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a Univ. of Arkansas Division of Agriculture Northeast Research and Extension Center, P.O. Box 48, Keiser, AR 72351
b Cotton Incorporated, 6399 Weston Pkwy., Cary, NC 27513
* Corresponding author (bourland{at}uark.edu).
ABSTRACT
Arkot 9623 (Reg. No. GP-908, PI 651858) and Arkot 9625 (Reg. No. GP-909, PI 651859) are noncommercial breeding lines of cotton (Gossypium hirsutum L.) released by the Arkansas Agricultural Experiment Station in January 2008. Both lines were derived from 1996 crosses using one common parent, Arkot 8712. The second parent of Arkot 9623 was DES 119 N Sm ne. The other parent of Arkot 9625 was ST 474. The lines were evaluated in 16 replicated tests in Arkansas from 2003 to 2006. Lint yields, lint percentage, and seed produced per area for each line were equal to two check cultivars. Both lines are early maturing, with Arkot 9625 significantly earlier than Arkot 9623 and the short-season cultivar SG 105, and Arkot 9623 equal to SG 105. Yield components of Arkot 9623 were similar to the checks, but Arkot 9625 produced larger seed with more lint per seed. Compared with check cultivars, Arkot 9623 fiber length was shorter and strength of both lines was weaker. Leaf pubescence and bract trichome density of the two lines were intermediate between the smooth-leaf and hairy-leaf check cultivars. Compared with the checks, both lines expressed improved resistance to bacterial blight [caused by Xanthomonas campestris pv. malvacearum (Smith) Dye], Fusarium wilt [caused by Fusarium oxysporum Schlect. F. sp. vasinfectum (Atk.) Snyd. & Hans.], root-knot nematode [Meloidogyne incognita (Kofoid & White, 1919) Chitwood, 1949], and a seedling disease pathogen. The relative yield, maturity, and line-specific host plant resistance traits make these lines valuable to cotton breeding programs.
Cotton (Gossypium hirsutum L.) originated as a perennial plant in tropical–subtropical regions but is now predominantly grown as an annual in temperate regions. Consequently, the development of early-maturing, high-yielding lines of cotton has been a long-time objective of many cotton breeding programs. The use of early-maturing lines has the potential to reduce inputs and risks associated with cotton production. Improvement of host plant resistance traits also has the potential to reduce inputs and risks. To be effective, both early-maturing and host plant resistance traits must be incorporated into germplasm without reducing yield.
Arkot 9623 (Reg. No. GP-908, PI 651858) and Arkot 9625 (Reg. No. GP-909, PI 651859) are noncommercial breeding lines of cotton that were released by the Arkansas Agricultural Experiment Station in January 2008. Both Arkot 9623 and Arkot 9625 were derived from 1996 crosses using one common parent, Arkot 8712 (PI 636101; Bourland et al., 2005). The second parent for Arkot 9623 was DES 119 N Sm ne (PI 603045; Meredith, 1998). The other parent of Arkot 9625 was ‘ST 474’ (PVP 9400152), a popular conventional cultivar in the 1990s. Both lines were released as part of an ongoing effort to develop improved germplasm lines having enhanced earliness, host plant resistance, yield, and fiber properties.
Methods
Early Generation Population and Line Development
Both lines were developed using the generalized procedures outlined by Bourland (2004). Within F1 populations grown at the Southeast Branch Station (Rohwer, AR) in 1997, bolls from visually superior individual plants were harvested and bulked. The F2 bulk populations were grown at the Northeast Research and Extension Center (Keiser, AR) in 1998, and superior individual plants were selected and harvested separately. Two of the individual plants were designated as 9623-21 and 9625-17 and were evaluated with other progenies at Keiser and Rohwer in 1999, 2000, and 2001. In 1999 and 2000, superior progenies were selected and advanced to the next year. Individual plant selections from the F5 generation in 2001 were evaluated as progenies in 2002. Two of these selections produced Arkot 9623 (tested as 9623-21-06) and Arkot 9625 (tested as 9625-17-04).
Field Test Evaluations
From 2003 to 2006, Arkot 9623 and Arkot 9625 were compared to ‘PSC 355’ (PVP 200000167) and ‘SG 105’ (PVP 9900190) in 16 replicated field tests at five Arkansas Agricultural Research Station sites and one on-farm site near Manila, AR (Table 1
). Experiment station test sites included the Northeast Research and Extension Center at Keiser in 2003 to 2006, the Delta Branch Experiment Station at Clarkedale in 2004, the Judd Hill Cooperative Research Site at Judd Hill in 2005 and 2006, the Lon Mann Cotton Research Station at Marianna in 2004 to 2006, and the Southeast Branch Experiment Station at Rohwer in 2003 to 2006. Since the Clarkedale and Judd Hill sites are only 42 km apart and have the same soil type, the two locations were considered the same location for analysis over years.
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Leaf and bracts were sampled at the Keiser, AR, test site each year. Leaf pubescence was rated using the rating system established by Bourland et al. (2003). Bracts were sampled, and marginal trichome density was determined using methods of Bourland and Hornbeck (2007). Leaf and bract data were analyzed by SAS v. 9.1 PROC GLM (SAS Institute, Cary, NC), with years and replications being random and entries being fixed.
Pest Resistance Evaluations
During selection, nurseries and seed increases of Arkot 9623 and Arkot 9625 were inoculated with multiple races of Xanthomonas campestris pv. malvacearum (Smith) Dye, the causal agent of bacterial blight. Susceptible plants were rogued from the early-generation populations and subsequent seed increases.
Percentages of wilted plants associated with Verticillium wilt (caused by Verticillium dahliae, Kleb) were visually estimated in field tests at Clarkedale in 2004 and at Judd Hill in 2005 and 2006. Both of these test sites have a history of naturally occurring infestations of this pathogen. Response of the lines to Fusarium wilt [caused by Fusarium oxysporum Schlect. F. sp. vasinfectum (Atk.) Snyd. & Hans.] was determined in the 2006 National Cotton Fusarium Wilt Test at Tallassee, AL.
Arkot 9623 and Arkot 9625 were evaluated for resistance to Rhizoctonia solani Kuehn (a seedling disease pathogen) 2004 and 2005 and for resistance to root-knot nematode [Meloidogyne incognita (Kofoid & White, 1919) Chitwood, 1949] in 2005 and 2006. Responses to R. solani and root-knot nematode were evaluated in greenhouse beds filled with soil (Routon-Dundee-Crevasse complex). Before cotton tests were planted, the beds were infested each spring with chopped tomato roots that were heavily galled by root-knot nematode. Roots of 1-mo-old cotton seedlings were examined for degree of galling (none, light, or heavy) of roots. An index was calculated by adding the number of seedlings with light galling (x50) and the number with heavy galling (x100), then dividing by the total number of seedlings. After root-knot nematode evaluation was completed, the soil was infested with R. solani–inoculated sorghum [Sorghum bicolor (L.) Moench] seed and then planted again with the cotton lines. One month after planting, the root-hypocotyl zones of seedlings were rated from 0 (no discoloration) to 70 (heavy discoloration). Dead seedlings were assigned a rating of 100. Ratings of seedlings were averaged to determine plot means. For both root-knot nematode and seedling disease evaluation, each plot was 0.6 m long by 0.3 m wide with approximately 10 plants plot–1. Plots were arranged in an RCB with four replications.
Response to tarnished plant bug [Lygus lineolaris (Palisot de Beauvois)] was determined in small plot field tests conducted at Keiser, AR, in 2005 and 2006. Single-row plots, 6 m by 1 m, were replicated 12 times and managed to encourage tarnished plant bug populations. White flowers were examined for plant bug damage, as indicated by discolored anthers, sequentially examined five to eight times over a 2-wk period in August of each year. A collective measure of percentage of damaged flowers over the sequential samples was determined for each plot.
All pest resistance data, except bacterial blight data, were analyzed using SAS v. 9.1 PROC GLM (SAS Institute, Cary, NC) with years and replications as random and entries being fixed.
Characteristics
Yield and Yield Components
Over locations, lint yields of Arkot 9623 and Arkot 9625 were relatively highest at the Manila site (Table 1). Arkot 9625 produced lowest relative yields at Clarkedale and Judd Hill, where the highest incidence of Verticillium wilt was observed. Relative yields of Arkot 9623 tended to be higher than Arkot 9625 at every site except on the clay soil at Keiser. When compared over locations and years, yields of both lines were statistically equal to each other and to the check cultivars (Table 2
).
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Morphological and Fiber Traits
Arkot 9623 was equal to PSC 355 in height but taller than SG 105 (Table 3
). Its maturity, as indicated by open bolls percentage, was intermediate to Arkot 9625 and the check cultivars. Arkot 9625 was equal to SG 105 in height and matured significantly earlier than either check cultivar.
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Micronaire and uniformity index of both lines were equal to the two check cultivars (Table 3). Fiber length of Arkot 9623 was significantly shorter than the check cultivars and Arkot 9625. Fiber length of Arkot 9625 was equal to PSC 355. Fiber strength of Arkot 9623 and Arkot 9625 was significantly weaker than either check cultivar.
Disease and Insect Resistance
During selection, Arkot 9623 and Arkot 9625 were screened for resistance to multiple races of Xanthomonas campestris pv. malvacearum (Smith) Dye, the causal agent of bacterial blight. Resistance to the multiple races conveys resistance to all known U.S. races of this pathogen. Both lines exhibited resistance to bacterial blight in annually produced seed increase blocks that were inoculated with the pathogen.
Incidence of Verticillium wilt on Arkot 9623 was equal to the check cultivars, but Arkot 9625 had significantly more wilted plants than Arkot 9623 or the check cultivars (Table 4 ). Due to earlier boll loading, early maturing lines tend to have higher incidence of wilted plants. The high incidence of wilted plants accompanied by relatively low yields at the Verticillium wilt test site (Table 1) suggests, however, that Arkot 9625 is more susceptible to Verticillium wilt than the check cultivars or Arkot 9623.
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In greenhouse tests, both lines appeared to be more susceptible to root-knot nematode than the check cultivars (Table 4). Arkot 9623 and Arkot 9625 had significantly higher galling indices than PSC 355. In other greenhouse tests, both lines were more resistant to seedling disease caused by R. solani than SG 105, but equal to PSC 355.
In field tests, both lines were more resistant to tarnished plant bug than SG 105 and the susceptible frego-bract check in tests conducted in 2005 and 2006 (Table 4). Arkot 9623 also had significantly fewer damaged flowers values than PSC 355. Damaged flowers values for Arkot 9625 did not differ significantly from Arkot 9623 or PSC 355.
Availability
Small quantities of Arkot 9623 and Arkot 9625 seed may be obtained for breeding purposes from the corresponding author. Unless specifically approved by the Arkansas Agricultural Experiment Station, the lines may not be used as recurrent parents in a breeding program.
Acknowledgments
Arkot 9623 and Arkot 9625 were developed with financial support from Arkansas Agricultural Experiment Station and Cotton Incorporated.
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 July 23, 2008.
References
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