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a USDA-ARS, Corn and Soybean Research Unit, and Dep. of Horticulture and Crop Science, 1680 Madison Ave., The Ohio State Univ., Wooster, OH 44691
b Dep. of Plant Pathology, The Ohio State Univ., Wooster, OH 44691
* Corresponding author (mian.3{at}osu.edu).
ABSTRACT
Strong-Rps1k (Reg. No. GP-359, PI 644025) soybean [Glycine max (L.) Merr.] germplasm line was developed jointly by the USDA-ARS and The Ohio Agricultural Research and Development Center (OARDC) at Wooster, OH. It was released because it carried the Rps1k gene for race specific resistance to Phytophthora root and stem rot (caused by Phytophthora sojae Kaufmann & Gerdemann) in the semidwarf background of cultivar Strong. Strong, which has the Rps1a gene, is well known for its adaptation to high yield environments where excessive vegetative growth and early lodging limit the yield potential of taller indeterminate soybean cultivars. Strong-Rps1k was developed by backcrossing the Rps1k gene from cultivar Sprite 87 into Strong. Seeds from the BC5F2:3 plants homozygous for the Rps1k gene were bulked to make Strong-Rps1k. It was evaluated for agronomic performance in local and regional tests. Performance of Strong-Rps1k was comparable to that of Strong. The Rps1k gene provides resistance against a wider range the P. sojae populations that are present in the North-Central U.S. soybean-growing region than does the Rps1a gene. Thus, Strong-Rps1k may be useful for breeders and researchers interested in developing new germplasm with resistance to Phytophthora root and stem rot with specific adaptation to high-yielding environments.
Strong-Rps1k (Reg. No. GP-359, PI 644025), an early maturity group IV (relative maturity 4.2) soybean [Glycine max (L.) Merr.] germplasm, was developed in Ohio by backcrossing the Rps1k gene from cultivar Sprite 87 (Cooper et al., 1991) into the early maturity group IV determinate semidwarf ‘Strong’ (Cooper et al., 2001). Semidwarf cultivars are specially adapted to highly productive environments where lodging frequently is a barrier to high soybean yields of the taller indeterminate cultivars (Cooper, 1981, 1985). The semidwarf soybean cultivars were developed by crossing high-yielding indeterminate (Dt1) cultivars grown in the northern United States with high-yielding determinate (dt1) cultivars grown in the southern United States followed by selection of high-yielding determinate lines adapted to the midwestern environments (Cooper, 1981). Phytophthora root and stem rot is the second leading disease for soybean yield loss in the United States (Burnham et al., 2003). Phytophthora root and stem rot (caused by Phytophthora sojae Kaufmann & Gerdemann) of soybean is mainly managed by deploying single genes that provide race specific resistance to the pathogen (Gordon et al., 2007). Many new races of P. sojae have developed in the past 15 yr, and Rps1a is no longer effective to many populations that exist in the North-Central region (Grau et al., 2004). While many fields have been identified in the North-Central region with isolates that have adapted to Rps1k (Grau et al., 2004), the proportion of such isolates in the field is still low and in most fields Rps1k is more effective than Rps1a (Dorrance et al., 2003). Cultivars with broad resistance to P. sojae populations are needed to reduce losses from this disease in infested soybean fields.
Materials and Methods
Parental Lines and Pedigree Information
Strong (Cooper et al., 2001) and Sprite 87 (Cooper et al., 1991) were used as parental lines in this research. Sprite 87 was developed by backcrossing the Rps1k gene from ‘Williams 82’ (Bernard and Cremeens, 1988) into ‘Sprite’ (Cooper et al., 1990). Sprite was developed from a cross between ‘Williams’ (Bernard and Lindahl, 1972) and ‘Ransom’ (Brim and Elledge, 1973). Strong, which has the Rps1a gene, is well known for its adaptation to high-yield environments where excessive vegetative growth and early lodging limit the yield potential of taller indeterminate soybean cultivars (Cooper et al., 2001; Cooper, 1981, 1985). Strong is derived from the cross Sprite 87 x HC85-6577. HC85-6577 is a semidwarf line from the cross HC78-350 x HC78-676. HC78-350 is a semidwarf line from the cross L72U-2567 x ‘Essex’ (Smith and Camper, 1973). L72U-2567 is a semidwarf line from the cross Williams x Ransom. HC78-676 is a semidwarf line from the cross L70T-543G x L74D-619. L70T-453G is an indeterminate line from the cross L15 x ‘Amsoy 71’ (Probst et al., 1972). L15 is a BC5–derived near-isogenic line of cultivar Wayne (Bernard, 1966). L74D-619 is a semidwarf line from the cross, Williams x Ransom.
Backcross Breeding and Phytophthora sojae Resistant Line Selection
The F1 plants from the Strong x Sprite 87 cross were backcrossed to the recurrent parent Strong, and the backcrossing cycle was repeated to obtain the BC5F1 seeds. In each backcross generation, progeny plants with resistance to P. sojae were selected by screening seedlings with race 3 (vir 1a, 7) of P. sojae in a greenhouse. Each 7-d-old seedling was inoculated by injecting inoculum culture suspensions into the hypocotyl with a hypodermic needle following the procedures described by Keen et al. (1971) and Schmitthenner and Bhat (1994). The resistant plants were crossed to the recurrent parent to obtain the next backcross generation. The BC5F2 seeds harvested from each resistant BC5F1 plant were planted in separate rows in a field in Wooster, OH, and 25 plants row–1 were harvested for further testing. Ten BC5F3 seedlings from the seeds of each selected F2 plant were grown in a large pot in a greenhouse, and the seedlings were tested against race 3 of P. sojae as described above. The F2 plants for which all 10 seedlings were resistant to P. sojae were considered homozygous resistant. The remnant seeds from each homozygous resistant F2 plant were grown in 1-row plots. The presence of Rps1k was then confirmed using the plants grown from BC5F2:4 seeds harvested from each row with P. sojae races 1 (vir 7), 4 (1a, 1c, 7), 7 (1a, 3a, 6, 7), 25 (1a, 1b, 1c, 1k, 7), and 30 (1a, 1b, 1k, 3a, 6, 7) of P. sojae as described above. Lines resistant to race 1, 4, and 7 and susceptible to race 25 and 30 carried the Rps1k gene. The remnant BC5F2:4 seeds from lines homozygous for the Rps1k gene were bulked and BC5F2:4 lines were tested as Strong-Rps1k for agronomic performance.
Evaluation of Agronomic Performance and Data Analysis
Strong-Rps1k was tested for agronomic performance in "Elite Tests" in high-yielding irrigated fields in Wooster for 3 yr (2002–2004). The experiment was conducted in a randomized complete block design with four replicates and 21 genotypes each year. Strong was included as a check for comparative purposes. Each plot was planted 6.4 m long with eight rows. The spacing between rows for the inside six rows was 19 cm, while the two border rows were 70 cm from the respective neighboring inside rows. After end trimming, 4.7 m of the six inside rows of each plot were harvested using a plot combine to measure seed yield. Plant height, maturity, and lodging data were collected before harvest. Analyses of variance (ANOVAs) for experimental data from the Elite Tests were conducted by using PROC GLM procedure of SAS version 9.1 (SAS Institute, 2002). For the combined analysis across years, only the data from genotypes common across the 3 yr were used. Years and genotypes were considered as fixed effects, and replications within year were considered as random effects. Differences among means were separated using the least significant difference (LSD) at P = 0.05 if their effects were found to be significant in the ANOVA. In 2003 Strong-Rps1k (originally designated as HC94-422BC) was entered in the USDA Northern Regional Uniform Test IV (Abney, 2003).
Results and Discussion
The year x genotype interactions were not significant for any of the traits; thus, only the results of the combined analyses across years are presented. Averaged over the 3 yr, the seed yield of Strong-Rps1k averaged 5385 kg ha–1, which was not significantly different from the recurrent parent Strong and other check cultivars (Table 1 ). The maturity date, lodging score, and plant height of Strong-Rps1k were also not different from Strong. The average data from the 2003 USDA Northern Uniform Regional Test is presented in Table 2 to demonstrate the numeric similarity of the agronomic performance of Strong and Strong-Rps1k. The seed yield of Strong-Rps1k was 2730 kg ha–1, while Strong yielded 2737 kg ha–1 (Table 2). Strong-Rps1k had the same maturity date and lodging score as Strong. The plant height, seed size, seed protein, and seed oil data of Strong-Rps1k were also quite similar to those of Strong (Table 2).
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Availability
Seed of this germplasm line will be deposited in the USDA soybean germplasm collection at Urbana, IL, where it will be available for research and development purpose, including development and commercialization of new cultivars. Seed will also be deposited in the USDA-ARS National Seed Storage Laboratory at Fort Collins, CO. Initially, small amounts of seeds may be obtained from the corresponding author. Appropriate recognition should be made if this material contributes to the development of a new germplasm or cultivar. No application for U.S. Plant Variety Protection will be made for Strong-Rps1k.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication September 11, 2007.
References
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