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Effects of Natural Selection in Segregating Soybean Populations Exposed to Phytophthora Rot or Soybean Cyst Nematodes¹

A study was conducted to determine the effects of natural selection in soybean [Glycine max (L.) Merr.] populations segregating for reaction to phytophthora rot (caused by Phytophthora megasperma Drechs. var. sojae Hildeb.) and soybean cyst nematode (SCN) (Heterodera glycines Ichinohe). Plantings of an F₂ population of ‘Forrest’ x ‘Tracy’ and 10 backcross F₃ families of Forrest (2) x Tracy were made at Stoneville, MS, where phytophthora rot caused injury to susceptible genotypes and at Ames Plantation in west Tennessee where SCN race 3 was present. The parent cultivar, Forrest, is resistant to SCN race 3 and moderately resistant to phytophthora rot, and Tracy is susceptible to SCN and resistant to phytophthora rot, races 1 through 9. The populations were advanced in bulk at each location for four generations.

Natural selection was not effective in increasing the percentage of plants having major genes for resistance to phytophthora rot in Stoneville plantings where susceptible cultivars show injury from the disease. The failure of natural selection to be effective was attributed to the presence of field resistant plants in the segregating population which competed effectively with plants having major genes for resistance.

Segregating populations grown on SCN infested soil showed a distinct increase in percentage of resistant plants as compared with populations grown on non-infested soil. On the basis of three recessive genes and 1 dominant gene being necessary for resistance to SCN race 3, 5.5% of the population would be expected to be resistant after the F₅ generation. The population of Forrest x Tracy grown on non-infested soil had 5% resistant plants after the F₅ generation, while the population advanced on SCN race 3 infested soil had 40% resistant plants. Backcross families of Forrest (2) x Tracy also had a distinctly higher percentage of SCN resistant plants when grown on infested soil than when grown on non-infested soil.

Key Words: Glycine max (L.) Merr. • Heterodera glycines Ichinoche • Phytophthora megasperma Drechs. var. sojae Hildeb.

² Supervisory research agronomist; research geneticist, USDA-SEA-AR, Stoneville, MS; research plant pathologist, USDA-SEA-AR, Jackson, TN; and agronomist, USDA-SEA-AR, Stoneville, MS.

Received for publication June 15, 1981.