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a USDA-ARS, Crop Genetics and Production Research Unit, Jackson, TN 38301
b (retired)
* Corresponding author (amengistu{at}ars.usda.gov)
Soybean [Glycine max (L.) Merr.] germplasm line D95–5048 (Reg. no. GP-329, PI 642958) was developed by the USDA-ARS, Stoneville, MS, in cooperation with the Mississippi Agricultural and Forestry Experiment Station, Stoneville, MS, and released in 2005. The reason for this release is because it carries the Rps1-b gene for resistance to Phytophthora rot [caused by Phytophthora sojae (M. J. Kaufmann & J. W. Gerdemann)] and has resistance to soybean cyst nematode (SCN, Heterodera glycines Ichinohe) HG type 0 (race 3) and moderate resistance to SCN HG Type 1.3.6.7 (race 14). Each previously released line in the set has a unique gene–allele combination in the same genetic background (Bedford) (Hartwig and Epps, 1978) which makes them useful as diagnostic tools. These released lines, each with a single different allele, are D92–6487, Rps1-k; D93–8664, Rps5; D94–6041, Rps4; D95–5246, Rps3-a; D96–1217, Rps1-c; D98–1216, Rps3-b (Kilen and Young, 1994, 1996, 1998, 2000, 2003a, 2003b), D98–1218, Rps2 (Mengistu et al., 2005), D99–2018, Rps6 (Donald et al., 2007), and D99–2065, Rps1-d (Mengistu et al., 2006). All these releases, including D95–5048, may be used in concert to pyramid genes for Phytophthora resistance. They also have the same level of resistance to SCN HG Type 0 and HG Type 1.3.6.7 as their recurrent parent, Bedford.
The Rps1-b allele confers resistance to races 1, 3 through 9, 13, 15, 18, 21, and 22 of P. sojae (Schmitthenner et al., 1994). D95–5048 was developed by backcrossing the allele Rps1-b into a Bedford background. The original donor of Rps1-b was germplasm line FC 31745, but ‘Tracy’ (Hartwig, 1974) was used as an intermediate donor because of its more desirable traits. Tracy was first crossed with ‘Forrest’ (Hartwig and Epps, 1973), but Bedford was later chosen as the recurrent parent. The pedigree of D95–5048 is Bedford (5) x sel. (Forest (3) x Tracy).
Race 1 of P. sojae was used to identify resistance. Race1 was verified by a standard set of soybean differentials [PI 130091, PI 171442, PI 340046, Williams 82, Harsoy 63, and PI 399073 (Buzzell and Anderson, 1992; Dorrance and Schmitthenner, 2000)] and breeding lines with known genes for resistance to P. sojae [FC 31745, Bedford, Freedom, Arksoy, D55–1492, D51–4863, D94–6041, D93–8664, D99–2004, and D98–1216 (Kilen et al., 1974; Kilen and Young, 1996, 1998)] and an array of P. sojae races. Seedlings were inoculated by the hypocotyl puncture method (Morgan and Hartwig, 1965), using race 1 of the pathogen. In each cycle of the backcrossing program, F2 plants were harvested. The reaction of 12 F3 plants from each F2 plant to pathogen inoculation was used to identify F2 plants homozygous for resistance to Phytophthora rot. Remnant seed from homozygous resistant F2 plants were used as pollen parents. After the fourth backcross to Bedford, an F3 plant that was uniformly resistant to P. sojae was increased for three generations (F3, F4, F5) by the pedigree method (Fehr, 1993). During these 4 yr, visual selection was made to complete the recovery of the morphological type of the recurrent parent. One hundred F5 plants were inoculated with race 1 to verify the reaction of the F3. Approximately 100 plants were also inoculated with race 1 at the F8 generation. D95–5048, composited in the BC4F3:8 generation, was evaluated for reaction to HG type 0 and HG Type 1.3.6.7 (Niblack et al., 2002) of SCN at Jackson, TN, using the method described by Young (1990). D95–5048 had female index values of 4 for each HG type 0 and 5 for type 1.3.6.7, indicating its level of resistance to these two races of SCN.
In replicated yield tests on clay soil (Vertic Haplaquept, very fine montmorillonitic, thermic) at Stoneville, MS, in 1999 through 2002, yields of D95–5048 and Bedford were 3756 vs. 3749 kg ha–1, 1566 vs. 1263 kg ha–1, 1646 vs. 1001 kg ha–1, and 3548 vs. 3265 kg ha–1 respectively. The 4-yr means were 2627 kg ha–1 for D95–5048 and 2318 kg ha–1 for Bedford. The differences in yield in the 4 yr were significant at the P 
0.01 level. A breeding line [D55–1492, from (Roanoke x Rose Non-pop) x (S100 x CNS)] grown in the same environment, but very susceptible to Phytophthora rot, showed symptoms of the disease all 4 yr, while D95–5048 showed no symptoms. Comparative yields on soils not prone to Phytophthora rot, however, were not measured.
D95–5048 is in maturity group V and is similar to its recurrent parent Bedford for all observable traits. Both D95–5048 and Bedford have white flowers and tawny pubescence. During the 3 yr of yield testing, D95–5048 had a mean plant height of 91 cm, a lodging score of 2.5 (on a scale of 1–5), and matured 125 d after planting. Bedford had a mean height of 90 cm, a lodging score of 2.7, and matured 126 d after planting.
Small amounts of seed may be obtained from the Crop Genetics and Production Research Unit, P. O. Box 345, Stoneville, MS 38776–0345 for research purposes, including development and commercialization of new cultivars. It is requested that appropriate acknowledgment and recognition be made if this germplasm contributes to the development of a new cultivar. Seed of D95–5048 has been deposited in the National Plant Germplasm System.
NOTES
Received for publication July 7, 2006.
REFERENCES
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