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a USDA-ARS, Corn and Soybean Research Unit, and Dep. of Horticulture and Crop Science, The Ohio State Univ., 215-A Williams Hall, 1680 Madison Ave., Wooster, OH 44691
b Dep. of Plant Pathology, The Ohio State Univ., Wooster, OH 44691
* Corresponding author (mian.3{at}osu.edu).
ABSTRACT
Stout-Rps1k soybean [Glycine max (L.) Merr.]) (Reg. No. GP-360, PI 644024) germplasm line was developed jointly by the USDA-ARS and the Ohio Agricultural Research and Development Center in Wooster, OH. It was released in October 2006 because it had the Rps1k gene for race-specific resistance to Phytophthora root and stem rot (caused by Phytophthora sojae, M.J. Kaufmann and J.W. Gerdemann) in the semidwarf cultivar Stout. Stout-Rps1k, a maturity group III (RM 3.3) germplasm, was developed by backcrossing the Rps1k gene from cultivar Sprite 87 into Stout. BC5–derived lines homozygous for the Rps1k gene were bulked and evaluated for agronomic performance in local and regional tests. Agronomic performance of Stout-Rps1k was similar to the recurrent parent Stout. The Rps1k gene conditions resistance against a wider range of P. sojae populations in the north-central soybean growing region of the United States than the Rps1a gene. Thus, Stout-Rps1k should be useful for breeders and researchers interested in developing new soybean lines with broad resistance to P. sojae populations and specific adaptation to high-yield environments.
Stout-Rps1k soybean [Glycine max (L.) Merr.]) (Reg. No. GP-360, PI 644024), maturity group III (relative maturity 3.3) germplasm, was developed in Ohio by backcrossing the Rps1k gene from ‘Sprite 87’ (Cooper et al., 1991) into ‘Stout’ (Cooper et al., 2001). Semidwarf soybean cultivars were selected from crosses of high-yielding indeterminate (Dt1) northern U.S. cultivars and high-yielding determinate (dt1) southern U.S. cultivars (Cooper, 1981). The semidwarf cultivars are particularly adapted to highly productive environments where lodging often is a barrier to high seed yields for the indeterminate tall cultivars (Cooper, 1981, 1985). Phytophthora root and stem rot (caused by Phytophthora sojae, M.J. Kaufmann and J.W. Gerdemann) is the number-two leading disease for soybean yield loss in the United States (Burnham et al., 2003). Phytophthora root and stem rot of soybean is mainly controlled by deploying cultivars with single genes with race-specific resistance to the pathogen (Gordon et al., 2007). A number of new P. sojae races have developed in the past 15 years, and Rps1a is not effective to many of the races that exist in the north-central region of the United States (Grau et al., 2004). New germplasm with broad resistance to current P. sojae populations are needed to reduce losses from Phytophthora root and stem rot in infested fields. 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).
Materials and Methods
Plant Materials and Pedigree
The F1 hybrids were obtained from a cross between Stout and Sprite 87. See Cooper et al. (2001) and Cooper et al. (1991) for a description of the development and breeding history of these cultivars. Stout was developed from the cross Sprite 87 x HC85-6577. HC85-6577 is a determinate semidwarf soybean 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 semidwarf line from the cross, L70T-453G 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. 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).
Breeding and Selection
The F1 progeny plants from the cross Stout x Sprite 87 were backcrossed to Stout, and the backcrossing cycle was continued to get the BC5F1 seeds. In each backcross generation, BC F1 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 the inoculum culture suspension into the hypocotyl with a hypodermic needle following the procedures of Keen et al. (1971) and Schmitthenner and Bhat (1994). The resistant BC F1 plants were crossed to the recurrent parent to get the next generation. The BC5F2 seeds from each resistant BC5F1 plant were planted in separate rows in a field in Wooster and 25 plants row–1 were harvested for further testing. Ten BC5F2:3 seedlings from the seeds of each selected BC5F2 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 BC5F2:3 plants for which all 10 seedlings were resistant to P. sojae were considered homozygous resistant. Remnant BC5F2:3 seeds from each homozygous resistant F2 plant were grown in 1-row plots. The lines homozygous for Rps1k were confirmed by testing seedlings grown from the 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 races 1, 4, and 7 and susceptible to races 25 and 30 had the Rps1k gene. The remnant BC5F2:4 seeds from 60 homozygous lines carrying the Rps1k gene were bulked to form Stout-Rps1k for evaluation of agronomic performance.
Evaluation of Field Performance and Data Analysis
The agronomic performance of Stout-Rps1k was evaluated in Elite Tests in high-yielding irrigated fields in Wooster, OH, for 3 yr (2002–2004). The experiments were conducted in a randomized complete block design with four replicates and 21 genotypes each year. 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 the seed yield. Plant height, maturity, and lodging data were collected before harvests. Analyses of variance for experimental data from the Elite Tests were conducted by using PROC GLM procedure of SAS ver. 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 as random effects. Differences among means were calculated using the least significant difference at P = 0.05 if their effects were found to be significant in the ANOVA. In 2003 Stout-Rps1k (designated as HC94-421BC) was entered in the USDA Northern Uniform Regional Test III (UT-III) (Abney, 2003).
Results and Discussion
The year x genotype interactions were not significant. Thus, only results from the combined analysis over years are presented. Averaged over the years, seed yield of Stout-Rps1k and Stout were similar. The average seed yield of Stout-Rps1k was 5380 kg ha–1, compared with 5268 kg ha–1 of Stout (Table 1 ). On average, the lodging score (1.9), plant height (63 cm), and maturity date (1 October) of Stout-Rps1k were similar to those of Stout. Like Stout, Stout-Rps1k has white flower, tawny pubescence, tan pods, dull yellow seeds with black hila, and semidwarf determinate growth habit. In the 2003 USDA Northern Uniform Regional Test III, Stout-Rps1k on average yielded the same as the semidwarf MG-III check cultivar Apex (Table 2 ). The seed yield of Stout-Rps1k was 2985 kg ha–1 against 3080 kg ha–1 of Apex. Lodging score (1.5), plant height (56 cm), and seed size (14.2 g 100–1 seeds) of Stout-Rps1k were similar to those of Apex. Stout-Rps1k had 31 g kg–1 higher seed protein than Apex, but the oil contents were similar for the two lines (Table 2). Stout-Rps1k should be useful for breeders interested in developing new germplasm with broad resistance to P. sojae populations and specific adaptation to high-yielding environments.
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Seed of this line will be deposited to the USDA Soybean Germplasm Collection at Urbana, IL and small amounts of seed for research purposes, including development and commercialization of new cultivars can be obtained by contacting the corresponding author. Seed will also be deposited in the USDA-ARS National Seed Storage Laboratory at Fort Collins, CO. This line will be available for research purposes, including development and commercialization of new cultivars. Appropriate recognition should be made if this material contributes to the development of a new breeding line or cultivar. No application for U.S. Plant Variety Protection will be made for Stout-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 November 14, 2007.
References
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