not known if both plant introductions carry the same gene. These sources of resistance may lessen genetic vulnerability should new virulent isolates overcome sources now used by breeders

Received for publication March 21, 1994.