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Analysis of Strategies for Transfer of an Allele for Resistance to Phytophthora Rot in Soybean

Sementes Dois Marcos, P.O. Box 7005, Brasilia, DF 71600, Brazil;
Department of Agronomy, Iowa State Univ., Ames, IA 50010;
Department of Agronomy, Iowa State Univ. and Univ of Puerto Rico, Mayaguez, PR 00708.

^* Corresponding author.

It has been reported that seven backcrosses are required to transfer a major allele for resistance to Phytophthora rot, caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Erwin, in soybean [Glycine max (L.) Merr.]. Recently, fewer backcrosses have been used successfully to transfer an allele for resistance to P. megasperma into soybean cultivars. To resolve the difference between the reported research and the practical experience of soybean breeders, this study was conducted to determine in what backcross generation a bulk of phenotypically similar, P. megasperma-resistant lines will provide the same yield as that of the recurrent parent. The dominant allele Rps1-k, conferring resistance to specific races of P. megasperma, was transferred from ‘Williams 82’ into two high-yielding, susceptible genotypes, A78-123018 and ‘Cumberland’. Thirty-six random resistant lines from each of five backcross generations of the two populations were evaluated for yield, maturity, lodging, and height in three Iowa environments. On average, 75% of the BC₀-derived lines did not differ significantly from the recurrent parent in yield. The BC₁ generation had an average yield equivalent to that of the recurrent parent. The results indicated that, if the donor and recurrent parents differ in yield by 10% or less, one backcross is sufficient to develop a population in which homozygous resistant, phenotypically similar lines can be bulked to replace the recurrent parent.

Key Words: Glycine max (L.) Merr. • Backross method • Plant breeding • Phytophthora megasperma f. sp. glycinea Kuan and Erwin

Received for publication April 27, 1987.